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Studies with inorganic ion-exchange membranes
Tolamz, Vol. 25, pp. 157-159.
Pergamon Rss, 1978.Rmtedin GreatBritain.
SHORT COMMUNICATIONS STUDIES S. K.
WITH INORGANIC
ION-EXCHANGE
MEMBRANES
SRIVASTAVA*, A. K. JAIN, SUSHMA AGRAWAL
and RAJ Chemistry Department, University, Roorkee, India
PAL
SINGH
(Received 18 May 1977. Accepted 18 October 1977) Summary-The pyridinium molybdoarsenate membrane shows a response to pyridinium ions and can be used to determine the concentration of these ions in the range 10d3-1M. The potentials generated across the membrane are reproducible and the response time is less than 1 min. There is no interference from certain inorganic and organic ions. The electrode can be used in the pH range 3-6 as well as in non-aqueous medium. Small additions of cetyltrimethylammonium bromide cause large shifts in the membrane potentials. A membrane, after being treated with this surfactant, shows a wider range of response to pyridinium ions. Precipitation titration of pyridinium nitrate has been monitored by using this membrane electrode.
The wide range of ion-selective electrodes may be extended by the use of membranes of new chemically and thermally stable inorganic ion-exchangers. We have earlier described the selectivity and performance of some inorganic ionexchange membranes. iV2This paper reports investigations with molybdoarsenate membranes, which are found to be highly selective for pyridinium ion over a fairly wide concentration range. EXPERIMENTAL
Reagents
Ammonium molybdate and arsenic pentoxide were reagent grade chemicals. All other reagents used were of analytical grade. Solutions were prepared in doubly distilled water. The pyridinium nitrate used in the preparation of the heteropoly salt was obtained as a white crystalline compound, by mixing nitric acid and distilled pyridine in equimolar quantities. It was crystallized from water and alcohol.
the electrodes were washed with distilled water to prevent cross-contamination. Procedure
The electrode assembly consisted of the membrane cemented between the two arms of a U-tube which formed the two compartments of a cell. The reference solution was O.lM pyridinium nitrate. Saturated calomel electrodes were used as reference electrodes. The emf readings were taken when they became steady. All measurements were made at a constant temperature of 30”. RESULTS
AND
DlSCUSSION
The potentials observed with the pyridinium molybdoarsenate membrane interposed between solutions of pyridinium nitrate are shown in Fig. l(a). The plots show the variation of electrode potential us. log concentration (or activity) of pyridinium ion. Activity coefficients in dilute solutions were calculated by the extended form of the Debye-Hiickel equation, while at appreciable concenPreparation of pyridinium molybdoarsen,ate trations the complete Hilckel equation3 was used. The Ammonium molybdate was dissolved in sodium hydroxvalues of the membrane potentials are smaller than the ide and the solution was heated to remove ammonia. It expected Nernstian value. The deviation from Nernstian was then cooled, and acidified with nitric acid, and arsenic behaviour may be due either to co-ion transfer or H* or pentoxide was added with stirring. Pyridinium nitrate was OH- competition with the electrolyte counter-ions.4 In then added and the precipitate formed was left overnight. spite of deviations from Nernstian behaviour, these memIt was then washed with distilled water and dried at 90”. branes are suitable for determining the pyridinium ion conThis compound was analysed (spectrophotometrically for centration from 1 to 10m3M [Fig. l(u)]. pyridine and molybdenum) and the values obtained for The precision was determined, the potential being repyridine, molybdenum and arsenic compare well with the peatedly measured in test solutions containing pyridinium theoretical values calculated on the basis of the formula ions in the range 10-3-10-1M. The results show a stan(C,HsNH),Mo,,AsO,,. Found; MO 54.4x, Py 11.4x, As dard deviation of 0.52 mV in the higher concentration 3.7%: calculated; MO 54.67x, Py 11.40~, As 3.56%. Therrange and 0.65 mV in the low concentration range. Thus mogravimetric analysis confirmed that the salt does not good reproducibility is shown over a working range of contain water. 3 orders of magnitude of pyridinium ion activity. The response time (static) is about 1 min in dilute soluPreparation of the membranes , tions and a little less in concentrated solutions. The potenThe membranes were prepared by mixing 0.65 g of the tials remain constant for 2-3 hr, after which a drift of heteropoly acid salt with 0.35 g of “Araldite”. The mixture 0.1 mV/min takes place. These membranes can be used for was spread thinly (about 0.5 mm) on filter paper and left 2 months, after which a decrease in potential of 4-8 mV/ to dry. The hardened membrane was cut into a circular decade is observed. This does not affect the response of disc 2.5 cm in diameter. It was then equilibrated with O.lM the membrane and the range of validity remains almost pyridinium nitrate for 34 days. Between measurements the same. The drift in potential is due to a shift in Ec. The pH-dependence of the electrode has been investigated in the range l-7; pH-values higher than this could * Present address: Indian Cooperation Mission, Kathnot be used owing to depolymerization of the heteropoly mandu, Nepal. 157
