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Electrochemical properties of metal complexes of dithio-acid derivatives and their application in extraction polarography
ELECTROCHEMICAL PROPERTIES OF METAL COMPLEXES OF DITHIO-ACID DERIVATIVES AND,THEIR APPLICATION IN EXTRACTION POLAROGRAPHY V. F.
TOROPOVA, R. G. K. BUDNIKOV
and N. A.
ULAKHOVICH
Faculty of Chemistry, V.LUl’yanov-Lenin State University, Lenina 18, Kazan, USSR (Received
9 Nwember
1977. Accepted
14 November
1977)
Summary-Metal complexes with dithiocarbamic (dtc), xanthic (xan) and dithiophosphoric (dtp) acids give rise to one or more polarographic waves in dimethylformamide and in mixtures of extracting solvents and ethanol. The electrons are found to be transferred stepwise in the case of unfilled d-shell metal complexes. The shift of half-wave potential depends on the ligand, increasing in the order dtp < xan < dtc. In solvents with low solvation power the electrode processes are more reversible. The linear dependence of the limiting current on the chelate concentration has been used for determining the metal in the organic phase without reextraction. Pb(II), Bi(III) and As(II1) have been separated with dtp as extractant, and the concentrations of
Co(II), Ni(I1) and Zn(II)‘compIexes (simultaneously extracted) have been determined polarographically.
Metal complexes formed with various substituted dithio-acids have recently found wide application in analytical chemistry. Dithio-acids and their simple salts are of great importance as reagents for extraction, separation and determination of metal ions by various methods, mainly photometric.‘-4 The use of dithioacids in extraction polarography can be successfully applied for solving new analytical problems. The effective application of this method requires a systematic and detailed investigation of the electrochemical behaviour of the metal complexes in non-aqueous media. This paper presents data on the electrochemical reduction of some metal complexes formed with the dithio-compounds given in Table 1. Cyclic chronovoltamperometry and d.c. and a.c. polarography were employed. Dimethylformamide (DMF), acetonitrile (AN), and 2:3 v/v mixtures of benzene, toluene, m-xylene, chloroform, ethylacetate or isoamyl alcohol with methanol were used as solvents. EXPERIMENTAL Appuratus
The polarographic measurements were performed with an LP-7 polarograph (Czechoslovakia), PO-5122 oscillographic polarograph ‘model 03 (USSR) and a.c. polarograph PPT-I
(USSR). All voltammetric measurements were made with a three-electrode cell at 25 &0.2”. The dropping mercury electrode (d.m.e.) was used as the working electrode for recording the current-voltage curves. At 25cm mercury pressure and zero applied voltage, the characteristics of the capillaries used were: m = 2.96 and 0.61 mg/sec and t = 4.81 and 7.5Osec respectively. The experiments were carried out with a drop-time of 0.5 set (electromechanical device). The auxiliary electrode was a mercury pool. The reference electrode consisted of a mercury pool in a DMF solution of tetraethylammonium perchlorate, placed in a glass tube ending in a cracked glass-bead junction with the bulk solution.
Reagents
The dithio-acids and their salts and metal chelates were synthesized by methods reported in the literature.“-” Their purity was checked by determining the m.p. and by elemental analysis. The organic solvents were purified by a standard technique. The supporting electrolytes used were O.lM tetraethylammonium perchlorate in DMF and 0.2M lithium perchlorate in ethanol. All solutions of the complexes were prepared in solvent previously deoxygenated with purified hydrogen. RESULTSAND DISCUSSION
The electrochemical behaviour of the complexes is best seen from the data obtained for DMF media. It is possible to identify two different systems of electrode reactions, corresponding to two types of process on the d.m.e. All the complexes formed by metals with filled delectron shells are reduced in DMF to the metal amalgams in one step. Electroreduction of the transition metal chelates formed by xan or dtp and the bivalent metal ions proceeds in a similar way. The diffusion-controlled current, id, is proportional to the concentration of chelate in the range 1-5 x IO-‘M, and practically independent of the mercury pressure head corrected for the back-pressure. Plots of log i, us. log V (ip is the limiting current on the cyclic voltamperograms, V is the scan-rate of potential) exhibit slopes of OS. These results also suggest that the limiting currents are diffusion-controlled. Plots of log i/(id-i) vs. E are straight lines with a slope of approximately 6O/nmV (n is the oxidation state of the central atom in the chelate) (Table 1). The limiting currents and microcoulometric analysis indicate that the waves for the Pb(I1). TI(I), Bi(III), Cu(I1) and Hg(I1) chelates correspond to the transfer of II electrons per molecule. From these results the wave appears to be reversible. This interpretation is supported by the parameters of 263
