Polarographic analysis using potassium iodide as a supporting electrolyte

ANALYTICA

192

POLAICOGRAPHIC

ANALYSIS

CHIMICA

USING

SUPPORTING I II.

THE

POI.ARO~;

RAI’H

POTASSIUM

IODIDE

AS

A

ELECTROLYTE*

IC BEHAVIOR

T1311<:1-II MhTStJiM~AE Governrruwl

ACTA

I?rduslviuI

(Rcccivcd

01’

AND

RYOZO

Iteseurch

Inslilutc,

May

goth,

‘TRIVAIXNT

ARSENIC

NAICASl-IIMA Ncrgoya

(Japan)

rgGo)

l’olarographic studies on trivalent arsenic in I M hydrochloric acid have been reported by scvcral authorsl-5. J
All polarograms * Parts 8 (19%)

were recorded by the Yanagimoto

I nncl 11 in this srrics 638; 5) (1960) 28.

were

prcscntccl

in the

pen-recording Rc/~ls.

Govt.

And.

Id.

Chiwt.

polarograph, ibsearctr

Aclrc,

Iml.,

24 (rgGx)

Model Nugoya,

lgz--1gg

POLAROGRAPHY

WITH

KI

AS SUPPORTING

ELECTROLYTE.

I93

III.

PB-4. The flow rate of mercury, m, and the drop-time were respectively 1.41 mg/scc and 3.41 set/drop at the applied potential of -0.80 V vs. SCE; the height of mercury column was It = 68.5 cm Hg. A saturated calomel electrode was used as a rcfercnce electrode and was connected to the solution through an agar bridge. All experiments were carried out in a thermostat maintained at 25 k 0.1~. The PH value of the solution was measured with a Horiba glass electrode pI*-mctcr, Model M-3. The 0.02 M arsenic stock solution was prepared by dissolving reagent-grade arsenic oxide in a minimal volume of z N potassium hydrosidc followed by acidification with hydrochloric acid and dilution to a known volume with distilled water. A 2 M potassium iodide solution was used as supporting electrolyte and was prcparcd from analytical grade reagent. Solutions of LNAC and other surface active substances used as maximum suppressors were o.oxo/o. The electrolytic sample solutions wcrc prcparccl by the following proccclurc. To a certain volume of the stock solution of arsenic, sufficient supporting clectrolytc and maximum suppressor were added to reach the final concentrations: 0.x III to 1.0 M and o.ooo5°/0 to o.oor5°/o, respectively. The 1~1value of the sample solutiolr was then adjustccl by aclding a little dilute hydrochloric acid or pot;kum hyclroxitlc. Dissolved oxygen was rcmovcd by bubbling pure nitrogen gas through the sample solution for about 15 min. Then the polarograms were recordccl. RI3ULTS

AND

DISCUSSION

The reduction waves of trivalent arsenic in 0.1 M potassium iodide containing LITAC as the maximum suppressor arc shown in Fig. I. The first step was a well-dcfincd wave

.02V ..-_ -___

-a3v I.

The reduction waves of nrscnic(lI1) in 0.x M ICI. (I) 1.2 mM AY+~, O.OOI~/~ DI’AC; 0.8 mM As+3, O.OOX~/~DTAC; (3) 0.4 mM As+“, O.OOI(~~ DTAC.

(2)

at concentrations of arsenic less than 0.8 rn.M (Curves z and 3), but the wave did not show a plateau pattern at concentrations of trivalent arsenic higher than I .o mM. The reduction current rose slowly and fell with increasing applied potential, i.e. the wave developed a roundish pattern (Curve I). Anal.

Chim.

Ada,

24 (x96x)

rgz-Igg

T. MATSUhlAE,

194

R. NAKASHIMA

‘The second wave, in the absence of a maximum suppressor, had a curious double maximum wave which could not bc suppressed by gclatinc (Fig. 2, Curve I) ; several surface-active substances were tcstcd as maximum suppressors and DTAC was found to be the most useful. The effects of IYKAC on the reduction wave of trivalent arsenic in 0.x M potassium iodide are sllown in Fig. 2. The same characteristics as in the first wave arc indicated.

Fig.

2.1 to 2.3.

mm2 on the reduction waves: As *a, 0.4 rnM; I< t, 0. I 0; (2) II-~-AC o.ooo_~o,~,; (3) 1x-x O.OOIO~,; (.I) rxa

2.

When the heights of mercury column were altcrccl, the limiting current of the first wave or the total wave was rccorclcd both in the abscncc and prcscncc of IXAC. Tables I, II ant1 III show the results. The limiting currents of the first wave in the abscncc of DTAC and those of tlic first wave or total wave in the prcscnce of ur.4c were proportional to the square root of the effective height on the mercury column (corrected for the back pressure). The tcmpcraturc cocfficicnts of the limiting current in the prescncc and absence of DTAC were found to bc 1.4’y0 and 1.7% per I%, rcspcctivcly. From thcsc results, it is consitlcrccl that the limiting currents in the cast described are diffusion-controllccl.

