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Voltammetry in dimethylsulphoxide—a review
Talanta, 1972. Vol. 19. pp. 989 to 1007. Pergamon Press. Printed in Northern Ireland
VOLTAMMETRY
IN DIMETHYLSULPHOXIDEA REVIEW
THOMASR. KOCH* and WILLIAM C. PURDY Department
of Chemistry,
University of Maryland,
College Park, Md. 20742, U.S.A.
(Received 15 November 1971. Accepted 27 December 1971) Summary-A review is given of voltammetric studies using dimethylsulphoxide as supporting electrolyte, and includes details of inorganic and organic systems studied and of the reference electrodes developed for use in this medium. IN RECENT YEARS the
scope of voltammetry has been expanded by an increase in the (DMSO) has of non-aqueous solvents employed. Dimethylsulphoxide been of great value in these voltammetric studies owing to its high dielectric constant and absence of ionizable protons. Many investigators have found purification of the solvent to be a necessity, owing to a cathodic wave caused by an impurity. This impurity can be removed by distillation; in addition, distillation over a drying agent helps to ensure a constant, low water level (usually about 0*03-0*05 %). Tetraalkylammonium salts are generally employed as supporting electrolytes to take full advantage of the extended cathodic range of DMSO as compared to water. While a variety of reference electrodes have been used, the most common is an aqueous saturated calomel electrode (SCE), separated from the sample compartment by a salt bridge. Several reference electrodes employing DMSO as a solvent have been described and they will be discussed later. number
EARLIER
WORK
DMSO were conducted in 1959 by Gutmann and Sch6ber.l With tetraethylammonium nitrate as supporting electrolyte, they reported a usable voltage range from -0.2 to -2.74 US. SCE. These workers described the frequent occurrence of maxima which, in some cases, were at least partially eliminated by varying the drop-rate. Conventional and derivative polarographic methods were used. Single, well-defined waves were found for the alkali metal ions and for the ammonium ion. The alkaline-earth metal ions were reported to give single, well-defined waves. Silicon tetrachloride exhibited two waves, well-formed and of equal height. Zirconium(N) and hafnium(IV) gave well-developed waves, each exhibiting a small first-order maximum. Titanium(N) gave a wave which was ideal in shape; derivative polarography showed three indistinct steps in this reduction, corresponding to Ti(IV) ---fTi(II1) -+ Ti(I1) -+ Ti(0). The
first
voltammetric
studies
in
Antimony(II1) exhibited three waves and niobium(V) one. The half-wave potential values reported by Gutmann and Schijber are included in Table I. These authors carried out no concentration studies. However, by use of a variety of reference * Present address: Erie County Laboratories, Grider Street, Buffalo, N.Y. 14214, U.S.A. 1
989
E. J. Meyer Memorial
Hospital
Division,
462
l.OM KClO,
0.1 M TEANO O*lMTBAP O+lMTEAP 0.1 M KClO,
Cr(C103,
CsCl cu(Clo‘)* cu(clo,)* cu(cloJ~
Cr(II1)
WI) CucrI) CWI) CUOI)
O*lMTEAP O*lMTEA_P 0.1 M TEANO* OalMTEANO, O*fM TEAP 0.1M TEAP @iMTBAP O*lM TBAP O-1M KCIOI O*lM KClO, 0.1 M KClO, l*OM KClO, l*OM KCIO, I *OM KClO, l*OM KClO, 1 *OM KCIO, O*lMTEAP O*lMTBAP 0~1MTEAP O.lMKCIOI l*OM KClO, l*OM KCIOI 1 *OM KCIOp l*OM KCIO, 0.1 M KClO,
O*lMTEAP
Inert electrolyte
::g; Cd0 CdOI) Ce(III) CoOI) Co@) Co@) Co0 CoOI) Co@) CooI) CrO
Cd(NO&,
BaCl, CaWU Ca(CQh Cd(ClO& CdfClO.), Cd&IO& Cd(ClO& Cd(ClWn
BaK104)a
AlCl*
Form used
Cd(ClO& Cd(ClO& WClOJ, Cd(ClWs Cd@Wz C&l* 0% Co(ClO& co(c1o3. Co(ClW* co(clo,)s co(clo$, Co(ClO& Cr(ClU
Cd(I1) Cd(H)
CaOU Cd011 C&W CdOO Cd(iO Cd(II)
CaOI>
BaOrI)
wm
AD3
Active species
!z:;; O-870(1) 1,603(2) 2.03 0.341 -0.05 0.212
::;5 2.09 2.30 2-38 0.58 o-931 0.966 0.820 0.820 0.63 0.789 0.794 &685 0.670 0.66 2.24 1.43 1.28 1.316 1.465 1.423 1.520 1,536
1*67(l) 1*95(2)
--Ez1,, v
TABLE I.-POLAROGRAPHIC DATA IN DMSO.
11
1: 35 11
SCE Hg Pool Ag DMSO Hg Pool
:: I1 11
:55 3s 25 SCE
::
Z = 1.96;
3;
0.096; 0.100 0.130; 0,097; 0,938
D = 4.83 D = 1.06
D = 0.734
n = 1.78
m = 0.130; D = 4.83 n = 1.95; z = I.95 m = 0.121; D = 7.12
m = m = m = m = ZJ =
0.040 0.048; 0.031; O-047 O-029
m = 0,030
D = 2.52 I3 = l-66
n=0*96; Z=l.48 n = 1.84; Z = 1.88 m = 0.033; D = 6.75 “D z ;;y”; D = 4.94
m = m = m = m = m = n=3
2: 25
;; 21 25
3: 25 25 25 25 25 35 25
ZB= 1.19
n = 1.97; Z = 2.07
z, = 0.86;
nx + ns = 2.90
Miscellaneous
3: 35 11 11 11 11 9 11 11 11 11 9 7
14 3s 1
25 ?
SCE Ag/DMSO SCE SCE AglDMSO Ag/DMSO Hg Pool Hg Pool Hg Pool SCE SCE Hg Pool Hg Pool SCE SCE SCE SCE SCE AgtDMSO SCE SCE SCE Hg Pool Hg Pool SCE a: ?
3s
?
Ag/DMSO
Ref.
T, “C
Reference electrode
A. INORGANIC MATE-
O*lMTEAP O*lM TEAP
0.1 M KClO,
O.lMTEAP O*lMTEAP O.lMTEAP
0.1 M TEAP O*lMTEAP 0.1M TEAP O*lMTEAP O.lMTEAP
O.lM TEAP O*lM TEAP O.lM TEAP
O.lM TEANO,
O*lMTEAP O*lMTEAP
O*lMTEAP O.IMTEAP O.lMTBAP 0.1 M TBAP 0.1 M TEANO*
Fe(ClO&
GdCl. HCl HCI
HClO, HClO, HISO. HISO, HCsHJ%
HCIHIOI TEA-HSOIHfCI,
HfCl,
HOC& KC1
KClO, KClO, KCIO, KCIO, KCIO,
FeOH)
GdO HO[) HO)
H(I) HO) Ha) H(I) H(I)
H(I) HSO,-
Hf (IV)
HfO
Ho(III) KO
Fe(II1)
Fe(ClO& Fe(ClO&
1 *OM KCIOl 1 *OM KClO, l*OM KCIOl 1 .OM KCIOI O*lMTEAP 0.1 M TEAP O*lMTEAP
FeOU
ErOW Eu(lII)
CUOI) DY(III)
cum cum
cu@)
02:;8(1) l-38(2) 1*07(l) 1*17(2) 2.09 --@01(l) 0*17(2) 2.22(3) 2.11 1.97 2.362 2.06 2.11
0.127 0.122 0.058 0.051 2.08 2.09 0.81(l) 2.12(2) 1.12 -0*05(l) 1*12(2) 0*722(l) 1.41 l(2) 2.16 1.08 -0*01(l) O-17(2) 1*14(3) 1.14 0.96 I.12 1.06 1*91(l) 2*22(2) 2.3
SCE Ag/DMSO Hg Pool SCE SCE
SCE SCE
SCE
SCE SCE SCE
SCE Ag/DMSO SCE SCE SCE
SCE SCE SCE
SCE
Ag/DMSO Ag/DMSO
Hg Pool Hg Pool SCE SCE SCE SCE SCE
f:
2:
?
21 ?
21
30 30 25
? ? ? 30 ?
21 30 ?
25
? ?
:: 7
23 35 11 9 1
7 23
1
3:
2:
23 35 23
7 5 23
11
::
::
11 11
0.100 I) = 2.46 D = 1.73
D = 1.630
tl=3 s1=200 sl=2020; ns=l sI = 2320; n, = 0.95 s = 2240; n = 0.94 n = 0.99; Z = 1.42 m = 0.060
xx = -0.054 x* = -0.15
s,=200 sI = 1260
s,=300 sl=2260; nl=l sJ = 2600; n, = 1.04 s = 2000; n = 0.95 II =0*75 s = 2400; n = 0.90
ml = O-052
m, = 0.037;
n, = 1.20
na=2
0.103; 0.102; 0.127
n = 1.60 n, = 0.80;
m = m = m = m = n=3 n=3 nt=l;
Ni(I1) Ni(I1) Ni(II) Ni(I1) Ni(I1) Ni(I1) Ni(I1) Ni(I1) Ni(TI) Ni(II) PWI) IWI) PWI)
:$I,
Li(1) WI) Li(1) Mg(II) WII) M&II) Mu(lI) Ma@) MnOI) MnoI) MuOIl Mn(I1) NaO Nag) N&I) NbCV) Nd(II1)
K(I) La(III)
Active species
Ni(ClO& Ni(CIO,)* Ni(CIO& Ni(ClO& Ni(ClO& Ni(NWa Ni(CIOJ, Ni(CIO& NWOJ, Ni(ClOJ, Pb(ClOJ~ Pb(C103, Pb(ClO$s
z:; NH, NH&l
~~~~~~ Mg(NW, Mn(ClO,), MntClO~~~ Mn(ClO& Mn(ClW~ Mu(ClO& MnClt NaC104 NaCIo, NaHSO&
LaCI, LiCl LiCIOI LiNOIl
?
Form used
O*lMTBAP O.lMTBAP O*lMTEAP @lM KClOp O*lM KCIOp O*lM KClO* I-OM KCIO, l*OM KCIO, l*OM KClO, l*OM KCIOI O+lM TBAP O*lM TEAP O-TMKClO&
O*lM TBAP 08IM TEAP 0.1M ‘WAN& O*lM TEAP O*lMTBAP O.lM TEAP t%lMTEANOs @lM TBAP l*OM KClO, l*OM KClO, l.OM KClOa l*OM KClO& 0.1M TEAP WlMTBAP O+lMTEANOs O,IMTEAI? @lM TEAP 0.1M TEANO. O*lM TEAP OelMTEAP O.lMTBAP
Inert electrofyte v
1.207 0.95 1.124 1.120 l-10 1.155 1,122 1*144 1.110 0.814 0.43 0,610
2070 226 245 238 2.38 2.44 2.28 2.19 1,683 1643 1*810 1,823 1.57 1.73 2.07 1.96 2.07 0.92 2.20 -0.02 0*17(l) :‘w&)
-Zh,
Hg Pool Hg Pool Ag/DMSO Hg Pool Hg Pool SCE Hg Pool Hg Pool SCE SCE Hg Pool Ag/DMsO Hg Pool
SCE SCE SCE Ag/DMSO SCE Ag/DMSO SCE SCE SCE SCE Hg Pool Hg Pool Ag/DMSO SCE SCE Ag/DMSO SCE SCE SCE SCE SCE
Reference electrode
TABLE 1 (Continues
:z 25 3s 25 3s 2s ? 2s
215
3”
2:” ? 1
;55 35 ? 25 21 ? 30
11 7 1 3s 9 3s
2s 21 21 ? 25 ? 21 25 2s
:: 11
:: 11 8 11 11 11 11
11 I1
273 23
3: 5 1
:: II 3s 9
; II
Ref.