V. F. TOROPOVA,R. G. K. BLJDNIKOVand N. A. ULAKHOVICH
264
Table
I. Polarographic
data for reduction -E,
Complexes
of ML. complexes
in DMF, concentration
2.V
id,PA
R
M
L
1.13 I.57 0.55 I.72 0.61 1.59 1.84 0.94 1.33 I .86 I .34 I.71 0.65 I .43 0.85 0.78 0.54 0.75 0.64
1.17 1.69 0.36 1.79 0.63 1.64 I.89 0.96 1.34 1.87 1.36 1.81 0.68 1.51 0.92 0.81 0.58 0.78 0.66
0.56 1.83 0.62 I .65 1.88 0.95 I .37 1.86 1.39 1.83 0.69 1.53 0.95 0.82 0.60 0.8 I 0.65
Me
Et
0.53 I.1 I 0.85 I.21 I.51 I.19 I.27 0.68 0.50
Et
Pr
Bu
0.28 0.51 0.30 0.64 0.33 0.31 0.31 0.29 0.29 0.32 0.31 0.37 0.67 0.63 0.46 0.58 0.38 0.61 0.85
0.28 0.71 0.31 0.33 0.34 0.29 0.3 I 0.33 0.30 0.34 0.68 0.52 0.37 0.57 0.26 0.55 0.74
0.26 0.48 0.25 0.54 0.26
1.84 0.97 1.39 1.88 1.38 1.82 0.71 1.53 0.94 0.85 0.61 0.82 0.71
0.58 I .92 0.61 1.60 1.86 0.94 1.36 I.81 1.34 1.70 0.70 I .49 0.90 0.79 0.55 0.77 0.62
0.28 0.54 0.27 0.62 0.32 0.28 0.29 0.30 0.29 0.33 0.33 0.38 0.68 0.65 0.46 0.60 0.34 0.64 0.98
0.48 0.33 0.32 0.35 0.32 0.42 0.54 0.51 0.31 0.54 0.27 0.52 0.68
0.36 0.78 0.33 0.31 0.33 0.34 0.34 0.36 0.28 0.29 0.7 I 0.65 0.42 0.56 0.35 0.77 I .05
Pr
Bu
Am
Me
Et
Pr
Bu
Am
0.55 I .08 0.89 I .22 1.50 1.20 1.29 0.71 0.53
0.55 1.09 0.92 1.65 1.18 1.30 0.72 0.56
0.57 I.14 I.14 I .33 1.64 1.23 1.27 0.69 0.59
0.64 I.13 I.21 I.39 I .66 1.28 I.28 0.74 0.61
0.42 I.01 0.35 0.77 0.66 0.56 0.53 0.62 I.11
0.44 0.92 0.34 0.78 0.62 0.57 0.52 0.59
1.os
0.41 0.93 0.32 0.61 0.65 0.54 0.51 0.61 0.98
0.39 0.8 I 0.29 0.53 0.61 0.57 0.51 0.58 0.99
0.36 0.87 0.31 0.67 0.63 0.51 0.47 0.59 0.95
Me
Et
Pr
Bu
i-Bu
Me
Et
Pr
Bu
i-Bu
0.57 1.09 I .os
0.61 1.12 0.89
0.61 I.15 1.13
0.64 1.21
0.51 0.94
0.47 1.06
0.45 0.98
0.43 0.98
0.47 0.94
Co(I1)
0.53 I .02 0.95
Ni(I1) Zn(I1) Tl(I)
0.55 0.90 0.48
0.56 0.89 0.50
0.54 0.88 0.51
0.56 0.91 0.55
1.10 0.53 0.92 0.52
0.70 0.75 0.74 0.46
0.68 0.77 0.72 0.47
0.69 0.81 0.77 0.45
0.65 0.72 0.68 0.44
0.64 0.74 0.73 0.48
Fe(lll)
CO(II1) dtc Ni(II) Cu(I1) Zn(I1) Cd(U) I-WI 1
-WI) Pb(Il) Bi(Il1)
Fe(II1) Co(II1) Ni(I1) Zn(I1) Cd(I1) Pb(I1) Bi(II1)
Fe(II1)
Pr
.
Me
Mn(lII)
dtp
Et
R Pip
Cr(lIl)
xan
Me
2 x IO-‘M
-
1.23
Bu 1.21 1.75 0.58 I .94 0.63
the cyclic chronovoltamperograms. With DMF medium ‘symmetric anodic-cathodic peaks are observed on the i = f’(E) curves (EP = 58/nmV). The ratio between the heights of the anodic and cathodic peaks of these chelates is approximately I.0 (in the range where V is less than 0.5 V/XX). The shape of both the d.c. polarograms and the cyclic curves is essentially dependent on the depolarizer concentration, because of the adsorptivity of the complexes on the d.m.e. Increasing the depolarizer concentration up to 1 x IO-‘A4 results in maxima appearing on the d.c. polarographic waves of the Bi(III), Cu(II), Pb(II)and TI(I)complexes. The ratio of anodic to cathodic current is less than unity in this case and log &/log V exceeds 0.5. The nature of the ligand affects the discharge characteristics of the chelate. The cathodic shift in E, ,2 with increasing size of R in a homologous series (Table
Pip
-
1) is due to increase in the electron-donor properties of the ligand. The second scheme involves a stepwise electrontransfer. Thus the first stages are reversible. Dithiocarbamate complexes of bivalent transition metal ions with unfilled d-shells and the complexes formed between tervalent metal ions and all the ligands investigated have the same reduction mechanism. Complexes with the central atom in a low oxidation state are formed as intermediate products. The polarographic curves of complexes of the M”+(R2NCS2), have II stages corresponding to the successive transfer of n electrons (M = Cr, Fe, Co, Ni). For the chelates FeL,, CoL,, CrL, and Mn(R,NCS,),, where L = ROCS; or (RO),PS;, the discharge proceeds in two stages with the transfer first of one electron and then of two electrons (Fig. 1). The stepwise reduction of the chelates is complicated by dissociation of the in-
Metal complexes of dithio-acid derivatives
265
n 6-
a x
4-
I3
.-
2-
V
-E.