RELATION

DISTWIZISN __.

L1MlTlNG

__.____.__.

__-_

h(m)

-__ -___--

53.5 x

______ICI, 0.1

G515 -.n!!;

CUIW3NT

_.-.-

___^ -_--

___. -._.-.--

ho,.

AND

-.-.. -..-

--...--.-.-.--

MMICUIIY .---..-.--.-.-_... _----

!w+a,

0.4

mA4;

it (lull __-.-.

3.02 3.08 3.10 3025

7.20 7.47 7.79 7*99 . ..__-_---_______---

DRI~SSURI~

Ox

THE

I’IRST

WAVE

------.----.-.-

il/lta*rr .._ _..._ .-.^--“.---.

-

O.&j20

0.41G 0,409 0~403 ^.____-_ __-_-----

_._.

_

pl-r,2.45.

And.

C/rim. Acta,

24 (1961)

Ig)Z-Igg

WITH

POLAROGRAPHY

RELATIOS

RETWEISS

LIMITlSG

I<1

AS

CURREST

SUPPORTING

ASD

ELECTROLYTE.

NERCURS

PRISSSt’RE

OS

III.

TlfE

195

FIRST

\VAVE

r,/ha..,

id 2

---

5.97 6.45

.-

63 67 7’ 7.5 0.1 nl;

I;I. ---..-

0.4

0.4 0.4

_._

--.-_-_-I_

____

_._

hfl~m)

63 67 7’

i)*r.u*, ___. _..

I;I.

r,715d,:

.__-_ 63 67 71 75 0. I

KI,

___.

___

.__.____.__..

__ -..-

he.,.

.” .____

s/fiTcts of pi

I)‘I’AC. -_-

_..-

OlL the

-_

AS” In,.lf) ..- . ..__._._ _._

--

043.53 0.350 0.3.5’) 0.3bO

-__-

_

.------_-.--

-_ _ .

t,

._^.

. - .._.. -.---.

(Il.4 I .__ ...__....

----__-_-

il’h,.., __ .._-__.-_

_.._. .

7.810 S.oGz H.,)26

0.8 0.H

I.!.77 13.17

I *G.( I.03

0.8

s.s.14 o.O~~~yj: 111i. ._._. - .__.. -_.--._.---_

0

‘3.53 ‘3.95

I,63

.8

. _.

.-.----.. 0. *+

6.362

X.4 26

O.‘#

8.544

0.4

6.573 6.704

I ‘$, pI1.

._.___

--.._

rcdrrctiov~

. ..-.. ..._----._-

..__..---.

-.

‘,*7”4 0.7s’) 0.7x0

G.121

H.062

_-

I.GI

-- .-- .- -

0.4

0.00

._-_.-

:..+H.

7.810

ILI:

---__--

.._._ _.___,..__. “___ .___-

_____.,.__..-_

---_.-_

0,795

2.75 2..57 3.03 3.0s

-_

--_-

______

__._

8.oG2

I)*rAc.-, 0.001 “:,; 1111,2.38.

0.763

6.79 -..

0.4

8.420 8.514

._

6.43

__-_.-.__-----

.-_--.-

7.810

0.764 0.800

0,7s5

z..;H. _

..._ -

..

.

__. _-_

. .^

_

-.

.

7Ckil)C

cffccts of plr on the half-wnvc potential and tllo diffusion currcn ts were studictl ; :haractcrs of the arscnic(II1) reduction wa\*c in the pII range I.(J) to 2.76 arc rn in Figs. 3 and 4. Helow plr 1.9, the starting point of the first wa\*e was so close IC rinoclic wave of dissolution of mercury that mcnsurcmcnts of tllc! w;~\‘c ldglit very difficult. ‘i’llc height of tllc first wave ciccrcnsctl with iIlcrcilsillg pII in tlic c I.C)O to 2.50, but the total wave: hcigllt sliowccl a constant value (Fig. 3). v’c pkI 3.00, the wave height nppixcntly clccrccisccl with increasing prr. ‘I’hc results :atc that tllc reduction of trivalent arsenic dccrcascs with 8 tlccrcnsc in the conration of Ilyclrogeu ions. ic half-wave potentials of botll the first anti scconcl waves sliiftcxl to more ncgativc cs with incrcilsing p1-I; the scconcl wave sliiftctl fnrthcr than the first (Fig. 4). relationship between the half-wave potential of the first wave and the prr was tr, with the graclicnt about 115 mV per plr unit. ‘I’!w rccluction of arscnious acid le clropplng mercury clcctroclc is consiclcrcd to proccctl as follows: IC

AsOx-+

.#H-+ +

3c

-+AsO-+

2ya0,

‘I I

x’forc,

the relationship

between ,

tr,

the half-wave

pkeritial Anat.

