Polarography of some anions in presence of aqueous organic solvent mixtures: Reduction of IO−3 ions in presence of various base electrolytes

Eherrochimiea ha

1977. Vol. 22 pp. 47Wg3. Pergamon Press. Printed m Great Britrrin

POLAROGRAPHY OF SOME ANIONS IN PRESENCE OF AQUEOUS ORGANIC SOLVENT MIXTURES: REDUCTION OF IO; IONS IN PRESENCE OF VARIOUS BASE ELECTROLYTES B. K. GUPTA,D. S. JAIN and J. N. GAUR Chemical Laboratories, University of Rajasthan, Jaipur-302004,India (Receivedin final form 10 December 1973) Abstract-The present studies treat with the reduction of IO; ions in different types of base electrolytes at the dropping mercury electrode. All the measurements were done in 0.1 M base electrolytes at 30°C. The effect of various sodium and potassium halides, different cations (alkali and alkaline earth metal chloride), various alkali metal nitrates and some ammonium salts, on the polarographic characteristics of iodate ions, have been studied in details, iR measurements have also been carried out in presence of various oxy-anions and sodium salts of organic acids. In all eases the reduction was found to be ditTwion controlled but irreversible. The kinetic parameters have been evaluated by Koutecky’s method. The reduction of iodate ions was iirst studied by Rylich[l] at dme who found a single reduction wave. Similar results were confirmed bv Kolthoff and Lingane[2,3]. Kolthoff and Orle&m[4] found that half wave potential is constant at pH and is independent of the iodate ion concentration. Coe and Rogers[S] found that addition of gelatin shifts the iodate wave to a more negative potential. The reduction potential of iodate is shifted markedly to more positive values in the presence of polyvalent cations. A. M. Shams El Din et czl[6] also studied the reduction of iodate ion at different pH and suggested the mechanism. V. I. Zykov[7] studied the effect of certain surface active organic cations on the reduction kinetics of iodate ions. H. A. Laitinen et a@] studied the effect of adsorbed films on kinetics of electrode reaction of iodate ion in acid and basic medium. S. I. Zhdanov et a@] determined the number of electrons involved in electrochemical reactions by microcoulometric. However, from survey of literature it appears that no attention has been so far turned towards the reduction of iodate ion in different base electrolyte. The similar studies have been done on chromate and bromate ions by authors[iO, 111. The present paper deals with the polarographic study of iodate ion in presence of different base electrolytes.

with applied potential of - 1.0 V us see) was used for all the measurements. The experiments were performed at 30°C which was maintained with a Haake type thermostat. The solutions were freed from dissolved oxygen by bubbling purified nitrogen gas for 20 min. The iR drop correction was applied. RESULT

AND

DISCUSSIONS

Current-potential curves were obtained in different supporting electrolytes, in each case a single well defined reduction wave appeared. The plots of id US ,/h (h = effective height of mercury column after applying back pressure correction), were linear and passes through origin for all supporting electrolytes, showing that electrode reaction is diffusion controlled. The same conclusions were confirmed by the plots of i,, vs BrIO; ion concentration which were linear and passed through origin. The plots of log i/i,, - i us Ede (-V 11ssee) were linear for all solutions and slopes were much higher (required for 6 electron irreversible reduction (1OmV)) showing that reduction is irreversible. The diffusion coefficients of IO; ion in all supporting electrolytes have been calculated by Ilkovic equation. The value of diffusion coefficient reported by the authors in 0.1 M KC1 of IO; ions comes to 1.01 x 10-5cm2/s which is in agreement with the value calculated by Kolthoff et ~947 (1.09 x 10m5cm’/s). The kinetic parameters are EXPERIMENTAL calculated by Koutecky’s method in each system. The value of KY,, was obtained by plotting a graph Stock solution of IOg (0.01 M) and other support-zero voltage ing electrolyte solutions (1.0 M) were prepared by dis- between -log KR us E( - V m__fl_&]__@ solving required weight of reagent grade KI03 and and the slope of these lines gave the value of in in electrolytes in double distilled water. Solutions con- each system. The standard rate constant cannot be taining 1 mM BrIO; ions and 0.1 M having different calculated because the value of standard potential is supporting electrolytes were prepared in 25 ml volu- not known for every system. However, the approximetric flasks. Solutions were also prepared having dif- mate values can be obtained by extrapolating the plots of -log F+, DSE( -V us nhe) to 0.26 V, which ferent IO; ions concentration in the same percentage of supporting electrolytes. The current-voltage (iR) is standard electrode potential for iodate ion in alkaline media[12]. The value of pH was measured after curves were obtained by manual set up. The dropping mercury electrode having m = l.l464mg/s and the experiment of each solutions and reported in t = 3.5 s (at 35cm of Hg pressure in 0.1 M NaOH the tables. 419