-E,
Fig. 1. Polarograms’of 1 x IOW4MCo(Etxan), (I), Pb(Et,dtc), (2) and Co(Et,dtc), (3) in DMF.
termediates, with formation of the anion L-, which may be detected by means of the anodic wave resulting from the interaction of this anion with mercury’ under the conditions of cyclic voltamperometry. The nature of the central atom has a considerable effect on this process. In general, the stability of the one-electron transfer products is correlated with the stability of the initial complexes. From theeffect on the cyclic i = f’(E) curves of changing the polarization amplitude.it is concluded that the products formed by the transfer of one electron to the Cr(III), Mn(II1) and Fe(II1) complexes then dissociate: ML,+ee[ML,]11
[ML,_,]“-“[ML,_,](l-P)-
+pL_
+e -t [ML,_,](2-P’[M1L”
](3-P)-
13M;o)+(3-P)L-
In the case of the Co(II1) and Ni(I1) complexes with dtc the splitting of the anion occurs at subsequent stages of the electrode process: ML,+e=[ML,][ML,_ 1]- +e + [MLJ2EMIL,_ J- zt-L-,
The E,,; of the reversible one-electron process becomes more negative as the electron-donor properties of the substituent are increased in accordance with the increase in stability of the complexes in the same series (found potentiometricaHy).‘” Though in some cases discharge of the chelates is complicated by adsorption effects, the ease of reduction of the investigated complexes formed between metals and dithio-acid derivatives depends on the nature of the central atom, and corresponds to the metallic characteristics of the metaHigand bond.” If we compare the reduction potentials of the complexes
V
Fig. 2. A.c. (1,2) and d.c. (3) polarograms of 5 x lO_“M Fe(Me,dtc), (I, 3) and Fe(Mexan), (2) in benzene-ethanol mixture, 0.2M in LiClO+
formed by ditl’erent dithio-compounds, we may notice a certain regularity in each case: a shift of E,,2 towards negative potentials, effect increasing with the ligands in the order dtp < xan < dtc. Data conceriling the effect of the’ nature of the solvent on the polarographic characteristics of complexes of Fe(II1) with different ligands are given in Table 2. The limiting currents decrease with increase in the solvent viscosity. Chelate solvation in a given solvent also plays a certain part. The change of id on transition from aromatic hydrocarbons to acetic esters reflects the growth of the degree of complex salvation leading to enlargement of the depolarizer molecule and, consequently, to decrease of the diffusion coefficient. The adsorption of complexes and the character of the electrode process are of great importance in practical application of dithio-acids and their salts in extraction polarography. The adsorption activity of complexes with dithio-compounds is greater in mixed solvents (ionizing solvent-methanof). This may be used for concentrating complexes on the dropping mercury electrode and for improving the sensitivity of determination in the case of the pulse techniques. The high reversibility of the electrode process, depending on the nature of the solvent, makes it possible to improve the sensitivity of determination when a.c. polarography is used (Fig. 2). Complexes of metals with filled d-shells gives polarographic waves of more reversible nature in benzene-methanol solvents (Fig. 3). It is therefore advisable to use aromatic hydrocarbons as extraction solvents ; polarographic measurements should be carried out in mixed solvents. The choice of a ligand depends on the nature of the problem and the objective in question. Greater sensitivity of determination by the a.c. polarographic method may be achieved by using dialkyldithiocarbamates as extractants; the dialkyldithiocarbamate complexes are more reversibly reduced. The difference in the degree of reversibility of the polarographic waves of the dtc and xan complexes may be accounted for by their great ability to form solvates, characterized by a slow charge transfer at the electrochemical stage
V. F. TOROPOVA,R.
266
G.
K. BUDNIKOVand N. A. ULAKHOVICH
Table 2. EtTect of solvents on the reduction of 1 x IO-‘M
Solvent
DN&,.,<
Viscosity, n1N WC.n,- 2
L (R=Et)
14.1
0.34
17.0
0.54 1.22
2.12 4.35
dtp
0.52 I.10
2.24 4.65
0.63 1.61 1.87 0.55 I .07 0.57 I .06
I .42 1.35 1.36 I .87 3.77 2.10 4.11
0.62 1.59 I.91 0.53 I.14 0.54 I.11
1.38 1.31 1.37 1.62 3.41 1.92 3.75
0.30
26.6
0.80
xan dtP
dtc DMSO
29.8
I .96
xan dtp
*Donor
number relative
I
I
0.7
I.3
-E, Fig.