T, ‘C
WO40; D = 2.90 O-034 0.038 0.038; D = 3.70 Z=lY%
m = m = m= m = m = n= m =
0.091 0*08S; 1) = 2.06 0.107; D = 0.845 0.087 0*030 1.99; Z = 2.33 OG46; D = 2.25
n== 3 s = 475 nl=l; s,=lS60 n*= 1; s,=l280 m=&OS3; D=S*86 m = 0.053; D = 7.74 z = 1.71 m = 0.067; D = 3.88 m = O-074; D = 5.66
n = 0.96; Z = 1.42
m = m = m = m = n-l;
n -= 2.22
n = O-73; Z = 1.61
m = O-062; D = 6.69 n==3
Mis~llaneous
O*lM TEAP
Th(CW,
ThCld
UCl,
ThOv)
Tho[V
Th(IW ‘WV) TKO WV)
znm ZnO znm Zn(li)
UCVI) Yb(II1)
UO*cNaJ,
O*lM TEAP
Sr(NO&
WI) MI) TbOIQ Th(IV)
YbCll
O+lM TEAP
SmCI,
Sm(nI)
TEAP TEANO% TBAP TEAP
0.IMTEAP GIMTBAP O*lM TBAP O*lM KCIO,
@lM TEAP 0.IMTEAP
O.lM O.lM O.lM O.lM
0.1&f TEANO O*lM TEAP OaIMTEAP O*lM TEAP
OvlM TEANO&
SiC1,
zgj PbOU Pbol1 Pb(II)
WV)
OvlM KCIOI O.lM KClO, 1 *OM KCIO, lGI4 KClO, l%%fKCIO, 1 .OM KCIOI O.lMTEAP O.lM TEANO, O-l&ZTEANO,
O*lM KCIO,
~~~) Sb@II)
PbGfO,),
Pb(CIO& Pb(CQ), Pb(CIW, Pb(CWs Pb(CIOJ, Pb(ClO& P&X, RbCl Sbl.
Pb(II) PbOI) 0.651 o-531 o*so 0.541 0.537 0,561 0,607 2.20 2.06 &36(l) @52(2) 1*13(3) @52(l) l-09(2) 2*02(l) 2*12(2) 210 2.20 2.19 1*55(l) 2*00(2) 1*75(l) 214(2) 1*57(l) 1.72 to 1*95(2) 2*07(3) 1*57 I.21 050 1*09(l) l-56(2) 0.53 1.48(l) 221(2) 0.95‘ . I-322 1.298 1.054 17
21 2: ? 21
SCE AglDMSO SCE AglDMSO
1: 16 7 35 I1 ::
;: 21 21 ? 25 :55
SCE SCE Ag/DMSO wg Pool wg Pool wg Pod
17
17 1
21
3:
7
21 21
SCE SCE SCE SCE
SCE
SCB
32
21
SCE
1
1; 11 11 11 7 1 1
21
:z
z 35
SCE
II 11
;: 21
35 25
Hg Pa01 SCE SCE SCE SCE Hg Pool Hg Pool SCE SCE SCE D = 1.60
D = 2.61
n1=1 o-22; n* = 3 0.056; n = 1 1 2 1.80; Z = l-94 &029; D = 3.38 m = @036; D = 3-58 m = @034; D = 5.38 x. = x = nl = n, = n = m =
q=o*o9;
n = 2.09; Z = 2.03 n=3 n* = 0.22; I = 0.68 n* = o-41 ; Z = 1.58
ni -= 1 n, = 2
m = 0.055; D = 2.19 n=3
m = 0.034; m = 0.060 m = 0.048
m = 0.052; ?n = 0*030
!Q
g
zg
$ G-
If
E
B
I
2.
c:
O*lM TEAP
0.1 M TEAP
O.lMTEAP
O*lM TEAP
@5M LiCl
O*lM TEAP
O*lMTEAP
O.lMTEAP
Al acetylacetonate
2-Aminoethanethiol
2-Aminoethanol
4Aminophenylsulphone
Anthraquinone
Asparagine Aspartic acid
Butylamine
Inert electrolyte
O*lMTEAP
ZrO(ClO&
Zr (IV)
Active species
O.lM TEAP
ZrDCll
ZrO
ZJWI) ZnO ZnOI) Zn(II)
Zr (IV) Zr (IV)
Inert electrolyte
l.OM KCIOl l*OM KClO, l.OM KClO, O*lM KCIOI O*lMTEANOa O*lM TEAP
Form used
Zn(C10J, Zn(ClW* Zn(ClW* Zn(ClW, ZrCl, ZrCl,
Active species I’
Ag/DMSO
SCE
Hg Pool Hg Pool SCE SCE SCE SCE
Reference electrode
V
2*2(l) 2*4(2) 0*17(l) 1.90(2) 2*40(3) -0*07(l) -@02(2) 0*17(l) 2*24(2) 0.33(l) 0*70(2) 2382 2*141(l) 2*618(2) 0.00
--&I,,
SCE
SCE SCE
Hg Pool
SCE
SCE
SCE
SCE
Reference electrode
B. Organic Materials
Y:Eg; 2*05(3)
i::(l) l-38(2) 0*98(l) l-38(2) 1.61(3)
1.089 1.082 1.099 1.066
-&I,,
?
25 25
?
?
?
?
25
T, “C
?
25
;: 35 21 25
25
T, “C
23
52 52
8
23
23
23
14
Ref.
35
32
11 11 11 11 1 32
Ref. = = = =
n = ni = n8 = n =
n1 = n2 = n,=l; ni + s1 = s1 = q =
x1 = x, = xx = xg = XI) = RI = na = n, =
m m m m
D = 3.18 D = 2.34
0.64; s = 4980 0.77; s1 = 6120 0.72; sz = 8660 1.80; s = 3500
2; s, = 4380 0.95; sa = 2.30 s,=2280 n, = 2.10 2120; s* = 1100 69 75
Miscellaneous
-0.054 -0.15 -0.86 -0.10 -0.14 0.57 0.21 0.40
0.108 0.067; 0.097; 0.068
Miscellaneous
%
g 4
.c
i
8
1 0, g
P
Voltammetry
in dimethylsulphoxide
995
0
0
II II II + I1 It II 1t II II II II + II II li II
~'~ O0 e ~ ~ " ~ ' ~ 0
~
00,,"~'~0 ~
~'- ~ ' ~ "
0 ~ ' ~ ~" ~
II II
~'~ ~"~ ~
II II Ii II II II II
~
~'~ r ' ~ . ~
~-~ r'~ t ~"
0
II li + II II II II
¢~ 0
~
~-"~'~0
~"~ ~"~ ~
0.5M LiCl
0.5M LiCl
acid
acid
acid
Nitrobenzene
m-Nitrobenzoic
o-Nitrobenzoic
p-Nitrobenzoic
0.5M LiCl
0.5M LiCl
0.5M LiCl
o-Nitrophenol
p-Nitrophenol
3-Nitrophthalic
0*5M LiCl
Nitroterephthalic
0.5M LiCl O.lMTEAP O.lMTEAP 0.5M LiCl
0.5M LiCl
0.5M LiCl
Oxalic acid Phenylalanine Proline pQuinone
Sodium m-nitrobenzoate
Sodiump-nitrophenolate
acid
0.5M LiCl
Nitroresorcinol
acid
0.5M LiCl
m-Nitrophenol
0.5M LiCl
0.5M LiCl
0.5M LiCl
Inert electrolyte
p-Nitrobenzaldehyde
Active species I’
8
8
? 25 25 ? ?
Hg Pool Hg Pool
Hg Pool Hg Pool Hg Pool
10
8 10
? ? 25 ? 25 25 25 25 ? ?
Hg Pool Hg Pool Hg Pool Hg Pool Hg Pool SCE SCE Hg Pool Hg Pool Hg Pool
10
10
52 52 8
a
10
8 25
Hg Pool
10
10
8
10
?
Hg Pool
Ref.
T, “C
Reference electrode
1 (Continued)
0*70(l) 0.90(2) 0.56(l) 1*O(2) 0.29(l) 0.59(2) O-87(3) 0*29(l) 0.70(2) O-65(1) 0.90(2) l-15(3) 0*95(l) l-15(2) 1.35(3) 1.45(4) 0*37(l) 0%7(2) 1*05(l) 1*5(2) 0.7(l) l-1(2) 0.37(l) 0.97(2) 0.7(l) 1 *I(2) 0.84 2.398 2.307 0.02(l) 0.28(2) 0*95(l) 1*25(2) 1.5
-&~a,
TABLE
n=0.93; s=5550 n = 1.03; s = 8810
Miscellaneous
s
bisulphate
Tetraethylammonium Threonine Tryptophan Tyrosine
O.lMTEAP
O*lM TEAP
O*
OP
0,
V
SCE SCE Hg PooI
Ag/DMSO
SCE
SCE SCE
SCE SCE SCE
Reference electrode
SCE SCE SCE SCE
SCE
I = diiIusion-current s = i&oncentration x = &I,&,.
Key to Table I
1.44 1.18 l-06(1) 1.53(2) 2.22 O-586(1) 1*308(2) 0.85(l) 2*20(2) 0.65(l) 1*70(2) o-77 o-777 1.156
--&a,
C. Gases
0.00(l) 0.17(2) 2-5 2.368 2.388 2.379
constant
? 25 25
?
?
::
25 25 25
T, “C
;:
;::
?
= i&malatllB
::
6
35
20
19 11
19 19 19
Ref.
5 52 52 52
23
TEAP = te~ae~ylammoni~ pen&orate TBAP = tetrabutyl~onium perchlorate Ag/DMSO = Ag/AgCl,- reference in DMSO SCE = saturated calomel electrode (aqueous) TEANO, = tetraethylammonium nitrate Hg Pool = mercury pool reference electrode The figures in brackets after the El/, values indicate the number of the wave.
m = slope of a plot of Ea,e vs. log &
n = number of electrons transferred in the half-cell D = diffusion coefficient in lo8 cm%ec.
O*lM TBAP O*lMTBAP O-XMTBAP
? 0.1 M KCIOI
NaO 02
x
? ? ?