&cl

Chim.

PH

Acta,

can 24

<

(L)

.

be writ&k

(1961)

Igz--Igg

T. MATSUMAE,

196 . .

El/z

Eo -

=

R. NAKASHIMA

0.059

--PH-t?a

(2)

where fIl and f. are the activities of the reductant and oxidant respectively, and DIZ and D 0 arc the corresponding diffusion coefficients. If the reaction proceeds reversibly, ?c = 3 and Itz = 4, and the value of AE 112per per unit should be 80 mV from cqn. (2). I3ut then value found showed greater shifts than would be expected if the reaction proceeded reversibly.

I

I

20

30

25 PH-

Fig. 3. ‘The cffcct of pn on the wave height: As+3, 1.0 mM; ICI, 0.1 M; IX-I = 1.9 to 3.5. 0 total wave; -o-_ofirst wave. --o-

Pig.

‘I. ‘Jk! cffcct of pH on the half wave potential: &+a, 1.0 mM; KI. 0.1 M . -) and I .o M (-o-o-) ; PH = I .9 (--oto 3.5.

113 t

E-

Fig.

5.

The

(2) PH =

plots

2.20,

Of

E -

(3) PH =

i)] of the first wave at. scvcral PH values: (I) PH = 2.00, (4) PH = 2.45, Tlw values shown at the foot of each curve are the reciprocal slopcs.

log[i/(id 2.25,

-

Anal.

Chim.

Ada,

24 (1961)

rgz-199

POLAROGRAPHY

WITH

KI AS SUPPORTING

ELECTROLYTE.

III.

197

In order to Ieafn more about the nature of the irreversible reduction of trivalent arsenic in potassium iodide solution, the reduction wave was analysed by a logarithmic plot. As shown in Fig. 5, the plots of E - log ii(id - i) of the first wave showed a linear dependence at pH values of 2.00, 2.20, 2.25 and 2.45; the gradients are indicated in Fig. 5. Therefore, one can conclude that the reaction As+3 --t As0 is irreversible, because the reciprocal slope of the first wave showed a valtlc greater than the thcoretical (three electrons). The reciprocal slope of the first wave at a certain PI-I is not a constant, the value decreasing with increasing PH. It is evident that the irreversible reaction of trivalent arsenic is dependent on the pi of the solution. Relation

betwmt

the rcdmtion

current and trivahtl

arscuic conccntvation

When concentrations varying from o.c;G mM to 4.o~ nlM of arsenious acid were adjusted to PH 2.0 to 2.4, and the reduction waves were recorded, the height of the first reduction wave increased proportionally to the concentration of arsenic only in the range from o.06 mM to 0.8 mM (Fig. 6, A ant1 13); however. above x.o mM of

1

COnCmtrotlon Pig.

6.

f

2 of

c

1

4-

3 AS”

(mf-f)

Rdation bctwccn the rctluction current and tlrc concentration of As+3 in 0.1 M ICI. The broken iinc shows the vrrluc cxtrapohtcd from the lincirr at low concentrations.

arsenic the relation between the reduction wave height and the concentration was not linear (Fig. 6, B and C), and the diffusion current constant decreased with incrcasing concentrations of arsenious acid. The reproducibilitics of the reduction current were not good in the range 1.0 mM to 2.5 mM of arscnious acid, but were better at higher concentrations. It is interesting that the point J3 at which the concentration began to show nonlinearity (in I’ig. 0) agreed with the point at which the plateau of the first wave changed to a roundish curve (Fig. I, Curve I). In an explanation suggested by MEITES~, ArtaZ. Chinr. Ada,

24

(x96x)

xgz--199

1’. MATSUIIAE,

198

R.

NAKASHIMA

it is considcrcd that a film of clemcntal arsenic adsorbed on the surface of the mercury drop limits the regular diffusion of trivalent arsenic. This adsorption phenomenon was confn-mcd from the relationship of the concentration and wave height. The clitfcrcncc hctwcen the obscrvcd current at non-linear relationships and the current cxtrapolatccl to a linear relationship bctwccn wave height and concentration of arsenic is consiclcrcd as an adsorption phenomenon. From a general equation of isothermal adsorption, the following ccluation can hc written ic -

i,n = i,‘ = LZC’I’~

‘I’licrcforc, lug iu =

log (I -+ f. log c 71

wlicrc i, is the current cstrapolotccl to a linear relation, im is the ohscrvcd current height at non-linear relation, i I, is the cliftcrcncc of tllcsc two currents (in Fig. 6), and C is the concentration of :irscnic. ‘I’lic plots of log i rr against log C show a linear rclationsliip (Fig. 7), conscqilcntly a non-linear rcl:Ltionsliip hctwccn tllc rctluction wave

I

0.2

03

3

05

IogCG-mM) Iii&.