PA
xan
xan
dtc DMF
id,
2,
2.25 2.17 2.2 I 2.32 4.57 2.40 4.93
dtp
Acetonc
V
0.61 1.59 I .89 0.55 I.13 ‘0.53 I .07
dtc AN
--Et
FeL,
to SbCI,
(Gutmann).
I
v
3. Cyclic voltammagrams of 5 x IO-“M Cd(Bu,dtd), ethanol-benzene mixture (I ) and in DMF (2).
Concentration in
proper. Dtp compounds are of greatest significance for use in extraction from strong acid media, their complexes being stable under these conditions. This is important for determining such elements as bismuth, antimony and arsenic which form complexes with dithio-acids only at low pH. The linear dependence of id on concentration, observed under certain conditions for all the com-
Fig. 4. Influence of pH on with Et,dtp. I-Cd. ~-CO,
HCI.
M
the extraction of metal complexes 3-Ni, As.
4-Zn,
5-Pb,
6-Bi,
7-
plexes investigated, may be used for determining the elements directly in the organic phase without reextraction. Because of the lack of information on the extraction of dithiophosphates and xanthates by aromatic hydrocarbons we have studied conditions for the toluene-extraction of complexes formed between metals and these ligands. Hydrochloric acid solutions
267
Metal complexes of dithio-acid derivatives Table 3. Determination of Ni(II) in presence of Co(H), reagent dtp (R = Et), solvent benzene Found, w/ml
Taken, w/mI
Error 3‘!0
co
Ni
co
Ni
co
Ni
0.00230 0.00620 0.00250
0.00270 0.00150 0.00840
0.0023 1 0.0064 I 0.00247
0.00266 0.00145 0.00841
3.1 5.1 4.1
2.7 4.8 1.4
Table 4. Determination of As(III) in presence of Bi(IIJ) and Pb(JJ), reagent dtp (R = Et), solvent toluene Taken, wlml
Found, m&l
Error, I:,,
As
Bi
Pb
As
Bi
Pb
As
Bi
Pb
0.00350 0.00520 0.00730 0.00250
0.00410 0.00550 0.00850 0.00850
0.00360 0.00600 0.00350 0.00600
0.00354 0.00519 0.00732 0.00254
0.00399 0.00557 0.0086 I 0.00862
0.00367 0.00607 0.00353 0.00605
6.9 3.8 3.4 10.5
3.6 3.3 2.9 2.6
4.0 2.3 9.6 5.6
thods of analysis of industrial and natural resources: the determination of lead in alloys, ofzinc in the blood of animals,‘* and of fungicides. ’ 3.‘4 6-
?.
REFERENCES 1. A. Hulanicki. Taluritu, 1967, 14, 1371. 2. D. J. Halls, Microchim. Acta, 1969.62. 3. Yu. A. Zolotov and N. M. Kuzmin, Soluenr E.~truction, p. 184. K himiya. Moscow, I97 I. 4. 1. P. Alimarin and Yu. A. Zolotov, Zh. Anulit. Khim.,
4-
: 2-
1975,30. I 02
0.6
-E.
I.0
1.4
V
Fig. 5. Polarograms of 1 x 10-3,%f M(Et,dtp), (M = Ni, Zn, Cojin benzene-ethanol mixture 0.2M in LiCIO,.
with a fivefold reagent excess were used for the extraction. Figure 4 shows sufficiently good separation of Pb(II), Bi(II1) and As(II1). If dtp is used as extraction reagent, the Co(II), Ni(II) and ‘Zn(I1) complexes can be jointly extracted together and determined polarographically in the same solution (Fig. 5). Results are presented in Tables 3 and 4. The data obtained have been used to develop some me-
1253.
5. D. Coucouvanis, Prog. Inorg. Chem., 1970,11,233. 6. J. R. Wasson, 1. M. Woltermann and H. J. Stoklosa, Fortschr. Chem. Forschung, 1973,35,65. 7. M. I. Kabachnik and T. A. Mastryukova, IX. Akod. Nauk USSR, 1953, 121. 8. T. A. Mastryukova, A. E. Shipov and M. I. Kabachnik. Zh. Obshch. Khim., 1961,31,507. 9. A. M. Bond, A. T. Casey and J. R. Thackeray, J. Electrochem. Sot., 1973, 120, 1502.
IO. V. F. Toropova, G. K. Budnikov and N. A. Ulakhovich, Z/r. Neorgan. Khim., 1974.19, 1852. 1I. R. Chant, A. R. Hendrickson, R. L. Martin and N. M. Rohde, Austral. J. Chum., 1973,26,2533. 12. G. K. Budnikov, V. F. Toropova, N. A. Ulakhovich and 1. P. Viter, Zh. Analit. Khim., 1975.30,2120. 13. Idem, ibid., 1974.29, 1204. 14. G. K. Budnikov, G. S. Supin, N. A. Ulakhovich and N. K. Shakurova, ibid., 1975.30, 2275.