480

8. K. GUWA,D. S. JAMAWJJ. N. GAUR

Table 1. Effect of IO; ion concentration on polarographic characteristics

Table 3. Effect of temperature on reduction of IO; ions Temperature

Concentration WI)

In 0.1 M NaOH 0.5 1.0 1.5 2.0 In 0.1 M NaCl 0.5 1.0 1.5 2.0

C

@A)

-

E1,2

(VUSSW)

Slope

(mv)

jd

(“C)

Slope

-h,2 (V

(PA)

vs

see)

W)

i&

8.4 16.8 24.8 33.6

1.265 1.270 1.270 1.270

95 85 80 80

16.8 16.8 16.5 16.8

8.36 16.62 25.01 33.88

1.345 1.357 1.357 1.357

105 100 1lXl 100

16.72 16.62 16.66 16.94

(ia) Efict of concentration of IO; ions in presence of 0.1 M NaOH und 0.1 M NaCl By increasing the IO; ion concentration in alkaline media (0.1 M NaOH) the id increases and halfwave potential almost remain the same. The ratio of &/C comes to a constant quantity showing that the reduction is diffusion controlled. This was also confirmed by plotting a graph between id vs C which results in straight line passing through origin. Similar results were obtained with 0.1 M NaCI. The results are summarized in Table I. (b) E&t ofheightof mercury reservoir. In presence of 0.1 M NaOH and 0.1 M NaCl, by increasing the height of dme i,, increases and halfwave potential shift towards to more negative values. The plots of id us J/I comes to linear and passing through origin, showing that the reduction is diffusion controlled. The value of i,./Jh was found to remain constant. The results are summarized in Table 2. (c) EfSect of temperature. The polarograms for 1 mM IO; ion in presence of 0.1 M NaCl and NaOH were recorded at 3 different temperatures. The difTusion current increases whereas halfwave potential almost remain unaltered with increase in temperature. The results are tabulated in Table 3.

(ii) Efict ofsodium and potassium halides on reduction @IO; (I mM) ions

In 0.1M NaCl 35 40 45 In 0.1M NaOH 35 40 45

19.16 20.77 20.90

1.357 1.360 1.360

95 102 100

20.67 21.23 22.09

1.275 1.272 1.270

80

the change in the size of the anion. The same conclusions were drawn by Zykov et al[7j. The reductions are irreversible so that kinetic parameters are obtained by Koutecky’s method. The results are tabulated in Table 4. The plots of -log K;, OSE,,= are presented in Fig. 1. of cations, alkali and alkaline earth metal

(iii) E’ct chlorides

By changing the nature of cations (from LiCl to CsCl and MgQ, CaCl,, SrClz and BaC12)the diusion current increases except in CsCl and halfwave potential shift to positive side. Similarly if the valency of cation increases the value of halfwave potential shift to more positive value and diffusion current slightly changes. The same results were drawn by Rylich[l] and Zykoy et a1[7]. From the analysis of plots of log t/&,- r usfide,the reductions were found to be irreversible, the kinetic parameters are calculated by Koutecky’s method and the results are summarised in Table 5. The plots of -log KTh us E( -V us nhe) are shown in Fig. 2. (iv) E#ect ions

of al!4

metals nitrute on reductiun on IO;

By changing the nitrate from Li to Cs, the halfwave potential and diffusion current changes with the change in cation from Li to Cs. The diffusion currents change slightly from Na to Cs while in LiNOs the

In the presence of different potassium and sodium halide solutions (0.1 M) the halfwave potential of reduction of 1mM IO; ions shift towards more positive values and id slightly changes in case of sodium halide solutions and remain constant in case potassium halide solutions. This may be explained due to Table 2. Effect of mercury

reservoir height on polarographic characteristics for the reduction of 1 mM IO; ions

Height (cm) In 0.1M NaOH

35 40

45 In 0.1 M NaCl 35 40 45

6 @A)

-k/z (V 0s see)

Slope (mV)

16.8

1.270

80

3.M)

19.16 20.67

1.272 1.275

80 80

3.03 3.08

16.62 17.54 19.16

1.357 1.360 1.357

100 100 95

2.80 2.78 2.80

id/jh

0

0.2

04

I

/

06

06

E, -Vvs

I

I

IO

I2

nhe

Fig. 1. Plots between -log

K& USE.