158
SHORT
COMMUNICATIONS
salt. The useful pH-range for the electrode IS 3-6. A drift in potential, at low pH-values, is observed at all concentrations of pyridinium ions from 1 to lo-‘M. The performance of the electrode has been assessed in the presence of various interfering ions in terms of the selectivity coefficient, K,,, defined by the following equations. For an electrode responding to the primary ion i of activity ai and charge z, in the presence of an interfering ion j of activity a, and charge y, the potential E is E = constant + (2.303 RT/F)log[a, + I(,,(ajy’p]
(I)
The potential in the presence of the primary ion i at activity ai is E’ = constant + (2.303 RT/zF) loga, From equations (1) and (2) K,, = (IO(=‘)zr/2 303 RT _
(2)
~J~;‘Y/~,
(3)
In the case of inorganic cations and anions as interfering ions, the selectivity coefficients, K, have been evaluated from cell potentials with 10e4M pyridinium ion and with 10m4M pyridinium + lo-*M interfering ion. The results are reported in Table 1. Selectivity coefficients of the organic ions picolinium, lutidinium and collidinium have been evaluated from the potentials of solutions having O.OlM concentration of the interfering cation along with varying concentrations of pyridinium ion, and the data are given in Table 2. The electrode preferentially responds to the prtmary ion if K,j < 1. The K, values of various univalent and bivalent inorganic ions (Table 1) show that these ions do not interfere. The organic cations picolinium, lutidinium and collidinium do not interfere (Table 2) unless the pyridinium ion concentration is too low. The response of the membrane electrode in a partially non-aqueous medium has been investigated. The potential us. activity plots (not shown here) are linear in the concentration range 10e3-1M in 10% alcohol or acetone solutions, and in 25% alcohol or acetone are linear down to 10m4M. The slopes of the curves are higher than those obtained in aqueous solution (Table 3). Thus the membrane shows a better response in 25% alcohol solution and this can be attributed to the low colloidal dispersion of molybdo and tungstoarsenates in aqueous media.’ This dispersion effect vanishes in non-aqueous media of low dielectric constant. The response time rises to 4 min and the drift in potential increases considerably in solutions containing 35% or more of alcohol. The electrode exhibits interesting behaviour in the presence of surfactant cations. A small addition of the cationic surfactant cetyltrimethylammonium bromide (CTAB) causes a shift in the membrane potential (in the presence
Table 2. Selectivity coefficient (K,,) values for organic cations (O.OlM) at varying pyridinium ion concentrations Concentration of pyridinium ion, M
0.20 0.02 0.001
K, a-Picolinium
Lutidinium
Collidinium
-0 -0 0.185
-0 -0 0.161
-0 -0 0.183
Table 3. Effect of solvent composition on calibration curves Solvent composition, via%
Slope dE/dpPy+ 40.0 33.0 34.0 40.0 45.0 40.0 42.0
Water Methanol 10% Methanol 25% Ethanol 10% Ethanol 25% Acetone 10% Acetone 25%
of low concentrations of pyridinium ion) to a more negative value and extends the range of linear response. The effect can be made permanent by treating the membrane with the surfactant solution (10e4M) for 1 hr. There is a slight change in the potential from that observed with the untreated electrode, and also a slight change in the slope of the response curve (Fig. lb). A similar effect has been reported6 for the calcium-selective membrane electrode, which is affected by the presence of an anionic surfactant, but becomes immune to the effect after conditioning for about an hour. The effect is attributed to initial adsorption of surfactant at the interface and subsequent extraction into the membrane phase. In the present case the ionexchanger in the membrane is negatively charged and thus (a) 50l-
_ (a)
Activity
30
Concentration
(b)
P
Table 1. Selectivity coefficients (Kij) values for various interfering ions (lo-‘M) in the presence of pyridinium ion (lO-+M) Interfering ion Na+ K+ Rb+ NH: Ag+ Tl+ Sra + Ca’+ PO:AsO:MOO:-
0 /
Ki, 7.50 4.90 2.64 4.56 10.0 3.48 1.65 2.19 1.20 2.41 3.52
x x x x X x x x x x x
10-a 10-a 1O-3 1O-3 10-a 10-a 1O-3 1O-3 10-s 10-a 1O-3
(0 )
w
-4.0
tb)
. log
cone,
mole
1-l
Fig. 1. (a) Plots of electrode potential VS.log of concentration of pyridinium ions. (b) Plot of electrode potential us. log of pyridinium ion concentration, in the presence of 2 x 10-4M CTAB.