TOROPOVA, R. G. K. BUDNIKOV
and N. A.
ULAKHOVICH
Faculty of Chemistry, V.LUl’yanov-Lenin State University, Lenina 18, Kazan, USSR (Received
9 Nwember
1977. Accepted
14 November
1977)
Summary-Metal complexes with dithiocarbamic (dtc), xanthic (xan) and dithiophosphoric (dtp) acids give rise to one or more polarographic waves in dimethylformamide and in mixtures of extracting solvents and ethanol. The electrons are found to be transferred stepwise in the case of unfilled d-shell metal complexes. The shift of half-wave potential depends on the ligand, increasing in the order dtp < xan < dtc. In solvents with low solvation power the electrode processes are more reversible. The linear dependence of the limiting current on the chelate concentration has been used for determining the metal in the organic phase without reextraction. Pb(II), Bi(III) and As(II1) have been separated with dtp as extractant, and the concentrations of
Co(II), Ni(I1) and Zn(II)‘compIexes (simultaneously extracted) have been determined polarographically.
Metal complexes formed with various substituted dithio-acids have recently found wide application in analytical chemistry. Dithio-acids and their simple salts are of great importance as reagents for extraction, separation and determination of metal ions by various methods, mainly photometric.‘-4 The use of dithioacids in extraction polarography can be successfully applied for solving new analytical problems. The effective application of this method requires a systematic and detailed investigation of the electrochemical behaviour of the metal complexes in non-aqueous media. This paper presents data on the electrochemical reduction of some metal complexes formed with the dithio-compounds given in Table 1. Cyclic chronovoltamperometry and d.c. and a.c. polarography were employed. Dimethylformamide (DMF), acetonitrile (AN), and 2:3 v/v mixtures of benzene, toluene, m-xylene, chloroform, ethylacetate or isoamyl alcohol with methanol were used as solvents. EXPERIMENTAL Appuratus
The polarographic measurements were performed with an LP-7 polarograph (Czechoslovakia), PO-5122 oscillographic polarograph ‘model 03 (USSR) and a.c. polarograph PPT-I
(USSR). All voltammetric measurements were made with a three-electrode cell at 25 &0.2”. The dropping mercury electrode (d.m.e.) was used as the working electrode for recording the current-voltage curves. At 25cm mercury pressure and zero applied voltage, the characteristics of the capillaries used were: m = 2.96 and 0.61 mg/sec and t = 4.81 and 7.5Osec respectively. The experiments were carried out with a drop-time of 0.5 set (electromechanical device). The auxiliary electrode was a mercury pool. The reference electrode consisted of a mercury pool in a DMF solution of tetraethylammonium perchlorate, placed in a glass tube ending in a cracked glass-bead junction with the bulk solution.
Reagents
The dithio-acids and their salts and metal chelates were synthesized by methods reported in the literature.“-” Their purity was checked by determining the m.p. and by elemental analysis. The organic solvents were purified by a standard technique. The supporting electrolytes used were O.lM tetraethylammonium perchlorate in DMF and 0.2M lithium perchlorate in ethanol. All solutions of the complexes were prepared in solvent previously deoxygenated with purified hydrogen. RESULTSAND DISCUSSION
The electrochemical behaviour of the complexes is best seen from the data obtained for DMF media. It is possible to identify two different systems of electrode reactions, corresponding to two types of process on the d.m.e. All the complexes formed by metals with filled delectron shells are reduced in DMF to the metal amalgams in one step. Electroreduction of the transition metal chelates formed by xan or dtp and the bivalent metal ions proceeds in a similar way. The diffusion-controlled current, id, is proportional to the concentration of chelate in the range 1-5 x IO-‘M, and practically independent of the mercury pressure head corrected for the back-pressure. Plots of log i, us. log V (ip is the limiting current on the cyclic voltamperograms, V is the scan-rate of potential) exhibit slopes of OS. These results also suggest that the limiting currents are diffusion-controlled. Plots of log i/(id-i) vs. E are straight lines with a slope of approximately 6O/nmV (n is the oxidation state of the central atom in the chelate) (Table 1). The limiting currents and microcoulometric analysis indicate that the waves for the Pb(I1). TI(I), Bi(III), Cu(I1) and Hg(I1) chelates correspond to the transfer of II electrons per molecule. From these results the wave appears to be reversible. This interpretation is supported by the parameters of 263