Inert electrolyte
05M TEAP 0,lMTEAP O*lMTEAP 0.1 M TEAP
O.lMTBAP
NO
Active species
chloride
Tetraethylammonium
m = 0.075 m = 0.069
In, = O-112 mp = O-086
Miscellaneous
n = 1.07; s = 6600 n = 1.02; s = 6180 n = 1.07; s = 4990
s1 = 180 s, = 2250
3 B l% B 2 s k E ccl g #
j
d F
998
THOMASR. KOCH and WILLIAMC. P~DY
ions they determined the diffusion potential between an aqueous SCE and a DMSO test solution.2 Iwamoto3 investigated the electro-oxidation of iodide. Chronopotentiometric studies at a platinum-foil electrode indicated a two-step oxidation. 61- --f 21, + 4e21, --f 31, + 2eThis is in contrast to the oxidation of iodide ion in water, which proceeds by a single step. Gutmann et al.4 extended their previous studies to several silicon compounds, including SiCI,, SiF,, and compounds of the type RCl,Si. The polarographic waves for the RCl,Si species in DMSO occurred at approximately the same potential, permitting polarographic analysis for organic silicon. Kolthoff and Reddy5 studied the polarographic behavior of several acids and of cobalt and nickel in DMSO. They reported the polarization limits with sodium perchlorate and tetraethylammonium perchlorate supporting electrolytes, using the rotating platinum electrode (RPE), the rotating mercury pool electrode (RMPE), and the dropping mercury electrode (DME). These polarization limits are presented in Table II. The polarographic characteristics of perchloric acid, sulphuric acid, hydrochloric acid, acetic acid, sodium bisulphate and tetraethylammonium bisulphate are included in Table I. Well-defined and identical current-voltage curves were observed for solutions of perchloric, hydrochloric, and sulphuric acids in O*lM sodium perchlorate; the half-wave potential was -0.67 f 0.02 V. The basic strength of DMSO is reported to be about the same as that of water. Because these acids gave the same polarographic wave, Kolthoff and Reddy concluded that sulphuric (Q, perchloric, and hydrochloric acids are completely dissociated in DMSO. Amperometric titrations of dilute strong acid solutions can be carried out in DMSO; the current is measured at an applied potential of -1.00 V. Kolthoff and Reddy calculated from polarographic data the diffusion coefficient of the hydrogen ion in DMSO solutions of strong acids and found it to be 4.4 x 10-s cm2/sec at 30”. On the basis of the specific conductance calculated from these data, the proton-diffusion mechanism in DMSO was shown to be different from that in water. Cobalt(I1) and nickel(I1) in O*lM sodium perchlorate gave sharp maxima, even in very dilute solution; no surface-active substance could be found which would entirely eliminate these maxima. Studies at the RMPE (120 rpm) gave well-defined waves without maxima. A dilute solution of sodium perchlorate in O*lM tetraethylammonium perchlorate (TEAP) gave a well-defined, nearly reversible wave for sodium with a half-wave potential of -2.07 V. In O*lM TEAP, quinhydrone and benzoquinone showed two cathodic waves of unequal height. The first wave was a one-electron reversible reduction, and the second wave was irreversible. When O*OlMhydrochloric acid, acetic acid, or water was added, the second wave was shifted to more positive potentials until only one wave with a maximum was observed. This wave had a height equal to the sum of the heights of the original waves. Duroquinone gave two waves, the first reversible, the second irreversible. Tetrachloroquinone gave two irreversible waves of equal height. The authors
Volt~met~
o
6
1
o
I
999
in d~thylsulphoxide
oooo
I
tilt
0
0
0
0
O0
0
000
0
0
0
..o
I I J I I t I I I I I I I I I I I I I I ! J ) I
0
. . . . .o
&&~&
I
~
& ~
! &~&~&&&&&
÷~&
I •
~
I ~&&&
0
0
U.l 0 0 0
zzzzz I111 II ~ 0 r~
0
0
0
~
0
0
0
0
0
0
0
0
0
0
0
0
0
~
0
0
0
0
~
0
0
0
~
0
0
0
0
1000
THOMAS R. KOCH
and
WILLIAM C. PURDY
concluded that the polarographic behavior of quinones in DMSO provided evidence that DMSO is a much weaker acid than water. In relatively acidic solvents such as water or acetic acid, the general reduction mechanism for the two-electron reduction of organic compounds at the DME was reported to be: R + e- -+ R-R=- + HS 3 RH. + SRH. + e- --+ RHRH-+HS-+RH,+SSince the electron affinity of RH- has been shown always to be greater than that of R, the two are reduced at the same potential in a single two-electron reduction wave, such as is observed for quinones in water. In DMSO, as well as in dimethylformamide and acetonitrile (all weakly acidic), the reduction mechanism for quinones was reported to be: R + e- -+ R*- (first wave) R*- + e- + R2- (second wave) Rs- diffuses into solution, where: R” + HS + RH- + S- (slow) RH- + HS --f RH, + S- (slow) A double wave results since R*- is reduced at a more negative potential than R. The second wave is somewhat smaller, probably because of limited protonation of R-or repulsion of R*- by the negatively-charged mercury drop. When a proton donor is added, it reacts with R2- but not with R*-, and the rate of the reaction Rs-+HA+RH-+Abecomes potential-determining. The second wave, therefore, shifts toward the first, and eventually merges with it. Oxygen gave two waves in DMSO, although only one drawn-out wave with a maximum was observed in concentrated solutions. Also, only one wave was observed in 0.1M sulphuric acid and in a 0.1M ammonium acetate and O.lM acetic acid buffer. The polarography of oxygen showed qualitatively the same effects observed with quinones, but more detailed studies were considered necessary. Peover and Whites*’ studied the reduction of oxygen to superoxide ion by a.c. and d.c. polarography. Their results, included in Table I, showed that the reduction is diffusion-controlled, and that the superoxide ion has considerable stability in DMSO. The reduction is reversible. On addition of a strong acid, the process reverts to the normal two-electron reduction. The preparative-scale reduction of oxygen to superoxide ion was studied by Maricle and Hodgson. s Their polarographic work was similar to that of Peover and White. Preparative-scale reductions produced both tetrabutylammonium superoxide and potassium superoxide. The polarographic reduction of oxygen in DMSO that was O-144 in TEAP was studied by Johnson et al9 As shown in Table I, two waves were obtained; in agreement with other workers, the authors found that the
Voltammetry in dimethylsulphoxide
1001
first wave was due to the reduction of oxygen to superoxide ion. The second wave, studied as a function of various cations at varying concentrations, was attributed to the combination of superoxide with one electron plus a cation to give a peroxide product. Jones and FritschelO used potential-sweep chronoamperometry at a hanging mercury drop electrode to study DMSO as an electrochemical solvent, and reported working ranges for various supporting electrolytes. The behaviour of oxygen differs from that reported by Johnson et al.,9 indicating, as the latter authors mentioned, that the reduction of oxygen in DMSO is very dependent on the nature and concentration of the supporting electrolyte. Cadmium and magnesium were also studied. In 1962, Burras” carried out an extensive study into the polarographic behaviour of a number of common cations in DMSO. The effects of various parameters were also investigated. Burras employed both aqueous saturated calomel and mercurypool reference electrodes, and used a variety of supporting electrolytes. The potentials of the electrocapillary maxima, and the usable potential ranges, are reported in Table II. The effects of variation of temperature and of supporting electrolyte concentration were studied. Also, the influence of a maximum suppressor and of water on the polarogram of cadmium, and of added chloride on the cadmium and lead polarograms, was investigated. The polarographic data for the various ions studied by Burras are included in Table I. Gutmann and SchSbeP studied the polarographic behaviour of various chromium(III)-anion systems. The equilibria of various co-ordination forms of chromium(II1) with chloride ion and DMSO were investigated. The co-ordination forms [Cr(DMSO)$+, [CrCI,(DMSO),]+, [CrCl,(DMSO)3], [CrCl,]-, [CrC1,‘J2-, and [CrCI,]” were observed. Schijber and Rahak13 discussed the application, to nonaqueous solutions, of methods used to establish reversibility in aqueous solution. The Cr(III)-Cr(I1) redox system in DMSO was investigated for reversibility. Within the limits of error of the experiment, both chromic nitrate and CrCl, were found to be entirely reversibly reduced in this solvent. Dehn et all4 studied the polarographic behaviour of aluminium chloride and aluminium acetylacetonate in DMSO. One wave was reported for the former, two for the latter; half-wave potentials are shown in Table I. The authors suggested that since aluminium can be extracted as the acetylacetonate, the polarographic method would be useful as an analytical tool for aluminium. The lanthanides (as chlorides) were investigated polarographically by Gritzner et all6 Lanthanum, cerium, praseodymium, neodymium, gadolinium, terbium, dysprosium, holmium, and erbium gave one wave; samarium, europium, and ytterbium gave two waves. All waves were diffusion-controlled and had no maxima. The polarographic behaviour of uranium compounds in DMSO was studied by Michlmayr et al. l6 Uranium tetrachloride shows two waves, the first involving one electron, the other three electrons. Both waves are quite irreversible and the second wave exhibits a maximum. However, both waves were found to be diffusioncontrolled. Uranyl compounds exhibit one diffusion-controlled, well-defined wave involving the one-electron reduction to UO, +. The effect of added water on the waves was also reported. Gritzner et a1.l’ investigated the polarography of thorium salts in DMSO. Thorium tetrachloride gives three waves at a concentrations below millimolar; at higher
1002
THOMAS R. KOCHand WILLIAM C. PURDY
concentrations, the third wave merges with the most cathodic wave. Thorium tetraperchlorate monohydrate and trihydrate give two waves, whereas thorium tetranitrate pentahydrate and thorium tetranitrate solvated with four DMSO molecules give only one wave. DMSO solutions of sodium chloride and sodium iodide were subjected to electrolysis at platinum electrodes. la The cathode reaction involved initial discharge of sodium ions, after which the sodium metal reacted with the solvent. An anodic reaction was also reported that involved reactions of the iodide and tri-iodide ions. Gritzner et a1.u found that nitrogen monoxide, dioxide and trioxide, and nitrous oxide could be determined polarographically in DMSO. Lindbeck and Young20 investigated the polarographic and controlled-potential coulometric determination of the nitric acid oxidation products of soil humic acid. Reduction waves were found which compared closely with the polarographic reduction waves of the nitrobenzoic acids. Although the authors discussed the advantages of the extended potential range of DMSO, their choice of a O*lM sodium hydroxide supporting electrolyte precluded the possibility of using it, by limiting the range to about that of water. Lindbeck and Freundzl studied substituted nitrobenzene compounds in DMSO by polarography and controlled-potential coulometry. The main purpose of their paper was to report n values for the nitrobenzene compounds. Polarography was used only to determine the proper potentials for use in constant-potential coulometry. Di Giacomo et aZ.22reported that phenylfluorone in DMSO gives two waves, as opposed to one in water. The second wave is higher than the first, and can be resolved by linear-sweep polarography into two steps; the more cathodic step shows kinetic characteristics. In the presence of benzoic acid, the two reduction waves are of equal height. A reduction mechanism is proposed. A number of sulphydryl, disulphide, amine compounds, and sulphur-amino acids were studied polarographically in DMSO by Giang et aZ.23 The half-wave potentials, n values, and id/C values given are shown in Table I. The authors reported that all of the waves were diffusion controlled and irreversible. An excellent review of electrochemistry in DMSO was published in 1967 by Butler.24 Both polarographic data and reference electrodes were discussed. REFERENCE
ELECTRODES
A reference electrode employing the silver-silver chloride couple in DMSO was constructed by Courtot-Coupez and Le Demezet. 25 The range of electroactivity in DMSO was tested, using a polished platinum electrode in the presence of sodium, potassium, lithium and quaternary ammonium perchlorates. Ranges varied from 4.18 to 4.80 V. Smyrl% determined the standard cell potential of Pt,,,/Li,,,/LiCl,,,,,/TlCl,,,/ T1rHg),(l)/Pt(s)in DMSO. The standard potential for the analogous calcium amalgam electrode was estimated. McMaster and co-workers2’ developed and tested a reference electrode consisting of a saturated solution of Zn(ClOJ,*4DMSO in contact with a saturated zinc amalgam. By comparing their half-wave potentials with those reported by other workers, the potential of this reference electrode was found to be -1.10 V us. aqueous SCE. Relative reduction-potential scales for several solvents were established.