Polarography

I

I

02

04

I

I

-V vs

I 12

1

08

06

E,

of some anions

IO

0

02

nhe

Fig. 2. Plots between -log

B&Se (0.1 M)

pH

(@A)

: 3 4 5 6 7

NaF NaCl NaBr NaI KCI KBr Kl

6.10 6.0 6.0 6.6 6.3 6.4 6.5

17.5 15.0 18.0 17.25 15.0 15.0 15.5

id

-El/Z (V DSSW)

1.362 1.357 1.318 1.314 1.360 1.340 1.322

06

6

A’ “s nhe

Slope (mV)

LOO 100 98 90 156 116 88

D"' x 10' (cm%) 3.560I 3.05 3.662 3.509 3.05 1 3.051 3.153

K; (cm/s) 2.51 1.22 5.01 7.07 3.55 1.78 1.99

id @A)

-EI,, (V us see)

Slope (mV)

D1'*x lo3

PH 7.0 6.0 6.0 h.3 7.0 5.6 6.4 6.3 6.0

11.50 12.0 15.0 15.0 8.0 8.0 15.0 12.0 15.0

1.091 1.370 1.357 1.360 1.290 1.042 1.062 l.OY3 1.112

78 114 138 156 93 47 88 88 92

2.34 2.44 3.05 3.05 1.62 1.62 3.05 3.44 3.05

(Cm’/s)

x x x x x x

No. : 3 4 5 6

Base electrolytes (0.1 M)

NH,NO, LiNO, NaNO, KNO, RbNO, CSNO,

pH

id (PA)

-El,, (V vs SW)

6.2 8.7 7.0 7.3 5.1 7.1

11.0 10.0 16.5 15.5 15.25 14.50

1.069 1.359 1.314 1.390 1.325 1.305

Slope (mV)

5.04 1.58 x 1.26 x 1.58 x 2.81 x 7.94 x 6.31 x

K; @m/s) 1.88 7.08 1.22 3.55 2.82 1.99 6.31 3.16 3.16

110 82 144 152 140 110

0"' x lo3 (cm’/s) 2.238 2.034 3.357 3.153 3.102 2.950

KR DSE.

K,q @m/s)

10-LL 10~12 10 I3 1o-‘1 lo-lo 10-12 lo-l3

x x x x x x x x x

10-l’ 10-‘O 10~” 10~” lomq lo-‘0 IO-”

0%

0.45 0.48 0.60 0.61 0.41 0.51 0.56

1mM IO, ions K,“.

(Cm/S)

lo-‘* lo-l2 lom’z lo-= lo-‘3 10-l” lo-‘2 lo-” 10-l’

Table 6. Effects of alkali metal nitrates on reduction of IO;

S.

IO

of IO; (1 mM) ions

Table 5. Effect of cations (alkali and alkaline earth metal chlorides) on reduction of

NH&l LiCl NaCl KC1 C&l MgC& CaCl, BaCI, SrCl,

08

Fig. 3. Plots between -log

KY,, 8s E.

Table 4. Effect of sodium and potassium halides on reduction s. No.

04

cm

3.16 2.51 5.03 2.82 9.44 1.56 5.62 5.01 8.91

x x x x x x x x x

lo-” IO-” 10-10 10-g 10~10 lo-” 10-‘O lo-l0 10-q

0.33 0.53 0.49 0.41 0.50 0.80 0.76 0.76 0.68

1.58 1.12 6.31 2.98 2.81 6.31

x x x x x

10-a 10-I’ 10-s lo+’ lo-” 10-l”

0.47 0.54 0.39 0.40 0.50 0.53

ions

K1) (cm/s)

3.98 1.99 7.94 3.16 6.31 1.78

x x x x x

1o-9 10-I’ 10-g 10-l’ 10-l’ 10~”

B. K. GUPTA, D. S.

482

JAIN AND

J. N. GAUR

I I 0

02

I

I 06

04

I

I 10

cle

02

0

12

Fig. 4. Plots between -log

0.6

0.4

E, -Vvs

E, -Vvs nhe

12 nhe

Fig. 5. Plots between -log Kyh vs E.