V. F. TOROPOVA,R. G. K. BLJDNIKOVand N. A. ULAKHOVICH
264
Table
I. Polarographic
data for reduction -E,
Complexes
of ML. complexes
in DMF, concentration
2.V
id,PA
R
M
L
1.13 I.57 0.55 I.72 0.61 1.59 1.84 0.94 1.33 I .86 I .34 I.71 0.65 I .43 0.85 0.78 0.54 0.75 0.64
1.17 1.69 0.36 1.79 0.63 1.64 I.89 0.96 1.34 1.87 1.36 1.81 0.68 1.51 0.92 0.81 0.58 0.78 0.66
0.56 1.83 0.62 I .65 1.88 0.95 I .37 1.86 1.39 1.83 0.69 1.53 0.95 0.82 0.60 0.8 I 0.65
Me
Et
0.53 I.1 I 0.85 I.21 I.51 I.19 I.27 0.68 0.50
Et
Pr
Bu
0.28 0.51 0.30 0.64 0.33 0.31 0.31 0.29 0.29 0.32 0.31 0.37 0.67 0.63 0.46 0.58 0.38 0.61 0.85
0.28 0.71 0.31 0.33 0.34 0.29 0.3 I 0.33 0.30 0.34 0.68 0.52 0.37 0.57 0.26 0.55 0.74
0.26 0.48 0.25 0.54 0.26
1.84 0.97 1.39 1.88 1.38 1.82 0.71 1.53 0.94 0.85 0.61 0.82 0.71
0.58 I .92 0.61 1.60 1.86 0.94 1.36 I.81 1.34 1.70 0.70 I .49 0.90 0.79 0.55 0.77 0.62
0.28 0.54 0.27 0.62 0.32 0.28 0.29 0.30 0.29 0.33 0.33 0.38 0.68 0.65 0.46 0.60 0.34 0.64 0.98
0.48 0.33 0.32 0.35 0.32 0.42 0.54 0.51 0.31 0.54 0.27 0.52 0.68
0.36 0.78 0.33 0.31 0.33 0.34 0.34 0.36 0.28 0.29 0.7 I 0.65 0.42 0.56 0.35 0.77 I .05
Pr
Bu
Am
Me
Et
Pr
Bu
Am
0.55 I .08 0.89 I .22 1.50 1.20 1.29 0.71 0.53
0.55 1.09 0.92 1.65 1.18 1.30 0.72 0.56
0.57 I.14 I.14 I .33 1.64 1.23 1.27 0.69 0.59
0.64 I.13 I.21 I.39 I .66 1.28 I.28 0.74 0.61
0.42 I.01 0.35 0.77 0.66 0.56 0.53 0.62 I.11
0.44 0.92 0.34 0.78 0.62 0.57 0.52 0.59
1.os
0.41 0.93 0.32 0.61 0.65 0.54 0.51 0.61 0.98
0.39 0.8 I 0.29 0.53 0.61 0.57 0.51 0.58 0.99
0.36 0.87 0.31 0.67 0.63 0.51 0.47 0.59 0.95
Me
Et
Pr
Bu
i-Bu
Me
Et
Pr
Bu
i-Bu
0.57 1.09 I .os
0.61 1.12 0.89
0.61 I.15 1.13
0.64 1.21
0.51 0.94
0.47 1.06
0.45 0.98
0.43 0.98
0.47 0.94
Co(I1)
0.53 I .02 0.95
Ni(I1) Zn(I1) Tl(I)
0.55 0.90 0.48
0.56 0.89 0.50
0.54 0.88 0.51
0.56 0.91 0.55
1.10 0.53 0.92 0.52
0.70 0.75 0.74 0.46
0.68 0.77 0.72 0.47
0.69 0.81 0.77 0.45
0.65 0.72 0.68 0.44
0.64 0.74 0.73 0.48
Fe(lll)
CO(II1) dtc Ni(II) Cu(I1) Zn(I1) Cd(U) I-WI 1
-WI) Pb(Il) Bi(Il1)
Fe(II1) Co(II1) Ni(I1) Zn(I1) Cd(I1) Pb(I1) Bi(II1)
Fe(II1)
Pr
.
Me
Mn(lII)
dtp
Et
R Pip
Cr(lIl)
xan
Me
2 x IO-‘M
-
1.23
Bu 1.21 1.75 0.58 I .94 0.63
the cyclic chronovoltamperograms. With DMF medium ‘symmetric anodic-cathodic peaks are observed on the i = f’(E) curves (EP = 58/nmV). The ratio between the heights of the anodic and cathodic peaks of these chelates is approximately I.0 (in the range where V is less than 0.5 V/XX). The shape of both the d.c. polarograms and the cyclic curves is essentially dependent on the depolarizer concentration, because of the adsorptivity of the complexes on the d.m.e. Increasing the depolarizer concentration up to 1 x IO-‘A4 results in maxima appearing on the d.c. polarographic waves of the Bi(III), Cu(II), Pb(II)and TI(I)complexes. The ratio of anodic to cathodic current is less than unity in this case and log &/log V exceeds 0.5. The nature of the ligand affects the discharge characteristics of the chelate. The cathodic shift in E, ,2 with increasing size of R in a homologous series (Table
Pip
-
1) is due to increase in the electron-donor properties of the ligand. The second scheme involves a stepwise electrontransfer. Thus the first stages are reversible. Dithiocarbamate complexes of bivalent transition metal ions with unfilled d-shells and the complexes formed between tervalent metal ions and all the ligands investigated have the same reduction mechanism. Complexes with the central atom in a low oxidation state are formed as intermediate products. The polarographic curves of complexes of the M”+(R2NCS2), have II stages corresponding to the successive transfer of n electrons (M = Cr, Fe, Co, Ni). For the chelates FeL,, CoL,, CrL, and Mn(R,NCS,),, where L = ROCS; or (RO),PS;, the discharge proceeds in two stages with the transfer first of one electron and then of two electrons (Fig. 1). The stepwise reduction of the chelates is complicated by dissociation of the in-
Metal complexes of dithio-acid derivatives
265
n 6-
a x
4-
I3
.-
2-
V
-E.