Voltammetry in dimethylsulphoxide
1003
A reference electrode employing the Ag/AgCl,- couple in DMSO was developed and tested by Courtot-Coupez and Le Demezet. 28 The regions of electroactivity of the platinized platinum, mercury, graphite and silver electrodes were determined, and are listed in Table II. A reference electrode of Ag/AgCl/LiCl in methanol was developed for potentiometric titrations in DMSO.%D Rallo and Ceccaroni30 described a reference electrode consisting of cadmium metal in contact with saturated DMSO solutions of CdCl,, CdCl,*H,O, or NaCl. The electrode was considered suitable for use in electrochemical studies. Synnott and Butlers1 developed and tested four chloride reference electrodes in DMSO. The half-cell Tl(Hg)/TlCl in DMSO appeared to be a good reference electrode for use in this solvent. RECENT STUDIES Polarographic investigations on DMSO solutions of zirconium and hafnium compounds were carried out by Michlmayr and Gutmann.3a The half-wave potentials, the nature of the limiting currents, the temperature coefficients of half-wave potentials and wave heights, the reversibility or irreversibility of the electrode processes, and the influence of water were determined. Schmid and Gutmann reported the halfwave potentials in DMSO (US.aqueous SCE) of thallium(I), rubidium, potassium, sodium, barium, zinc, cadmium, manganese(II), cobalt(II), and nickel perchlorates. These potentials were referred to that of bis(biphenyl)chromium(I) iodide. Reduction mechanisms, various analytical aspects of the system, and the effects of water were discussed. Pyatnitskii and Ruzhanskaya34 reported half-wave potentials for the 8-hydroxyquinolinates of copper( lead, iron(III), cadmium, and bismuth in mixtures of methanol and DMSO, and with eleven other solvents. Lithium chloride was used as the supporting electrolyte. No correlation was found between the half-wave potential and the dielectric constant of the solvent. Kumar and Pantony investigated the polarographic behaviour of a number of anhydrous inorganic perchlorates in DMSO. Gritzner et ~1.~ applied oscillographic polarography to the study of DMSO solutions of the lanthanides. These results were compared with those of classical polarography. The peak potentials are shown in Table III. Haynes and Sawyers7 have employed chronopotentiometry, controlled-potential coulometry, gas chromatography, and pH titrations to investigate the electrochemical reduction of carbon dioxide in DMSO solutions at gold and mercury electrodes. At a gold electrode, irreversible but well-defined chronopotentiograms were observed; the reduction is a one-electron process and is diffusion-controlled. The diffusioncontrolled reduction at a mercury electrode, complicated by equilibria involving water has un = O-64. Controlled-potential coulometry at a mercury electrode indicates an overall one-electron process. In anhydrous medium, the reduction products at both electrodes are carbon monoxide and carbonate ion; some bicarbonate and formate ions are formed in the presence of water. Johnson et al.% observed a unique reduction wave, at potentials less cathodic than the reduction wave of either reducible species alone, in DMSO solutions containing both oxygen and one of the metal ions, cadmium, zinc, strontium, thallium(I), or yttrium. The variation of the height of this wave was studied as a function of concentration of oxygen and the various metal ions. Studies of this type revealed
1004
THOMAS R. KOCHand WILLIAM C. PURDY TABLE III.-OSCILLOORAPHIC PEAK POTENTIALS IN DMS03E -E
Species La(II1) Ce(II1) Pr(II1) Nd(III) Sm(II1) Eu(II1) Gd(II1) Tb(III) Dy(III) Ho(III) Er(II1) Yb(III)
Reduction potential, V 2.39 2.39 2.39 2.42 2*11(l) 2.30(2) 0+6(l) 2.28(2) 2.42 2.42 2.42 2.40 2.42 2.39
-E
Oxidation potential, V 2.31 2.39 2.30 2.29 2.05 0.73(l) 2.11(2) 2.36 2.34 2.33 2.36 2.33 2.20
the stoichiometry of the materials which caused the reduction wave. Controlledpotential coulometry was employed to determine values of n. Massive preparation of reduction products by controlled-potential electrolysis, along with the above information, revealed the probable reduction mechanism. In the cases of zinc, strontium, and thallium(I), the reduction was of metal ion plus oxygen to the superoxide; cadmium and yttrium with oxygen were reduced to the peroxides. Fujinaga et aL39 found that in DMSO containing between 10 and 30% water, the first oxygen wave (reduction to superoxide ion) becomes a two-electron wave, because of protonation by water. The polarographic diffusion coefficients of oxygen in the mixed solvent were calculated. Pasadas et aL40 studied the kinetics of hydrogen evolution on an iron electrode as excess of potassium perchlorate a function of hydrochloric acid concentration; was employed. Temperatures were varied from 20” to 45”. A two-step mechanism was proposed in which the simple discharge of solvated hydrogen ions was ratedetermining. The function of a hydrogen electrode in DMSO was studied by Courtot-Coupez and Le Demezet.41 The authors determined the pK values of a number of acids, and reported the ion product of the solvent to be pK, = 33.3 f 0.05. This differs greatly from the value reported by Reddv2 (pK, = 17*3), but is in virtual agreement with the value determined spectrophotometrically by Steine?44 (pK, = 32). Rallo et a1?5 studied the polarographic reduction of nitric oxide in O-U4 TEAP. Solutions with varying NO/N, content were investigated. One wave appeared when there was less than 10% NO in the gas mixture; with more than 10% NO, a second, more anodic wave appeared, which was not diffusion-controlled and was thought to be due to the reduction of a dimer. The rate-determining dimerization which preceded reduction was thought to control the wave height. Michlmayr and Sawyer46 investigated the electrochemical behaviour of the hydrogen halides at platinum electrodes in DMSO by chronopotentiometry, controlled-potential coulometry, and cyclic voltammetry. The results indicated that the oxidation products of the hydrogen halides reacted with the solvent. In the cases of HCI and
Voltammetry in dimethylsulphoxide
1005
HBr, a catalytic process occurred which enhanced the value of iGf2 by a factor of 5-10. The electrochemical oxidation of formate has been studied at gold and platinum electrodes, by chronopotentiometry, controlled-potential coulometry and cyclic voltammetry.47 Well-defined chronopotentiograms were obtained, with quarterwave potentials of 0.33 and -0.07 V vs. SCE at a gold and a platinum electrode respectively. The process is diffusion-controlled. The oxidation products were carbon dioxide and hydrogen ions, which react with formate ion. Thus, the overall reaction is a one-electron oxidation in which half a mole of carbon dioxide and half a mole of formic acid are formed for each mole of formate ion electrolysed. Jacobson and Sawye? studied the electrochemical oxidation of oxalate at a gold electrode. The chronopotentiometric quarter-wave potential was 1.25 V us. SCE. The final oxidation product was carbon dioxide, but an unstable intermediate was formed during the oxidation process. In the absence of excess of hydroxide ion, oxalate ion is partially hydrolysed to the hydrogen oxalate ion, which is oxidized to carbon dioxide and hydrogen ions. Buchta and Evans49 examined the electrochemical behaviour of 1,3-diphenyl1,3-propanedione, as well as a number of other p-diketones. Controlled-potential coulometry at the potential for the first wave gave an n-value of 0.55. The products were found to be the enolate of 1,3-diphenyl-1,3-propanedione and the pinacole 1+dibenzoyl-2,3-diphenyl-2,3-butanediol. Further electrolysis resulted in an increase in current, followed by a decrease to the residual value. This was caused by an electrolytic autocatalytic decomposition of the pinacol in a base-catalysed reverse aldol condensation to benzil and acetophenone. The presence of the benzil radical anion was confirmed by ESR spectroscopy. Other B-diketones behaved in an analogous fashion. Goolsby and Sawye$O studied the electrochemistry of hydroxylamine at gold and platinum electrodes in 0.2M lithium perchlorate in DMSO. The electroactive species, NH,OH, is oxidized irreversibly in a diffusion-controlled, two-electron process. The chronopotentiometric El14 values were -0.84 V US.SCE at a platinum electrode and -1.18 V at a gold electrode. The major products are water, hydrogen ion and nitrous oxide. The rate-controlling step is a one-electron process which follows a preceding chemical reaction of a hydroxide ion with hydroxylamine. The electrochemical oxidation, at platinum electrodes, of hydrazine, 1,l-dimethylhydrazine, and 1,2_dimethylhydrazine in DMSO was studied by Michlmayr and Sawyer. 51 In each case the oxidation involved one electron. Chronopotentiometric quarter-wave potentials were reported as 0.00,0.02, and 0.03 V US.SCE, for the three compounds in the order listed above. The oxidation products were identified and mechanisms proposed. Koch and Purdy 52studied the polarographic behaviour of amino-acids in DMSO. Nine amino-acids, asparagine, hydroxyproline, isoleucine, methionine, phenylalanine, proline, threonine, tryptophan, and tyrosine, gave single waves involving the one-electron reduction of the proton of the carboxyl group. Five amino-acids, dihydroxyphenylalanine, glutamic acid, aspartic acid, glutamine, and cysteine gave multiple waves. The electrical double-layer in DMSO solutions was studied by Payne.53 He noted a strong resemblance in electrocapillary curves and double-layer capacities of DMSO 2
1006
THOMAS
R. KOTCH and WILLIAMC. PURDY
and aqueous solutions. Anions are specifically adsorbed from DMSO solutions in the order I- > Br- > Cl- > NO,- > ClO, > PF,-; cations are not significantly adsorbed. The electrocapillary maximum is shifted in the positive direction in DMSO as compared to water, and a large hump appears close to the limit of anodic polarization in solutions where the anion is not strongly adsorbed. The influence of diffusionlayer capacity is more marked in DMSO. The interfacial tension at the electrocapillary maximum is about 60 dyn/cm lower (370.5 dyn/cm) than in water. Payne54 reported the occurrence of capacity humps on the electrocapillary curve for 0-W KPFs solutions in DMSO. The hump appears on the anodic side of the electrocapillary maximum. Synnott and Butler 55 studied the equilibria of silver chloride in DMSO-water mixtures containing excess of chloride. They used potentiometric results to determine equilibrium constants for the system. Gutman# reviewed the effect of the donor properties of various solvents, including DMSO. Also discussed were the effects of the acceptor properties of metal ions, solvation of metal ions and complex formation on half-wave potentials and the thermodynamics of reduction processes. Zusannnenfassnng-Es wird eine Ubersicht iiber voltametrische Untersuchungen mit Dimethylsulfoxid als Triigerelektrolyt gegeben. Sie umfa8t Einzelheiten tiber die untersuchten anorganischen und organischen Systeme sowie tiber die fur dieses Medium entwickelten Bezugselektroden. R&me-On presente une revue des etudes voltamm&iques utilisant le dimethylsulfoxyde comme electrolyte-support, et celle comprend des details de systemes mineraux et organiques Studies et des electrodes de reference tlaborees pour l’emploi dam ce milieu. REFERENCES 1. V. Gutmann and G. Schbber, Z. Anal. Chem., 1959,171,339. 2. G. Schiiber and V. Gutmann, Monatsh., 1959,90,897. R. T. Iwamoto, Anal. Chem., 1959,31,955. :: G. Schbber, V. Gutmann and P. Heilmayer, Monatsh., 1961,92,240. I. M. Kolthoff and T. B. Reddy, J. Electrochem. Sot., 1961,108,980. :: M. E. Peover and B. S. White, Chem. Commun., 1965,183. Idem, Electrochim. Acta, 1966,11, 1061. :: D. L. Maricle and W. G. Hodgson, Anal. Chem., 1965,37,1562. 9. E. L. Johnson, K. H. Pool and R. E. Hamm, ibid., 1966,38,183. 10. 5. L. Jones and H. A. Fritsche, Jr., J. Electroanal. Chem., 1966,12, 334. 11. R. T. Burras, Ph.D. Dissertation, University of Tennessee, 1962. V. Gutmann and G. Schiiber, Monash., 1962,93,212. : f G. Schbber and G. Rahak, ibid., 1962,93,445. 14: H. Dehn. V. Gutmann and G. Schiiber, ibid., 1962, 93,453. 15. G. Gritzner, V. Gutmann and G. Schober, ibid., 1965,%, 1056. 16. M. Michlmavr. G. Gritzner and V. Gutmann. Inorp. Nucl. Chem. Letters. 1966. 2.227. 17. G. Gritzner,‘V: Gutmann and M. Michlmayr; Z. &al. Chem., 1967,224; 245. . . 18. M. C. Giordano, J. C. Bazan and A. J. Arvia, Electrochim. Acta, 1966, 11, 741. 19. G. Gritzner, V. Gutmann and G. Schober, Mikrochim. Acta, 1964, 193. 20. M. R. Lindbeck and J. L. Young, Soil Sci., 1966,101,366. 21. M. R. Lindbeck and H. Freund, Anal. Chim. Acta, 1966,35,74. 22. F. DiGiacomo, F. Rallo and L. Rampazzo, Ric. Sci., 1967,37, 1085. 23. B. Y. Giang, G. D. Christian and W.-C. Purdy, J. Pilarographic Sot., 1967,13, 17. 24. J. N. Butler. J. Electroanal. Chem.. 1967. 14. 89. 25. J. Courtot-Coupez and M. Le Demezct, Co&pt. Rend., Ser. C, 1966,263,997. 26. W. H. Smyrl, Diss. Abstr. B, 1967,28,637.
Voltammetry
in dimethylsulphoxide
1007
27. D. L. McMasters, R. B. Dunlap, J. R. Kuempel, L. W. Kreiger and T. R. Shearer, Anal. Chem., 1967,39, 103. 28. J. Courtot-Coupez and M. Le Demexet, Bull. Sot. Chem. France, 1967,39,888. 29. T. Jasinski and E. Kwiatkowski, Zesz. Nauk. Mat. Fiz., Khim., Wyzsza Szk. Pedagog. Gnaa!vku, 1968,8,101. 30. F. Rail0 and G. Ceccaroni, Ric. Sci., 1968,38, 1067. 31. J. C. Syrmott and J. N. Butler, Anal. Chem., 1969,41,1890. 32. M. Michlmayr and V. Gutmann, Electrochim. Acta, 1968, W, 1071. 33. R. S&mid and V. Gutmann, Chem. Zuesti, 1969,23,746. 34. I. V. Pyatnitskii and P. Ruxhanskaya, Zh. Analit. Khim., 1970,25,1063. 35. G. P. Kumar and D. A. Pantony, J. Polarographic Sot., 1968,14,84. 36. G. Gritxner, V. Gutmarm and R. Schmid, Electrochim. Acta, 1968,13,919. 37. L. V. Haynes and D. T. Sawyer, Anal. Chem., 1967,39,332. 38. W. L. Johnson, K. H. Pool and R. E. Hamm, ibid., 1967,39,888. 39. T. Fujinaga, K. Ixutsu and T. Adachi, Bull. Chem. Sot. Japan, 1969,42,140. 40. D. Pasadas, J. J. Podesta and A. J. Arvia, EIectrochim. Acta, 1970,15,1225. 41. J. Courtot-Coupez and M. Le Demexet, Compt. Rend, Ser. C, 1968,266, 1438. 42. T. B. Reddy, Ph.D. Dissertation, University of Minnesota, 1960. 43. E. C. Steiner and J. D. Starkey, J. Am. Chem. Sot., 1963,85,3054. 44. E. C. Steiner, J. D. Starkey, J. M. Tralmer and R. 0. Trucks, Div. Petrol. Chem., Amer. Chem. Sot., Preprints, 1967,12, C-11. 45. F. Rallo, L. Rampauo and F. DiGiacomo, Ric. Sci., 1969,38, 1085. 46. M. Michlmayr and D. T. Sawyer, J. ElectroanaI. Chem., 1969,23,387. 47. E. Jacobsen, J. L. Roberts and D. T. Sawyer, ibid., 1968,16,351. 48. E. Jacobsen and D. T. Sawyer, ibid., 1968,16,361. 49. R. C. Buchte and D. H. Evans, Anal Chem., 1968,40,2181. 50. C. D. Goolsby and D. T. Sawyer, J. Electroanal. Chem., 1968, 19,405. 51. M. Michlmayr and D. T. Sawyer, ibid., 1969,23, 375. 52. T. R. Koch and W. C. Purdy, Anal. Chim. Acta, 1971,54,271. 53. R. Payne, J. Am. Chem. Sot., 1967,89,489. 54. Idem. J. Phys. Chem., 1967,71,1548. 55. J. C. Synnott and J. N. Butler, ibid., 1969,73,1470. 56. V. Gutmann, A&. Prakt. Chem., 1970,21,116.