K> USE.

id was found to be very low. The polarographic characteristics and kinetic parameters are tabulated in Table 6. The graph between -log K& us E(- V us nhe) is presentd in Fig. 3.

(v) Reduction in presence of sodium salt of organic acids of

The reduction of IO; ions is studied in presence sodium salts of different organic acids such as

Table 7. Effect of various sodium salts of organic acids on reduction of IO; ions S. NO.

1 2 3 4 5 6 7 8

Base electrolytes (0.1 M) Sodium formate Sodium acetate Sodium tartrate Sodium malonate Sodium salicylate Sodium citrate Sodium benzoate Potassium oxalate

id

El,,

Slope rJ”Z x 103

pH

@A) (-V 11sSW) (mV)

7.6

15.0 18.5 16.0 11.0 13.50 15.0 14.75 13.5

6.0 8.5 8.0 7.4 7.9 5.4 7.9

1.338 1.320 1.366 1.356 1.362 1.412 1.388 0.698

94 99 93 124 122 136 168 94

KY

(cm+)

(cm/s)

3.051 3.763 3.255 2.441 2.146 3.051 3.001 2.746

5.24 X 10-l’ 6.31 X lo-” 5.01 X lo-‘* 1.56 x lo-‘2 1.12 x lo-‘2 3.16 x lo-” 1.00 x 10-m 1.26 x 10-c

K,4

(cm/s) 2.81 3.1 1.99 7.08 5.01 1.78 1.00 8.91

x X x x x x x x

lo-r0 lo~ra

lo-” IO-”

10-10 10-r“ 10-s lo-*

an 0.48 0.51 0.50 0.48 0.43 0.37 0.48 0.38

Table 8. Reduction of IO; ions in presence of oxy-anions

s. No. 1 2 3 4

Supporting electrolyte

Sodium arsenite . Sodium arsenate Sodium tellurate Sodium tellurite 5 Sodium vanadate 6 Sodium tungstate 7 Sodium molybdate 8 Sodium selenate 9 Monosodium phosphate 10 Disodium phosphate 11 Sodium sulphite 12 Sodium thiosulphate 13 Sodium sulphate 14 Sodium nitrate 15 Sodium nitrite

pH

ir @A)

8.5 8.5 7.5 7.7 8.6 8.1 7.2 8.7 8.0 8.7 8.6 7.2 9.1 7.0 7.0

13.0 13.5 4.0 12.0 4.0 13.0 13.0 10.0 18.5 10.0 13.0 10.5 14.0 16.5 14.0

Slope -Et,2 (V USSW) (mV) 1.410 1.333 1.515 1.420 1.370 1.431 1.331 1.351 1.282 1.283 1.347 1.300 1.343 1.314 1.365

126 100 140 130 130 110 40 63 116 98 164 144 120

D’” X 103 (cn+) 2.644 2.146 0.813 2.441 0.848 2.644 2.644 2.034 3.763 2.034 2.644 2.136 2.848 3.357 2.848

KT (cm/s)

K,” (cm/s)

1.78 x IO-“’ 1.78 x IO-’ 7.94 X 1o-9 7.94 X 10-s ill defined stretched wave 2.34 X lo-‘0 2.34 x 10m9 not well defined wave 1.58 x 10-r’ 1.58 x lo-“’ 5.88 x lo-’ 8.91 x lo-’ 1.995 x 1Om’4 3.98 x lo-l2 1.78 x lo-* 2.81 x lo-’ 8.91 x 10-r’ 2.51 X lo-‘* 5.01 x lo-‘3 1.99 x lo-‘2 2.51 X lo-‘2 5.62 x lo-” 1.99 x lo-” 1.12 x lo-‘* 3.16 X 10-q 3.16 x 10-s 2.23 x lo-” 1.00 x 10‘”

* Due to limited solubility of sodium tellurate, the measurements were done in 0.01 M instead of 0.1 M

an 0.38 0.38 0.37 0.36 0.32 0.63 0.30 0.59 0.54 0.61 0.48 0.31 0.50

4x3

Polarography of some anions Table 9. Reduction of IO; ions in presence of various base electrolytes S. No.