7.

‘I’ht!

~~tbWr~~ti~lll

tlic arsenic concimtmtion reduction

780 E(mV

c!ffl!ct

currcmt.

of

on Iho

vs

c

800 S.~E.)

IQ. 8. ‘I’lic cffcct of the nrscnic thcst:mting potcntid of the slxonll

conccntr;rtion reduction

on

wave.

height and the conccntrntion can be consiclcrcd as some cffcct of an adsorption phenomcnon upon the rccluction process ot arsenic at the surface of the mercury drop. The rccluction current unclcr tllc conclitions of the aclsorption phenomenon formed a round pattern with increasing ncgativc potential; the second reduction current appearccl sucldcnly at the cncl of the drop in the rounclish pattern. Tlic curvature of the rounclecl wave incrcasecl lvith increasing concentrations of arsenious acid, and

POLAROGRAPHY

WITH

KI

AS SUPPORTING

ELECTROLYTE.

III.

199

the starting potential of the second reduction wave also shifted to negative potentials with increasing concentrations of arsenious acid. The results are given in Fig. 8. The phenomenon is considered as follows. The amount of arsenic adsorbed on the surface of mercury drop increases with shifting negative potentials and the reduction current decreased’. At the potential corresponding to the active energy for reduction of the second wave, a sharp rcductign current appears. The phenomenon is found notably with increasing concentrations ot arsenic; it is considered that the starting potentials of the second wave become more negative, increasing active cncrgy being necessary for reduction of the second wave. The relationship between the total wave height (the first and second wave heights) and the concentration of arscnious acid is shown in Table IV; the relationship below

RICLATION

UISTWERN

As+”

-

Tlil5

TOTAL

AS13

X.601

3.271 6.593 9.735 12.#

I .oo

rG.G3

2.00

33.58 51.11 07.04

3.00 4.00 --

. ..-......._..

ICI, 0.1 IPI; In-AC,

----.---_-___._..

ptl, 2.1 to 2.3;

__I__

--_____

10.g1

‘0.77 IO.87 11.03 IO.80

0,3G

-

.--__ --..-_ -. ___..... -. .._ ._.-__

0.001q:;

I

ARSENIC

‘0.33 JO.33 IO.59 ro.GH 10.5r

16.35 16.48 16.23 X6.23 I G.63 10.78 17.03 rG.79 :I:

OF

_-

IG.01

rG..+s

.-- _.--..--

CONCISNTRATION

IS.95

0.957

0.20 0.40 0.60 0.80

TIIR

id/C

-__--

0.10

_----__-...

flISICIIT

ia f/IA)

fmhl)

0.06

Mean

WAVE

10.65

.__._ ----

rrr2/n fll0,

f

0.23

___- _._____. _____“__

1.543.

the concentration 4.0 mM is linear and the total wave height at constant concentration is a constant value over the pr-Irange 1.0 to 3.5. Thercforc, the polarographic dctcrmination of micro amounts of arsenic in potassium iodicle solution seems to be possible. SUMRIAKY The polnroyraplly

of nrscnic(II1)

in 0.1 or 1.0 AVI potassium

iodide

SOlUtilJIIS

has been studied.

RlbUMl? Les autcurs out cffcctuh unc dtudc du comportcmcnt l’iodurc dc potissium commc dlcctrolytc dc b:rsc.

polaro~raphicluc dc I’nrscnic(lII),

cn utilisant

%USAMMENl~ASSUNG Ucschrcibung eincr Untcrsuchung libcr tins ~~olarograplliscllc Vcrhaltcn wcndung von Kaliumjodid als Grundclcktrolyt.

1 IC. KACIRKO~A, Collccfion Ctechoslov. Chern. Cotrrntu~~s., 1 (1929) 477. a J. J, LINGANE, lnd. Eng. Cherrt., AnuZ. Ed., 15 (rg43) 583. 8 L. MLSITES, J. Atn. C/rem. Sot., 7G (1954) 5927. 4 1). A. EVEREST AND C. W. FINCH, J. Clrmr. Sot., (1955) 704. 0 G. I?. HAIGHT, A~taZ.Circm., 2G (1954) 593. 0 'r. fifATSUhIAE, Dcnki Kagakac, 27 (x959) 549, 604. 7 T. MATSUMAE AND IL NAKASSIIhlA, RePIs. Govf. Iud. Researck Inst.. (rg6o) 28.

von Arsen (I 1I) bci Vcr-

Nagoya,

8 (rggg)

638;

Alaat. Ckim. Acta, 24 (196x) xgz-xgg

g