VOLTAMMETRY
IN DIMETHYLSULPHOXIDEA REVIEW
THOMASR. KOCH* and WILLIAM C. PURDY Department
of Chemistry,
University of Maryland,
College Park, Md. 20742, U.S.A.
(Received 15 November 1971. Accepted 27 December 1971) Summary-A review is given of voltammetric studies using dimethylsulphoxide as supporting electrolyte, and includes details of inorganic and organic systems studied and of the reference electrodes developed for use in this medium. IN RECENT YEARS the
scope of voltammetry has been expanded by an increase in the (DMSO) has of non-aqueous solvents employed. Dimethylsulphoxide been of great value in these voltammetric studies owing to its high dielectric constant and absence of ionizable protons. Many investigators have found purification of the solvent to be a necessity, owing to a cathodic wave caused by an impurity. This impurity can be removed by distillation; in addition, distillation over a drying agent helps to ensure a constant, low water level (usually about 0*03-0*05 %). Tetraalkylammonium salts are generally employed as supporting electrolytes to take full advantage of the extended cathodic range of DMSO as compared to water. While a variety of reference electrodes have been used, the most common is an aqueous saturated calomel electrode (SCE), separated from the sample compartment by a salt bridge. Several reference electrodes employing DMSO as a solvent have been described and they will be discussed later. number
EARLIER
WORK
DMSO were conducted in 1959 by Gutmann and Sch6ber.l With tetraethylammonium nitrate as supporting electrolyte, they reported a usable voltage range from -0.2 to -2.74 US. SCE. These workers described the frequent occurrence of maxima which, in some cases, were at least partially eliminated by varying the drop-rate. Conventional and derivative polarographic methods were used. Single, well-defined waves were found for the alkali metal ions and for the ammonium ion. The alkaline-earth metal ions were reported to give single, well-defined waves. Silicon tetrachloride exhibited two waves, well-formed and of equal height. Zirconium(N) and hafnium(IV) gave well-developed waves, each exhibiting a small first-order maximum. Titanium(N) gave a wave which was ideal in shape; derivative polarography showed three indistinct steps in this reduction, corresponding to Ti(IV) ---fTi(II1) -+ Ti(I1) -+ Ti(0). The
first
voltammetric
studies
in
Antimony(II1) exhibited three waves and niobium(V) one. The half-wave potential values reported by Gutmann and Schijber are included in Table I. These authors carried out no concentration studies. However, by use of a variety of reference * Present address: Erie County Laboratories, Grider Street, Buffalo, N.Y. 14214, U.S.A. 1
989
E. J. Meyer Memorial
Hospital
Division,
462
l.OM KClO,
0.1 M TEANO O*lMTBAP O+lMTEAP 0.1 M KClO,
Cr(C103,
CsCl cu(Clo‘)* cu(clo,)* cu(cloJ~
Cr(II1)
WI) CucrI) CWI) CUOI)
O*lMTEAP O*lMTEA_P 0.1 M TEANO* OalMTEANO, O*fM TEAP 0.1M TEAP @iMTBAP O*lM TBAP O-1M KCIOI O*lM KClO, 0.1 M KClO, l*OM KClO, l*OM KCIO, I *OM KClO, l*OM KClO, 1 *OM KCIO, O*lMTEAP O*lMTBAP 0~1MTEAP O.lMKCIOI l*OM KClO, l*OM KCIOI 1 *OM KCIOp l*OM KCIO, 0.1 M KClO,
O*lMTEAP
Inert electrolyte
::g; Cd0 CdOI) Ce(III) CoOI) Co@) Co@) Co0 CoOI) Co@) CooI) CrO
Cd(NO&,
BaCl, CaWU Ca(CQh Cd(ClO& CdfClO.), Cd&IO& Cd(ClO& Cd(ClWn
BaK104)a
AlCl*
Form used
Cd(ClO& Cd(ClO& WClOJ, Cd(ClWs Cd@Wz C&l* 0% Co(ClO& co(c1o3. Co(ClW* co(clo,)s co(clo$, Co(ClO& Cr(ClU
Cd(I1) Cd(H)
CaOU Cd011 C&W CdOO Cd(iO Cd(II)
CaOI>
BaOrI)
wm
AD3
Active species
!z:;; O-870(1) 1,603(2) 2.03 0.341 -0.05 0.212
::;5 2.09 2.30 2-38 0.58 o-931 0.966 0.820 0.820 0.63 0.789 0.794 &685 0.670 0.66 2.24 1.43 1.28 1.316 1.465 1.423 1.520 1,536
1*67(l) 1*95(2)
--Ez1,, v
TABLE I.-POLAROGRAPHIC DATA IN DMSO.
11
1: 35 11
SCE Hg Pool Ag DMSO Hg Pool
:: I1 11
:55 3s 25 SCE
::
Z = 1.96;
3;
0.096; 0.100 0.130; 0,097; 0,938
D = 4.83 D = 1.06
D = 0.734
n = 1.78
m = 0.130; D = 4.83 n = 1.95; z = I.95 m = 0.121; D = 7.12
m = m = m = m = ZJ =
0.040 0.048; 0.031; O-047 O-029
m = 0,030
D = 2.52 I3 = l-66
n=0*96; Z=l.48 n = 1.84; Z = 1.88 m = 0.033; D = 6.75 “D z ;;y”; D = 4.94
m = m = m = m = m = n=3
2: 25
;; 21 25
3: 25 25 25 25 25 35 25
ZB= 1.19
n = 1.97; Z = 2.07
z, = 0.86;
nx + ns = 2.90
Miscellaneous
3: 35 11 11 11 11 9 11 11 11 11 9 7
14 3s 1
25 ?
SCE Ag/DMSO SCE SCE AglDMSO Ag/DMSO Hg Pool Hg Pool Hg Pool SCE SCE Hg Pool Hg Pool SCE SCE SCE SCE SCE AgtDMSO SCE SCE SCE Hg Pool Hg Pool SCE a: ?
3s
?
Ag/DMSO
Ref.
T, “C
Reference electrode
A. INORGANIC MATE-
O*lMTEAP O*lM TEAP
0.1 M KClO,
O.lMTEAP O*lMTEAP O.lMTEAP
0.1 M TEAP O*lMTEAP 0.1M TEAP O*lMTEAP O.lMTEAP
O.lM TEAP O*lM TEAP O.lM TEAP
O.lM TEANO,
O*lMTEAP O*lMTEAP
O*lMTEAP O.IMTEAP O.lMTBAP 0.1 M TBAP 0.1 M TEANO*
Fe(ClO&
GdCl. HCl HCI
HClO, HClO, HISO. HISO, HCsHJ%
HCIHIOI TEA-HSOIHfCI,
HfCl,
HOC& KC1
KClO, KClO, KCIO, KCIO, KCIO,
FeOH)
GdO HO[) HO)
H(I) HO) Ha) H(I) H(I)
H(I) HSO,-
Hf (IV)
HfO
Ho(III) KO
Fe(II1)
Fe(ClO& Fe(ClO&
1 *OM KCIOl 1 *OM KClO, l*OM KCIOl 1 .OM KCIOI O*lMTEAP 0.1 M TEAP O*lMTEAP
FeOU
ErOW Eu(lII)
CUOI) DY(III)
cum cum
cu@)
02:;8(1) l-38(2) 1*07(l) 1*17(2) 2.09 --@01(l) 0*17(2) 2.22(3) 2.11 1.97 2.362 2.06 2.11
0.127 0.122 0.058 0.051 2.08 2.09 0.81(l) 2.12(2) 1.12 -0*05(l) 1*12(2) 0*722(l) 1.41 l(2) 2.16 1.08 -0*01(l) O-17(2) 1*14(3) 1.14 0.96 I.12 1.06 1*91(l) 2*22(2) 2.3
SCE Ag/DMSO Hg Pool SCE SCE
SCE SCE
SCE
SCE SCE SCE
SCE Ag/DMSO SCE SCE SCE
SCE SCE SCE
SCE
Ag/DMSO Ag/DMSO
Hg Pool Hg Pool SCE SCE SCE SCE SCE
f:
2:
?
21 ?
21
30 30 25
? ? ? 30 ?
21 30 ?
25
? ?
:: 7
23 35 11 9 1
7 23
1
3:
2:
23 35 23
7 5 23
11
::
::
11 11
0.100 I) = 2.46 D = 1.73
D = 1.630
tl=3 s1=200 sl=2020; ns=l sI = 2320; n, = 0.95 s = 2240; n = 0.94 n = 0.99; Z = 1.42 m = 0.060
xx = -0.054 x* = -0.15
s,=200 sI = 1260
s,=300 sl=2260; nl=l sJ = 2600; n, = 1.04 s = 2000; n = 0.95 II =0*75 s = 2400; n = 0.90
ml = O-052
m, = 0.037;
n, = 1.20
na=2
0.103; 0.102; 0.127
n = 1.60 n, = 0.80;
m = m = m = m = n=3 n=3 nt=l;
Ni(I1) Ni(I1) Ni(II) Ni(I1) Ni(I1) Ni(I1) Ni(I1) Ni(I1) Ni(TI) Ni(II) PWI) IWI) PWI)
:$I,
Li(1) WI) Li(1) Mg(II) WII) M&II) Mu(lI) Ma@) MnOI) MnoI) MuOIl Mn(I1) NaO Nag) N&I) NbCV) Nd(II1)
K(I) La(III)
Active species
Ni(ClO& Ni(CIO,)* Ni(CIO& Ni(ClO& Ni(ClO& Ni(NWa Ni(CIOJ, Ni(CIO& NWOJ, Ni(ClOJ, Pb(ClOJ~ Pb(C103, Pb(ClO$s
z:; NH, NH&l
~~~~~~ Mg(NW, Mn(ClO,), MntClO~~~ Mn(ClO& Mn(ClW~ Mu(ClO& MnClt NaC104 NaCIo, NaHSO&
LaCI, LiCl LiCIOI LiNOIl
?
Form used
O*lMTBAP O.lMTBAP O*lMTEAP @lM KClOp O*lM KCIOp O*lM KClO* I-OM KCIO, l*OM KCIO, l*OM KClO, l*OM KCIOI O+lM TBAP O*lM TEAP O-TMKClO&
O*lM TBAP 08IM TEAP 0.1M ‘WAN& O*lM TEAP O*lMTBAP O.lM TEAP t%lMTEANOs @lM TBAP l*OM KClO, l*OM KClO, l.OM KClOa l*OM KClO& 0.1M TEAP WlMTBAP O+lMTEANOs O,IMTEAI? @lM TEAP 0.1M TEANO. O*lM TEAP OelMTEAP O.lMTBAP
Inert electrofyte v
1.207 0.95 1.124 1.120 l-10 1.155 1,122 1*144 1.110 0.814 0.43 0,610
2070 226 245 238 2.38 2.44 2.28 2.19 1,683 1643 1*810 1,823 1.57 1.73 2.07 1.96 2.07 0.92 2.20 -0.02 0*17(l) :‘w&)
-Zh,
Hg Pool Hg Pool Ag/DMSO Hg Pool Hg Pool SCE Hg Pool Hg Pool SCE SCE Hg Pool Ag/DMsO Hg Pool
SCE SCE SCE Ag/DMSO SCE Ag/DMSO SCE SCE SCE SCE Hg Pool Hg Pool Ag/DMSO SCE SCE Ag/DMSO SCE SCE SCE SCE SCE
Reference electrode
TABLE 1 (Continues
:z 25 3s 25 3s 2s ? 2s
215
3”
2:” ? 1
;55 35 ? 25 21 ? 30
11 7 1 3s 9 3s
2s 21 21 ? 25 ? 21 25 2s
:: 11
:: 11 8 11 11 11 11
11 I1
273 23
3: 5 1
:: II 3s 9
; II
Ref.