-E,
Fig. 1. Polarograms’of 1 x IOW4MCo(Etxan), (I), Pb(Et,dtc), (2) and Co(Et,dtc), (3) in DMF.
termediates, with formation of the anion L-, which may be detected by means of the anodic wave resulting from the interaction of this anion with mercury’ under the conditions of cyclic voltamperometry. The nature of the central atom has a considerable effect on this process. In general, the stability of the one-electron transfer products is correlated with the stability of the initial complexes. From theeffect on the cyclic i = f’(E) curves of changing the polarization amplitude.it is concluded that the products formed by the transfer of one electron to the Cr(III), Mn(II1) and Fe(II1) complexes then dissociate: ML,+ee[ML,]11
[ML,_,]“-“[ML,_,](l-P)-
+pL_
+e -t [ML,_,](2-P’[M1L”
](3-P)-
13M;o)+(3-P)L-
In the case of the Co(II1) and Ni(I1) complexes with dtc the splitting of the anion occurs at subsequent stages of the electrode process: ML,+e=[ML,][ML,_ 1]- +e + [MLJ2EMIL,_ J- zt-L-,
The E,,; of the reversible one-electron process becomes more negative as the electron-donor properties of the substituent are increased in accordance with the increase in stability of the complexes in the same series (found potentiometricaHy).‘” Though in some cases discharge of the chelates is complicated by adsorption effects, the ease of reduction of the investigated complexes formed between metals and dithio-acid derivatives depends on the nature of the central atom, and corresponds to the metallic characteristics of the metaHigand bond.” If we compare the reduction potentials of the complexes
V
Fig. 2. A.c. (1,2) and d.c. (3) polarograms of 5 x lO_“M Fe(Me,dtc), (I, 3) and Fe(Mexan), (2) in benzene-ethanol mixture, 0.2M in LiClO+
formed by ditl’erent dithio-compounds, we may notice a certain regularity in each case: a shift of E,,2 towards negative potentials, effect increasing with the ligands in the order dtp < xan < dtc. Data conceriling the effect of the’ nature of the solvent on the polarographic characteristics of complexes of Fe(II1) with different ligands are given in Table 2. The limiting currents decrease with increase in the solvent viscosity. Chelate solvation in a given solvent also plays a certain part. The change of id on transition from aromatic hydrocarbons to acetic esters reflects the growth of the degree of complex salvation leading to enlargement of the depolarizer molecule and, consequently, to decrease of the diffusion coefficient. The adsorption of complexes and the character of the electrode process are of great importance in practical application of dithio-acids and their salts in extraction polarography. The adsorption activity of complexes with dithio-compounds is greater in mixed solvents (ionizing solvent-methanof). This may be used for concentrating complexes on the dropping mercury electrode and for improving the sensitivity of determination in the case of the pulse techniques. The high reversibility of the electrode process, depending on the nature of the solvent, makes it possible to improve the sensitivity of determination when a.c. polarography is used (Fig. 2). Complexes of metals with filled d-shells gives polarographic waves of more reversible nature in benzene-methanol solvents (Fig. 3). It is therefore advisable to use aromatic hydrocarbons as extraction solvents ; polarographic measurements should be carried out in mixed solvents. The choice of a ligand depends on the nature of the problem and the objective in question. Greater sensitivity of determination by the a.c. polarographic method may be achieved by using dialkyldithiocarbamates as extractants; the dialkyldithiocarbamate complexes are more reversibly reduced. The difference in the degree of reversibility of the polarographic waves of the dtc and xan complexes may be accounted for by their great ability to form solvates, characterized by a slow charge transfer at the electrochemical stage
V. F. TOROPOVA,R.
266
G.
K. BUDNIKOVand N. A. ULAKHOVICH
Table 2. EtTect of solvents on the reduction of 1 x IO-‘M
Solvent
DN&,.,<
Viscosity, n1N WC.n,- 2
L (R=Et)
14.1
0.34
17.0
0.54 1.22
2.12 4.35
dtp
0.52 I.10
2.24 4.65
0.63 1.61 1.87 0.55 I .07 0.57 I .06
I .42 1.35 1.36 I .87 3.77 2.10 4.11
0.62 1.59 I.91 0.53 I.14 0.54 I.11
1.38 1.31 1.37 1.62 3.41 1.92 3.75
0.30
26.6
0.80
xan dtP
dtc DMSO
29.8
I .96
xan dtp
*Donor
number relative
I
I
0.7
I.3
-E, Fig.