1 2 3 4 5 6

Slope o”* x IO3 -E,,z (V USSW) (mv) (cm’is)

KT

K;,

(cm/s)

(cm/s)

an

2.848 2.848 3.357 3.662

1.05 x lo-‘” 5.01 X lo-” 3.98 x lo-l4 7.94 x 10-l”

2.98 x 10-l’ 1.41 X lo-‘* 7.58 x 10-l’ 1.78 x lo-”

0.50 0.5x 0.64 0.61

168

3.458

6.60 x 10-10

7.08 x 1O-9

0.36

156

3.255

1.04 x lo~‘O

1.26 x 10m9 0.35

Base electrolytes

pH

id (PA)

LiCIOL NaClO, NaOH KCNS Tetraethylammonium perchlorate Tetra methyl ammonium bromide

8.0 6.5 12.1 5.2

14.0 14.0 16.50 18.0

1.316 1.360 1.270 1.326

112 110 84 88

7.6

17.0

1.540

7.5

16.0

1.450

sodium formate; sodium acetate; sodium tartrate; sodium benzoate; potassium oxalate erc and was found to be diffusion controlled and irreversible. The kinetic parameters are calculated by Koutecky’s method and the results are summarised in Table 7. Figure 4 presents the graph between -log K& us E( - V USnhe). (vi) Reduction in presence of unrious oxy-anions The iodate ions reduces in different oxy-anions such as arscnite; arsenate; tellurate; tellurite; mono and diphosphate; sulphite; thiosulphate; sulphate; vanadate; nitrate; nitrite; tungstate; molybdate; selenate etc. Again the reduction was found to be diffusion controlled and irreversible in each case. The kinetic parameters are calculated by Koutecky’s method as usual and polarographic characteristics and kinetic parameters are summarised in Table 8. The graph between -log KR us E( -V us nhe) presented in Fig. 5. (vii) Reducrion in dresence of various base electrolytes The reduction of 1 mM IO3 ions in presence of various 0.1 M base electrolytes are studied. The reductions are found to be diffusion controlled and irreversible. The polarographic characteristics and kinetic parameters are tabulated in Table 9. The electrode reaction of IO; ions in alkaline or neutral media has been explained by Kolthoff et al[13] IO; + 3H,O t 3e- = I’ + 60Hor IO; t 6H+ + 6e- = I- + 3H20 In acid medium the reduction of IO; ion is similar to that for bromate ion. The possible mechanism is

as fo1lows: H+ t IO, = HI03 HIOj + e = HIO; HIO;

t 5H’ t 5e = I- + 3H,O

according to this 6 electrons are involved. It is obvious from present results that the polarographic characteristics (E 1,2; i,, and slope etc) for reduction of IO; ion changes a little with the variation of different base electrolytes, suggesting that the electrode reaction essentially remain the same. REFERENCES

A. Rylich, C&R Czrch. them. Communs.7, 288 (1935). 2. I. M. Kolthoff and J. J. Lingane, Chum.Rev. 24, l-94 (1939). 3. 1. M. Kolthoff and J. J. Lingane, J. Am. them. Sot. 61, 825 (1939). 4. I. M. KoltholI and E. E. Orlemann, J. Am. them. Sot. 64, 1044, 1970(1942). 5. R. H. Coe and L. B. Rogers, J. Am. &em. Sot. 70, 3276 (1948). 6. A. M. Shams El Din et nl, J. electroanal. Chei. 36(2), 4 11425 (1972). 7. V. I. Zykov, Zh. Fiz. Khim, 35, 35G2 (1961). 8. H. P. Laitinen et al, .I. Am. &em. Sot. 80, 2623-8 (1958). 9. S. I. Zhdanov and V. I. Zykov; lBudy inst. Fiz-Khim; 1.

Akad. Nauk SSSR 6, Nooy Metody, Fiz. Khim Issledo-

uanii 2, 29-38 (1957). 10. B.K. Gupta, D. S. Jain and J. N. Gaur, J. electrochem. Sot. India u(i), 35 (1974). Il. B. K. Gupta, D. S. Jain and J. N. Gaur, J. inorg nucl. Chem. 37, 133 (1975). 12. Hundbook oJ Electrochemical Constants (R. Persons), p. 73, Table No. 63. Butterworths Scientific Publications, London (1959). 13. I. M. Kolthoff and J. J. Lingane, Poloroyraphy, Vol. 2, (2nd ed) p. 573. Interscience, New York.