T, ‘C
WO40; D = 2.90 O-034 0.038 0.038; D = 3.70 Z=lY%
m = m = m= m = m = n= m =
0.091 0*08S; 1) = 2.06 0.107; D = 0.845 0.087 0*030 1.99; Z = 2.33 OG46; D = 2.25
n== 3 s = 475 nl=l; s,=lS60 n*= 1; s,=l280 m=&OS3; D=S*86 m = 0.053; D = 7.74 z = 1.71 m = 0.067; D = 3.88 m = O-074; D = 5.66
n = 0.96; Z = 1.42
m = m = m = m = n-l;
n -= 2.22
n = O-73; Z = 1.61
m = O-062; D = 6.69 n==3
Mis~llaneous
O*lM TEAP
Th(CW,
ThCld
UCl,
ThOv)
Tho[V
Th(IW ‘WV) TKO WV)
znm ZnO znm Zn(li)
UCVI) Yb(II1)
UO*cNaJ,
O*lM TEAP
Sr(NO&
WI) MI) TbOIQ Th(IV)
YbCll
O+lM TEAP
SmCI,
Sm(nI)
TEAP TEANO% TBAP TEAP
0.IMTEAP GIMTBAP O*lM TBAP O*lM KCIO,
@lM TEAP 0.IMTEAP
O.lM O.lM O.lM O.lM
0.1&f TEANO O*lM TEAP OaIMTEAP O*lM TEAP
OvlM TEANO&
SiC1,
zgj PbOU Pbol1 Pb(II)
WV)
OvlM KCIOI O.lM KClO, 1 *OM KCIO, lGI4 KClO, l%%fKCIO, 1 .OM KCIOI O.lMTEAP O.lM TEANO, O-l&ZTEANO,
O*lM KCIO,
~~~) Sb@II)
PbGfO,),
Pb(CIO& Pb(CQ), Pb(CIW, Pb(CWs Pb(CIOJ, Pb(ClO& P&X, RbCl Sbl.
Pb(II) PbOI) 0.651 o-531 o*so 0.541 0.537 0,561 0,607 2.20 2.06 &36(l) @52(2) 1*13(3) @52(l) l-09(2) 2*02(l) 2*12(2) 210 2.20 2.19 1*55(l) 2*00(2) 1*75(l) 214(2) 1*57(l) 1.72 to 1*95(2) 2*07(3) 1*57 I.21 050 1*09(l) l-56(2) 0.53 1.48(l) 221(2) 0.95‘ . I-322 1.298 1.054 17
21 2: ? 21
SCE AglDMSO SCE AglDMSO
1: 16 7 35 I1 ::
;: 21 21 ? 25 :55
SCE SCE Ag/DMSO wg Pool wg Pool wg Pod
17
17 1
21
3:
7
21 21
SCE SCE SCE SCE
SCE
SCB
32
21
SCE
1
1; 11 11 11 7 1 1
21
:z
z 35
SCE
II 11
;: 21
35 25
Hg Pa01 SCE SCE SCE SCE Hg Pool Hg Pool SCE SCE SCE D = 1.60
D = 2.61
n1=1 o-22; n* = 3 0.056; n = 1 1 2 1.80; Z = l-94 &029; D = 3.38 m = @036; D = 3-58 m = @034; D = 5.38 x. = x = nl = n, = n = m =
q=o*o9;
n = 2.09; Z = 2.03 n=3 n* = 0.22; I = 0.68 n* = o-41 ; Z = 1.58
ni -= 1 n, = 2
m = 0.055; D = 2.19 n=3
m = 0.034; m = 0.060 m = 0.048
m = 0.052; ?n = 0*030
!Q
g
zg
$ G-
If
E
B
I
2.
c:
O*lM TEAP
0.1 M TEAP
O.lMTEAP
O*lM TEAP
@5M LiCl
O*lM TEAP
O*lMTEAP
O.lMTEAP
Al acetylacetonate
2-Aminoethanethiol
2-Aminoethanol
4Aminophenylsulphone
Anthraquinone
Asparagine Aspartic acid
Butylamine
Inert electrolyte
O*lMTEAP
ZrO(ClO&
Zr (IV)
Active species
O.lM TEAP
ZrDCll
ZrO
ZJWI) ZnO ZnOI) Zn(II)
Zr (IV) Zr (IV)
Inert electrolyte
l.OM KCIOl l*OM KClO, l.OM KClO, O*lM KCIOI O*lMTEANOa O*lM TEAP
Form used
Zn(C10J, Zn(ClW* Zn(ClW* Zn(ClW, ZrCl, ZrCl,
Active species I’
Ag/DMSO
SCE
Hg Pool Hg Pool SCE SCE SCE SCE
Reference electrode
V
2*2(l) 2*4(2) 0*17(l) 1.90(2) 2*40(3) -0*07(l) -@02(2) 0*17(l) 2*24(2) 0.33(l) 0*70(2) 2382 2*141(l) 2*618(2) 0.00
--&I,,
SCE
SCE SCE
Hg Pool
SCE
SCE
SCE
SCE
Reference electrode
B. Organic Materials
Y:Eg; 2*05(3)
i::(l) l-38(2) 0*98(l) l-38(2) 1.61(3)
1.089 1.082 1.099 1.066
-&I,,
?
25 25
?
?
?
?
25
T, “C
?
25
;: 35 21 25
25
T, “C
23
52 52
8
23
23
23
14
Ref.
35
32
11 11 11 11 1 32
Ref. = = = =
n = ni = n8 = n =
n1 = n2 = n,=l; ni + s1 = s1 = q =
x1 = x, = xx = xg = XI) = RI = na = n, =
m m m m
D = 3.18 D = 2.34
0.64; s = 4980 0.77; s1 = 6120 0.72; sz = 8660 1.80; s = 3500
2; s, = 4380 0.95; sa = 2.30 s,=2280 n, = 2.10 2120; s* = 1100 69 75
Miscellaneous
-0.054 -0.15 -0.86 -0.10 -0.14 0.57 0.21 0.40
0.108 0.067; 0.097; 0.068
Miscellaneous
%
g 4
.c
i
8
1 0, g
P
Voltammetry
in dimethylsulphoxide
995
0
0
II II II + I1 It II 1t II II II II + II II li II
~'~ O0 e ~ ~ " ~ ' ~ 0
~
00,,"~'~0 ~
~'- ~ ' ~ "
0 ~ ' ~ ~" ~
II II
~'~ ~"~ ~
II II Ii II II II II
~
~'~ r ' ~ . ~
~-~ r'~ t ~"
0
II li + II II II II
¢~ 0
~
~-"~'~0
~"~ ~"~ ~
0.5M LiCl
0.5M LiCl
acid
acid
acid
Nitrobenzene
m-Nitrobenzoic
o-Nitrobenzoic
p-Nitrobenzoic
0.5M LiCl
0.5M LiCl
0.5M LiCl
o-Nitrophenol
p-Nitrophenol
3-Nitrophthalic
0*5M LiCl
Nitroterephthalic
0.5M LiCl O.lMTEAP O.lMTEAP 0.5M LiCl
0.5M LiCl
0.5M LiCl
Oxalic acid Phenylalanine Proline pQuinone
Sodium m-nitrobenzoate
Sodiump-nitrophenolate
acid
0.5M LiCl
Nitroresorcinol
acid
0.5M LiCl
m-Nitrophenol
0.5M LiCl
0.5M LiCl
0.5M LiCl
Inert electrolyte
p-Nitrobenzaldehyde
Active species I’
8
8
? 25 25 ? ?
Hg Pool Hg Pool
Hg Pool Hg Pool Hg Pool
10
8 10
? ? 25 ? 25 25 25 25 ? ?
Hg Pool Hg Pool Hg Pool Hg Pool Hg Pool SCE SCE Hg Pool Hg Pool Hg Pool
10
10
52 52 8
a
10
8 25
Hg Pool
10
10
8
10
?
Hg Pool
Ref.
T, “C
Reference electrode
1 (Continued)
0*70(l) 0.90(2) 0.56(l) 1*O(2) 0.29(l) 0.59(2) O-87(3) 0*29(l) 0.70(2) O-65(1) 0.90(2) l-15(3) 0*95(l) l-15(2) 1.35(3) 1.45(4) 0*37(l) 0%7(2) 1*05(l) 1*5(2) 0.7(l) l-1(2) 0.37(l) 0.97(2) 0.7(l) 1 *I(2) 0.84 2.398 2.307 0.02(l) 0.28(2) 0*95(l) 1*25(2) 1.5
-&~a,
TABLE
n=0.93; s=5550 n = 1.03; s = 8810
Miscellaneous
s
bisulphate
Tetraethylammonium Threonine Tryptophan Tyrosine
O.lMTEAP
O*lM TEAP
O*
OP
0,
V
SCE SCE Hg PooI
Ag/DMSO
SCE
SCE SCE
SCE SCE SCE
Reference electrode
SCE SCE SCE SCE
SCE
I = diiIusion-current s = i&oncentration x = &I,&,.
Key to Table I
1.44 1.18 l-06(1) 1.53(2) 2.22 O-586(1) 1*308(2) 0.85(l) 2*20(2) 0.65(l) 1*70(2) o-77 o-777 1.156
--&a,
C. Gases
0.00(l) 0.17(2) 2-5 2.368 2.388 2.379
constant
? 25 25
?
?
::
25 25 25
T, “C
;:
;::
?
= i&malatllB
::
6
35
20
19 11
19 19 19
Ref.
5 52 52 52
23
TEAP = te~ae~ylammoni~ pen&orate TBAP = tetrabutyl~onium perchlorate Ag/DMSO = Ag/AgCl,- reference in DMSO SCE = saturated calomel electrode (aqueous) TEANO, = tetraethylammonium nitrate Hg Pool = mercury pool reference electrode The figures in brackets after the El/, values indicate the number of the wave.
m = slope of a plot of Ea,e vs. log &
n = number of electrons transferred in the half-cell D = diffusion coefficient in lo8 cm%ec.
O*lM TBAP O*lMTBAP O-XMTBAP
? 0.1 M KCIOI
NaO 02
x
? ? ?