PA
xan
xan
dtc DMF
id,
2,
2.25 2.17 2.2 I 2.32 4.57 2.40 4.93
dtp
Acetonc
V
0.61 1.59 I .89 0.55 I.13 ‘0.53 I .07
dtc AN
--Et
FeL,
to SbCI,
(Gutmann).
I
v
3. Cyclic voltammagrams of 5 x IO-“M Cd(Bu,dtd), ethanol-benzene mixture (I ) and in DMF (2).
Concentration in
proper. Dtp compounds are of greatest significance for use in extraction from strong acid media, their complexes being stable under these conditions. This is important for determining such elements as bismuth, antimony and arsenic which form complexes with dithio-acids only at low pH. The linear dependence of id on concentration, observed under certain conditions for all the com-
Fig. 4. Influence of pH on with Et,dtp. I-Cd. ~-CO,
HCI.
M
the extraction of metal complexes 3-Ni, As.
4-Zn,
5-Pb,
6-Bi,
7-
plexes investigated, may be used for determining the elements directly in the organic phase without reextraction. Because of the lack of information on the extraction of dithiophosphates and xanthates by aromatic hydrocarbons we have studied conditions for the toluene-extraction of complexes formed between metals and these ligands. Hydrochloric acid solutions
267
Metal complexes of dithio-acid derivatives Table 3. Determination of Ni(II) in presence of Co(H), reagent dtp (R = Et), solvent benzene Found, w/ml
Taken, w/mI
Error 3‘!0
co
Ni
co
Ni
co
Ni
0.00230 0.00620 0.00250
0.00270 0.00150 0.00840
0.0023 1 0.0064 I 0.00247
0.00266 0.00145 0.00841
3.1 5.1 4.1
2.7 4.8 1.4
Table 4. Determination of As(III) in presence of Bi(IIJ) and Pb(JJ), reagent dtp (R = Et), solvent toluene Taken, wlml
Found, m&l
Error, I:,,
As
Bi
Pb
As
Bi
Pb
As
Bi
Pb
0.00350 0.00520 0.00730 0.00250
0.00410 0.00550 0.00850 0.00850
0.00360 0.00600 0.00350 0.00600
0.00354 0.00519 0.00732 0.00254
0.00399 0.00557 0.0086 I 0.00862
0.00367 0.00607 0.00353 0.00605
6.9 3.8 3.4 10.5
3.6 3.3 2.9 2.6
4.0 2.3 9.6 5.6
thods of analysis of industrial and natural resources: the determination of lead in alloys, ofzinc in the blood of animals,‘* and of fungicides. ’ 3.‘4 6-
?.
REFERENCES 1. A. Hulanicki. Taluritu, 1967, 14, 1371. 2. D. J. Halls, Microchim. Acta, 1969.62. 3. Yu. A. Zolotov and N. M. Kuzmin, Soluenr E.~truction, p. 184. K himiya. Moscow, I97 I. 4. 1. P. Alimarin and Yu. A. Zolotov, Zh. Anulit. Khim.,
4-
: 2-
1975,30. I 02
0.6
-E.
I.0
1.4
V
Fig. 5. Polarograms of 1 x 10-3,%f M(Et,dtp), (M = Ni, Zn, Cojin benzene-ethanol mixture 0.2M in LiCIO,.
with a fivefold reagent excess were used for the extraction. Figure 4 shows sufficiently good separation of Pb(II), Bi(II1) and As(II1). If dtp is used as extraction reagent, the Co(II), Ni(II) and ‘Zn(I1) complexes can be jointly extracted together and determined polarographically in the same solution (Fig. 5). Results are presented in Tables 3 and 4. The data obtained have been used to develop some me-
1253.
5. D. Coucouvanis, Prog. Inorg. Chem., 1970,11,233. 6. J. R. Wasson, 1. M. Woltermann and H. J. Stoklosa, Fortschr. Chem. Forschung, 1973,35,65. 7. M. I. Kabachnik and T. A. Mastryukova, IX. Akod. Nauk USSR, 1953, 121. 8. T. A. Mastryukova, A. E. Shipov and M. I. Kabachnik. Zh. Obshch. Khim., 1961,31,507. 9. A. M. Bond, A. T. Casey and J. R. Thackeray, J. Electrochem. Sot., 1973, 120, 1502.
IO. V. F. Toropova, G. K. Budnikov and N. A. Ulakhovich, Z/r. Neorgan. Khim., 1974.19, 1852. 1I. R. Chant, A. R. Hendrickson, R. L. Martin and N. M. Rohde, Austral. J. Chum., 1973,26,2533. 12. G. K. Budnikov, V. F. Toropova, N. A. Ulakhovich and 1. P. Viter, Zh. Analit. Khim., 1975.30,2120. 13. Idem, ibid., 1974.29, 1204. 14. G. K. Budnikov, G. S. Supin, N. A. Ulakhovich and N. K. Shakurova, ibid., 1975.30, 2275.