Inert electrolyte
05M TEAP 0,lMTEAP O*lMTEAP 0.1 M TEAP
O.lMTBAP
NO
Active species
chloride
Tetraethylammonium
m = 0.075 m = 0.069
In, = O-112 mp = O-086
Miscellaneous
n = 1.07; s = 6600 n = 1.02; s = 6180 n = 1.07; s = 4990
s1 = 180 s, = 2250
3 B l% B 2 s k E ccl g #
j
d F
998
THOMASR. KOCH and WILLIAMC. P~DY
ions they determined the diffusion potential between an aqueous SCE and a DMSO test solution.2 Iwamoto3 investigated the electro-oxidation of iodide. Chronopotentiometric studies at a platinum-foil electrode indicated a two-step oxidation. 61- --f 21, + 4e21, --f 31, + 2eThis is in contrast to the oxidation of iodide ion in water, which proceeds by a single step. Gutmann et al.4 extended their previous studies to several silicon compounds, including SiCI,, SiF,, and compounds of the type RCl,Si. The polarographic waves for the RCl,Si species in DMSO occurred at approximately the same potential, permitting polarographic analysis for organic silicon. Kolthoff and Reddy5 studied the polarographic behavior of several acids and of cobalt and nickel in DMSO. They reported the polarization limits with sodium perchlorate and tetraethylammonium perchlorate supporting electrolytes, using the rotating platinum electrode (RPE), the rotating mercury pool electrode (RMPE), and the dropping mercury electrode (DME). These polarization limits are presented in Table II. The polarographic characteristics of perchloric acid, sulphuric acid, hydrochloric acid, acetic acid, sodium bisulphate and tetraethylammonium bisulphate are included in Table I. Well-defined and identical current-voltage curves were observed for solutions of perchloric, hydrochloric, and sulphuric acids in O*lM sodium perchlorate; the half-wave potential was -0.67 f 0.02 V. The basic strength of DMSO is reported to be about the same as that of water. Because these acids gave the same polarographic wave, Kolthoff and Reddy concluded that sulphuric (Q, perchloric, and hydrochloric acids are completely dissociated in DMSO. Amperometric titrations of dilute strong acid solutions can be carried out in DMSO; the current is measured at an applied potential of -1.00 V. Kolthoff and Reddy calculated from polarographic data the diffusion coefficient of the hydrogen ion in DMSO solutions of strong acids and found it to be 4.4 x 10-s cm2/sec at 30”. On the basis of the specific conductance calculated from these data, the proton-diffusion mechanism in DMSO was shown to be different from that in water. Cobalt(I1) and nickel(I1) in O*lM sodium perchlorate gave sharp maxima, even in very dilute solution; no surface-active substance could be found which would entirely eliminate these maxima. Studies at the RMPE (120 rpm) gave well-defined waves without maxima. A dilute solution of sodium perchlorate in O*lM tetraethylammonium perchlorate (TEAP) gave a well-defined, nearly reversible wave for sodium with a half-wave potential of -2.07 V. In O*lM TEAP, quinhydrone and benzoquinone showed two cathodic waves of unequal height. The first wave was a one-electron reversible reduction, and the second wave was irreversible. When O*OlMhydrochloric acid, acetic acid, or water was added, the second wave was shifted to more positive potentials until only one wave with a maximum was observed. This wave had a height equal to the sum of the heights of the original waves. Duroquinone gave two waves, the first reversible, the second irreversible. Tetrachloroquinone gave two irreversible waves of equal height. The authors
Volt~met~
o
6
1
o
I
999
in d~thylsulphoxide
oooo
I
tilt
0
0
0
0
O0
0
000
0
0
0
..o
I I J I I t I I I I I I I I I I I I I I ! J ) I
0
. . . . .o
&&~&
I
~
& ~
! &~&~&&&&&
÷~&
I •
~
I ~&&&
0
0
U.l 0 0 0
zzzzz I111 II ~ 0 r~
0
0
0
~
0
0
0
0
0
0
0
0
0
0
0
0
0
~
0
0
0
0
~
0
0
0
~
0
0
0
0
1000
THOMAS R. KOCH
and
WILLIAM C. PURDY
concluded that the polarographic behavior of quinones in DMSO provided evidence that DMSO is a much weaker acid than water. In relatively acidic solvents such as water or acetic acid, the general reduction mechanism for the two-electron reduction of organic compounds at the DME was reported to be: R + e- -+ R-R=- + HS 3 RH. + SRH. + e- --+ RHRH-+HS-+RH,+SSince the electron affinity of RH- has been shown always to be greater than that of R, the two are reduced at the same potential in a single two-electron reduction wave, such as is observed for quinones in water. In DMSO, as well as in dimethylformamide and acetonitrile (all weakly acidic), the reduction mechanism for quinones was reported to be: R + e- -+ R*- (first wave) R*- + e- + R2- (second wave) Rs- diffuses into solution, where: R” + HS + RH- + S- (slow) RH- + HS --f RH, + S- (slow) A double wave results since R*- is reduced at a more negative potential than R. The second wave is somewhat smaller, probably because of limited protonation of R-or repulsion of R*- by the negatively-charged mercury drop. When a proton donor is added, it reacts with R2- but not with R*-, and the rate of the reaction Rs-+HA+RH-+Abecomes potential-determining. The second wave, therefore, shifts toward the first, and eventually merges with it. Oxygen gave two waves in DMSO, although only one drawn-out wave with a maximum was observed in concentrated solutions. Also, only one wave was observed in 0.1M sulphuric acid and in a 0.1M ammonium acetate and O.lM acetic acid buffer. The polarography of oxygen showed qualitatively the same effects observed with quinones, but more detailed studies were considered necessary. Peover and Whites*’ studied the reduction of oxygen to superoxide ion by a.c. and d.c. polarography. Their results, included in Table I, showed that the reduction is diffusion-controlled, and that the superoxide ion has considerable stability in DMSO. The reduction is reversible. On addition of a strong acid, the process reverts to the normal two-electron reduction. The preparative-scale reduction of oxygen to superoxide ion was studied by Maricle and Hodgson. s Their polarographic work was similar to that of Peover and White. Preparative-scale reductions produced both tetrabutylammonium superoxide and potassium superoxide. The polarographic reduction of oxygen in DMSO that was O-144 in TEAP was studied by Johnson et al9 As shown in Table I, two waves were obtained; in agreement with other workers, the authors found that the
Voltammetry in dimethylsulphoxide
1001
first wave was due to the reduction of oxygen to superoxide ion. The second wave, studied as a function of various cations at varying concentrations, was attributed to the combination of superoxide with one electron plus a cation to give a peroxide product. Jones and FritschelO used potential-sweep chronoamperometry at a hanging mercury drop electrode to study DMSO as an electrochemical solvent, and reported working ranges for various supporting electrolytes. The behaviour of oxygen differs from that reported by Johnson et al.,9 indicating, as the latter authors mentioned, that the reduction of oxygen in DMSO is very dependent on the nature and concentration of the supporting electrolyte. Cadmium and magnesium were also studied. In 1962, Burras” carried out an extensive study into the polarographic behaviour of a number of common cations in DMSO. The effects of various parameters were also investigated. Burras employed both aqueous saturated calomel and mercurypool reference electrodes, and used a variety of supporting electrolytes. The potentials of the electrocapillary maxima, and the usable potential ranges, are reported in Table II. The effects of variation of temperature and of supporting electrolyte concentration were studied. Also, the influence of a maximum suppressor and of water on the polarogram of cadmium, and of added chloride on the cadmium and lead polarograms, was investigated. The polarographic data for the various ions studied by Burras are included in Table I. Gutmann and SchSbeP studied the polarographic behaviour of various chromium(III)-anion systems. The equilibria of various co-ordination forms of chromium(II1) with chloride ion and DMSO were investigated. The co-ordination forms [Cr(DMSO)$+, [CrCI,(DMSO),]+, [CrCl,(DMSO)3], [CrCl,]-, [CrC1,‘J2-, and [CrCI,]” were observed. Schijber and Rahak13 discussed the application, to nonaqueous solutions, of methods used to establish reversibility in aqueous solution. The Cr(III)-Cr(I1) redox system in DMSO was investigated for reversibility. Within the limits of error of the experiment, both chromic nitrate and CrCl, were found to be entirely reversibly reduced in this solvent. Dehn et all4 studied the polarographic behaviour of aluminium chloride and aluminium acetylacetonate in DMSO. One wave was reported for the former, two for the latter; half-wave potentials are shown in Table I. The authors suggested that since aluminium can be extracted as the acetylacetonate, the polarographic method would be useful as an analytical tool for aluminium. The lanthanides (as chlorides) were investigated polarographically by Gritzner et all6 Lanthanum, cerium, praseodymium, neodymium, gadolinium, terbium, dysprosium, holmium, and erbium gave one wave; samarium, europium, and ytterbium gave two waves. All waves were diffusion-controlled and had no maxima. The polarographic behaviour of uranium compounds in DMSO was studied by Michlmayr et al. l6 Uranium tetrachloride shows two waves, the first involving one electron, the other three electrons. Both waves are quite irreversible and the second wave exhibits a maximum. However, both waves were found to be diffusioncontrolled. Uranyl compounds exhibit one diffusion-controlled, well-defined wave involving the one-electron reduction to UO, +. The effect of added water on the waves was also reported. Gritzner et a1.l’ investigated the polarography of thorium salts in DMSO. Thorium tetrachloride gives three waves at a concentrations below millimolar; at higher
1002
THOMAS R. KOCHand WILLIAM C. PURDY
concentrations, the third wave merges with the most cathodic wave. Thorium tetraperchlorate monohydrate and trihydrate give two waves, whereas thorium tetranitrate pentahydrate and thorium tetranitrate solvated with four DMSO molecules give only one wave. DMSO solutions of sodium chloride and sodium iodide were subjected to electrolysis at platinum electrodes. la The cathode reaction involved initial discharge of sodium ions, after which the sodium metal reacted with the solvent. An anodic reaction was also reported that involved reactions of the iodide and tri-iodide ions. Gritzner et a1.u found that nitrogen monoxide, dioxide and trioxide, and nitrous oxide could be determined polarographically in DMSO. Lindbeck and Young20 investigated the polarographic and controlled-potential coulometric determination of the nitric acid oxidation products of soil humic acid. Reduction waves were found which compared closely with the polarographic reduction waves of the nitrobenzoic acids. Although the authors discussed the advantages of the extended potential range of DMSO, their choice of a O*lM sodium hydroxide supporting electrolyte precluded the possibility of using it, by limiting the range to about that of water. Lindbeck and Freundzl studied substituted nitrobenzene compounds in DMSO by polarography and controlled-potential coulometry. The main purpose of their paper was to report n values for the nitrobenzene compounds. Polarography was used only to determine the proper potentials for use in constant-potential coulometry. Di Giacomo et aZ.22reported that phenylfluorone in DMSO gives two waves, as opposed to one in water. The second wave is higher than the first, and can be resolved by linear-sweep polarography into two steps; the more cathodic step shows kinetic characteristics. In the presence of benzoic acid, the two reduction waves are of equal height. A reduction mechanism is proposed. A number of sulphydryl, disulphide, amine compounds, and sulphur-amino acids were studied polarographically in DMSO by Giang et aZ.23 The half-wave potentials, n values, and id/C values given are shown in Table I. The authors reported that all of the waves were diffusion controlled and irreversible. An excellent review of electrochemistry in DMSO was published in 1967 by Butler.24 Both polarographic data and reference electrodes were discussed. REFERENCE
ELECTRODES
A reference electrode employing the silver-silver chloride couple in DMSO was constructed by Courtot-Coupez and Le Demezet. 25 The range of electroactivity in DMSO was tested, using a polished platinum electrode in the presence of sodium, potassium, lithium and quaternary ammonium perchlorates. Ranges varied from 4.18 to 4.80 V. Smyrl% determined the standard cell potential of Pt,,,/Li,,,/LiCl,,,,,/TlCl,,,/ T1rHg),(l)/Pt(s)in DMSO. The standard potential for the analogous calcium amalgam electrode was estimated. McMaster and co-workers2’ developed and tested a reference electrode consisting of a saturated solution of Zn(ClOJ,*4DMSO in contact with a saturated zinc amalgam. By comparing their half-wave potentials with those reported by other workers, the potential of this reference electrode was found to be -1.10 V us. aqueous SCE. Relative reduction-potential scales for several solvents were established.
Voltammetry in dimethylsulphoxide
1003
A reference electrode employing the Ag/AgCl,- couple in DMSO was developed and tested by Courtot-Coupez and Le Demezet. 28 The regions of electroactivity of the platinized platinum, mercury, graphite and silver electrodes were determined, and are listed in Table II. A reference electrode of Ag/AgCl/LiCl in methanol was developed for potentiometric titrations in DMSO.%D Rallo and Ceccaroni30 described a reference electrode consisting of cadmium metal in contact with saturated DMSO solutions of CdCl,, CdCl,*H,O, or NaCl. The electrode was considered suitable for use in electrochemical studies. Synnott and Butlers1 developed and tested four chloride reference electrodes in DMSO. The half-cell Tl(Hg)/TlCl in DMSO appeared to be a good reference electrode for use in this solvent. RECENT STUDIES Polarographic investigations on DMSO solutions of zirconium and hafnium compounds were carried out by Michlmayr and Gutmann.3a The half-wave potentials, the nature of the limiting currents, the temperature coefficients of half-wave potentials and wave heights, the reversibility or irreversibility of the electrode processes, and the influence of water were determined. Schmid and Gutmann reported the halfwave potentials in DMSO (US.aqueous SCE) of thallium(I), rubidium, potassium, sodium, barium, zinc, cadmium, manganese(II), cobalt(II), and nickel perchlorates. These potentials were referred to that of bis(biphenyl)chromium(I) iodide. Reduction mechanisms, various analytical aspects of the system, and the effects of water were discussed. Pyatnitskii and Ruzhanskaya34 reported half-wave potentials for the 8-hydroxyquinolinates of copper( lead, iron(III), cadmium, and bismuth in mixtures of methanol and DMSO, and with eleven other solvents. Lithium chloride was used as the supporting electrolyte. No correlation was found between the half-wave potential and the dielectric constant of the solvent. Kumar and Pantony investigated the polarographic behaviour of a number of anhydrous inorganic perchlorates in DMSO. Gritzner et ~1.~ applied oscillographic polarography to the study of DMSO solutions of the lanthanides. These results were compared with those of classical polarography. The peak potentials are shown in Table III. Haynes and Sawyers7 have employed chronopotentiometry, controlled-potential coulometry, gas chromatography, and pH titrations to investigate the electrochemical reduction of carbon dioxide in DMSO solutions at gold and mercury electrodes. At a gold electrode, irreversible but well-defined chronopotentiograms were observed; the reduction is a one-electron process and is diffusion-controlled. The diffusioncontrolled reduction at a mercury electrode, complicated by equilibria involving water has un = O-64. Controlled-potential coulometry at a mercury electrode indicates an overall one-electron process. In anhydrous medium, the reduction products at both electrodes are carbon monoxide and carbonate ion; some bicarbonate and formate ions are formed in the presence of water. Johnson et al.% observed a unique reduction wave, at potentials less cathodic than the reduction wave of either reducible species alone, in DMSO solutions containing both oxygen and one of the metal ions, cadmium, zinc, strontium, thallium(I), or yttrium. The variation of the height of this wave was studied as a function of concentration of oxygen and the various metal ions. Studies of this type revealed
1004
THOMAS R. KOCHand WILLIAM C. PURDY TABLE III.-OSCILLOORAPHIC PEAK POTENTIALS IN DMS03E -E
Species La(II1) Ce(II1) Pr(II1) Nd(III) Sm(II1) Eu(II1) Gd(II1) Tb(III) Dy(III) Ho(III) Er(II1) Yb(III)
Reduction potential, V 2.39 2.39 2.39 2.42 2*11(l) 2.30(2) 0+6(l) 2.28(2) 2.42 2.42 2.42 2.40 2.42 2.39
-E
Oxidation potential, V 2.31 2.39 2.30 2.29 2.05 0.73(l) 2.11(2) 2.36 2.34 2.33 2.36 2.33 2.20
the stoichiometry of the materials which caused the reduction wave. Controlledpotential coulometry was employed to determine values of n. Massive preparation of reduction products by controlled-potential electrolysis, along with the above information, revealed the probable reduction mechanism. In the cases of zinc, strontium, and thallium(I), the reduction was of metal ion plus oxygen to the superoxide; cadmium and yttrium with oxygen were reduced to the peroxides. Fujinaga et aL39 found that in DMSO containing between 10 and 30% water, the first oxygen wave (reduction to superoxide ion) becomes a two-electron wave, because of protonation by water. The polarographic diffusion coefficients of oxygen in the mixed solvent were calculated. Pasadas et aL40 studied the kinetics of hydrogen evolution on an iron electrode as excess of potassium perchlorate a function of hydrochloric acid concentration; was employed. Temperatures were varied from 20” to 45”. A two-step mechanism was proposed in which the simple discharge of solvated hydrogen ions was ratedetermining. The function of a hydrogen electrode in DMSO was studied by Courtot-Coupez and Le Demezet.41 The authors determined the pK values of a number of acids, and reported the ion product of the solvent to be pK, = 33.3 f 0.05. This differs greatly from the value reported by Reddv2 (pK, = 17*3), but is in virtual agreement with the value determined spectrophotometrically by Steine?44 (pK, = 32). Rallo et a1?5 studied the polarographic reduction of nitric oxide in O-U4 TEAP. Solutions with varying NO/N, content were investigated. One wave appeared when there was less than 10% NO in the gas mixture; with more than 10% NO, a second, more anodic wave appeared, which was not diffusion-controlled and was thought to be due to the reduction of a dimer. The rate-determining dimerization which preceded reduction was thought to control the wave height. Michlmayr and Sawyer46 investigated the electrochemical behaviour of the hydrogen halides at platinum electrodes in DMSO by chronopotentiometry, controlled-potential coulometry, and cyclic voltammetry. The results indicated that the oxidation products of the hydrogen halides reacted with the solvent. In the cases of HCI and
Voltammetry in dimethylsulphoxide
1005
HBr, a catalytic process occurred which enhanced the value of iGf2 by a factor of 5-10. The electrochemical oxidation of formate has been studied at gold and platinum electrodes, by chronopotentiometry, controlled-potential coulometry and cyclic voltammetry.47 Well-defined chronopotentiograms were obtained, with quarterwave potentials of 0.33 and -0.07 V vs. SCE at a gold and a platinum electrode respectively. The process is diffusion-controlled. The oxidation products were carbon dioxide and hydrogen ions, which react with formate ion. Thus, the overall reaction is a one-electron oxidation in which half a mole of carbon dioxide and half a mole of formic acid are formed for each mole of formate ion electrolysed. Jacobson and Sawye? studied the electrochemical oxidation of oxalate at a gold electrode. The chronopotentiometric quarter-wave potential was 1.25 V us. SCE. The final oxidation product was carbon dioxide, but an unstable intermediate was formed during the oxidation process. In the absence of excess of hydroxide ion, oxalate ion is partially hydrolysed to the hydrogen oxalate ion, which is oxidized to carbon dioxide and hydrogen ions. Buchta and Evans49 examined the electrochemical behaviour of 1,3-diphenyl1,3-propanedione, as well as a number of other p-diketones. Controlled-potential coulometry at the potential for the first wave gave an n-value of 0.55. The products were found to be the enolate of 1,3-diphenyl-1,3-propanedione and the pinacole 1+dibenzoyl-2,3-diphenyl-2,3-butanediol. Further electrolysis resulted in an increase in current, followed by a decrease to the residual value. This was caused by an electrolytic autocatalytic decomposition of the pinacol in a base-catalysed reverse aldol condensation to benzil and acetophenone. The presence of the benzil radical anion was confirmed by ESR spectroscopy. Other B-diketones behaved in an analogous fashion. Goolsby and Sawye$O studied the electrochemistry of hydroxylamine at gold and platinum electrodes in 0.2M lithium perchlorate in DMSO. The electroactive species, NH,OH, is oxidized irreversibly in a diffusion-controlled, two-electron process. The chronopotentiometric El14 values were -0.84 V US.SCE at a platinum electrode and -1.18 V at a gold electrode. The major products are water, hydrogen ion and nitrous oxide. The rate-controlling step is a one-electron process which follows a preceding chemical reaction of a hydroxide ion with hydroxylamine. The electrochemical oxidation, at platinum electrodes, of hydrazine, 1,l-dimethylhydrazine, and 1,2_dimethylhydrazine in DMSO was studied by Michlmayr and Sawyer. 51 In each case the oxidation involved one electron. Chronopotentiometric quarter-wave potentials were reported as 0.00,0.02, and 0.03 V US.SCE, for the three compounds in the order listed above. The oxidation products were identified and mechanisms proposed. Koch and Purdy 52studied the polarographic behaviour of amino-acids in DMSO. Nine amino-acids, asparagine, hydroxyproline, isoleucine, methionine, phenylalanine, proline, threonine, tryptophan, and tyrosine, gave single waves involving the one-electron reduction of the proton of the carboxyl group. Five amino-acids, dihydroxyphenylalanine, glutamic acid, aspartic acid, glutamine, and cysteine gave multiple waves. The electrical double-layer in DMSO solutions was studied by Payne.53 He noted a strong resemblance in electrocapillary curves and double-layer capacities of DMSO 2
1006
THOMAS
R. KOTCH and WILLIAMC. PURDY
and aqueous solutions. Anions are specifically adsorbed from DMSO solutions in the order I- > Br- > Cl- > NO,- > ClO, > PF,-; cations are not significantly adsorbed. The electrocapillary maximum is shifted in the positive direction in DMSO as compared to water, and a large hump appears close to the limit of anodic polarization in solutions where the anion is not strongly adsorbed. The influence of diffusionlayer capacity is more marked in DMSO. The interfacial tension at the electrocapillary maximum is about 60 dyn/cm lower (370.5 dyn/cm) than in water. Payne54 reported the occurrence of capacity humps on the electrocapillary curve for 0-W KPFs solutions in DMSO. The hump appears on the anodic side of the electrocapillary maximum. Synnott and Butler 55 studied the equilibria of silver chloride in DMSO-water mixtures containing excess of chloride. They used potentiometric results to determine equilibrium constants for the system. Gutman# reviewed the effect of the donor properties of various solvents, including DMSO. Also discussed were the effects of the acceptor properties of metal ions, solvation of metal ions and complex formation on half-wave potentials and the thermodynamics of reduction processes. Zusannnenfassnng-Es wird eine Ubersicht iiber voltametrische Untersuchungen mit Dimethylsulfoxid als Triigerelektrolyt gegeben. Sie umfa8t Einzelheiten tiber die untersuchten anorganischen und organischen Systeme sowie tiber die fur dieses Medium entwickelten Bezugselektroden. R&me-On presente une revue des etudes voltamm&iques utilisant le dimethylsulfoxyde comme electrolyte-support, et celle comprend des details de systemes mineraux et organiques Studies et des electrodes de reference tlaborees pour l’emploi dam ce milieu. REFERENCES 1. V. Gutmann and G. Schbber, Z. Anal. Chem., 1959,171,339. 2. G. Schiiber and V. Gutmann, Monatsh., 1959,90,897. R. T. Iwamoto, Anal. Chem., 1959,31,955. :: G. Schbber, V. Gutmann and P. Heilmayer, Monatsh., 1961,92,240. I. M. Kolthoff and T. B. Reddy, J. Electrochem. Sot., 1961,108,980. :: M. E. Peover and B. S. White, Chem. Commun., 1965,183. Idem, Electrochim. Acta, 1966,11, 1061. :: D. L. Maricle and W. G. Hodgson, Anal. Chem., 1965,37,1562. 9. E. L. Johnson, K. H. Pool and R. E. Hamm, ibid., 1966,38,183. 10. 5. L. Jones and H. A. Fritsche, Jr., J. Electroanal. Chem., 1966,12, 334. 11. R. T. Burras, Ph.D. Dissertation, University of Tennessee, 1962. V. Gutmann and G. Schiiber, Monash., 1962,93,212. : f G. Schbber and G. Rahak, ibid., 1962,93,445. 14: H. Dehn. V. Gutmann and G. Schiiber, ibid., 1962, 93,453. 15. G. Gritzner, V. Gutmann and G. Schober, ibid., 1965,%, 1056. 16. M. Michlmavr. G. Gritzner and V. Gutmann. Inorp. Nucl. Chem. Letters. 1966. 2.227. 17. G. Gritzner,‘V: Gutmann and M. Michlmayr; Z. &al. Chem., 1967,224; 245. . . 18. M. C. Giordano, J. C. Bazan and A. J. Arvia, Electrochim. Acta, 1966, 11, 741. 19. G. Gritzner, V. Gutmann and G. Schober, Mikrochim. Acta, 1964, 193. 20. M. R. Lindbeck and J. L. Young, Soil Sci., 1966,101,366. 21. M. R. Lindbeck and H. Freund, Anal. Chim. Acta, 1966,35,74. 22. F. DiGiacomo, F. Rallo and L. Rampazzo, Ric. Sci., 1967,37, 1085. 23. B. Y. Giang, G. D. Christian and W.-C. Purdy, J. Pilarographic Sot., 1967,13, 17. 24. J. N. Butler. J. Electroanal. Chem.. 1967. 14. 89. 25. J. Courtot-Coupez and M. Le Demezct, Co&pt. Rend., Ser. C, 1966,263,997. 26. W. H. Smyrl, Diss. Abstr. B, 1967,28,637.
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27. D. L. McMasters, R. B. Dunlap, J. R. Kuempel, L. W. Kreiger and T. R. Shearer, Anal. Chem., 1967,39, 103. 28. J. Courtot-Coupez and M. Le Demexet, Bull. Sot. Chem. France, 1967,39,888. 29. T. Jasinski and E. Kwiatkowski, Zesz. Nauk. Mat. Fiz., Khim., Wyzsza Szk. Pedagog. Gnaa!vku, 1968,8,101. 30. F. Rail0 and G. Ceccaroni, Ric. Sci., 1968,38, 1067. 31. J. C. Syrmott and J. N. Butler, Anal. Chem., 1969,41,1890. 32. M. Michlmayr and V. Gutmann, Electrochim. Acta, 1968, W, 1071. 33. R. S&mid and V. Gutmann, Chem. Zuesti, 1969,23,746. 34. I. V. Pyatnitskii and P. Ruxhanskaya, Zh. Analit. Khim., 1970,25,1063. 35. G. P. Kumar and D. A. Pantony, J. Polarographic Sot., 1968,14,84. 36. G. Gritxner, V. Gutmarm and R. Schmid, Electrochim. Acta, 1968,13,919. 37. L. V. Haynes and D. T. Sawyer, Anal. Chem., 1967,39,332. 38. W. L. Johnson, K. H. Pool and R. E. Hamm, ibid., 1967,39,888. 39. T. Fujinaga, K. Ixutsu and T. Adachi, Bull. Chem. Sot. Japan, 1969,42,140. 40. D. Pasadas, J. J. Podesta and A. J. Arvia, EIectrochim. Acta, 1970,15,1225. 41. J. Courtot-Coupez and M. Le Demexet, Compt. Rend, Ser. C, 1968,266, 1438. 42. T. B. Reddy, Ph.D. Dissertation, University of Minnesota, 1960. 43. E. C. Steiner and J. D. Starkey, J. Am. Chem. Sot., 1963,85,3054. 44. E. C. Steiner, J. D. Starkey, J. M. Tralmer and R. 0. Trucks, Div. Petrol. Chem., Amer. Chem. Sot., Preprints, 1967,12, C-11. 45. F. Rallo, L. Rampauo and F. DiGiacomo, Ric. Sci., 1969,38, 1085. 46. M. Michlmayr and D. T. Sawyer, J. ElectroanaI. Chem., 1969,23,387. 47. E. Jacobsen, J. L. Roberts and D. T. Sawyer, ibid., 1968,16,351. 48. E. Jacobsen and D. T. Sawyer, ibid., 1968,16,361. 49. R. C. Buchte and D. H. Evans, Anal Chem., 1968,40,2181. 50. C. D. Goolsby and D. T. Sawyer, J. Electroanal. Chem., 1968, 19,405. 51. M. Michlmayr and D. T. Sawyer, ibid., 1969,23, 375. 52. T. R. Koch and W. C. Purdy, Anal. Chim. Acta, 1971,54,271. 53. R. Payne, J. Am. Chem. Sot., 1967,89,489. 54. Idem. J. Phys. Chem., 1967,71,1548. 55. J. C. Synnott and J. N. Butler, ibid., 1969,73,1470. 56. V. Gutmann, A&. Prakt. Chem., 1970,21,116.