SHORT
159
COMMUNICATIONS
60
56 54 52 50 46 46
0
36
L:
36
5 1.0 2.0
3.0
4.0
volume
5.0
6.0
of ocld
7.0
6.0
added.
> -
9.0
there is a possibility of exchange of surfactant cations with the pyridinium ions at the membrane. However, it is thought that in this case only the adsorption at the interface takes place and that there is no uptake of surfactant cations by the membrane phase. This view is supported by the fact that the behaviour of the treated membrane towards surfactant cations at various concentrations is erratic. Also, in an earlier paper5 on the ion-exchange properties of some heteropoly salts we observed that the crystal lattice of such compounds is not sufficiently open to permit the exchange of very large cations such as the cetyltrimethylammonium ion. Membranes treated with a more concentrated solution of the surfactant ion (> lo- 3bf) gave noisy signals and non-reproducible potentials, probably because of formation of micelles and their binding at the interface. This electrode has also been used as an end-point indicator in potentiometric titrations in aqueous and alcoholic media. The titration of aqueous O.OlM pyridinium nitrate with 1Zmolybdophosphoric acid (0.037N) and the same titration in 25% ethanol (pyridinium nitrate 0.005M and molybdophosphoric acid 0.027N) is illustrated in Fig. 2. There should be three breaks corresponding to the three stages of precipitation: ---+PyH,PMo,,Odo
Py+ + PyH2PMo,204,,--+Py2HPMo1~0~0 Py’ + Py2HPMo12040 --+ Py,PMo,zO,,,
I
34 (0)
32
ml
Fig. 2. (a) Titration of 25 ml of O.OlM pyridinium nitrate acid in aqueous solution, with molybdophosphoric (0.037N). (b) Titration of 25 ml of 0.005M pyridinium nitrate in 25% ethanol solution, with molybdophosphoric acid (0.027N).
Py+ + H3PMo,Z0,,
0
‘\ i
+ H+ + H+ + H+
The breaks in the titration curve (in aqueous media) are not very sharp. A Gran plot’ of the data was constructed for both titrations, lO’(V, + V)lOEF/*3RT us. V ml of the acid added (V, = initial solution volume) being plotted. Breaks in the Gran plot (Fig. 3) are quite sharp and easily detected, but the first two breaks do not correspond to
26 24 (tl)
2000
.
.
.
Volume
.
of ood added,
ml
Fig. 3. (a) Gran plot of the titration data for aqueous medium. (b) Gran plot of the titration data for ethanolic medium. stoichiometric ratios and it is only for the third step, that is, the complete precipitation of the salt as pyridinium molybdophosphate, that the correct stoichiometric ratio is realized. This, however, does not affect the utility of this electrode as an indicator electrode in precipitation titrations involving pyridinium ions. Acknowledgement-The financial aid received from the U.G.C. India for carrying out this work is gratefully acknowledged.
REFERENCES
1. W. U. Malik, S. K. Srivastava, V. M. Bhandari and S. Kumar, J. Colloid Interface Sci., 1974, 1, 47. 2. W. U. Malik, 9. K. Srivastava, P. Razdan and S. Kumar, J. Electroanal. Chem., 1976, 72, 111. 3. S. Glasstone, Electrochemistry of Solutions, p. 145. Van Nostrand, New York, 1965. 4. G. J. Moody and J. D. R. Thomas, Talanta, 1972, 19, 623. 5. W. U. Malik, S. K. Srivastava and S. Kumar, ibid., 1976, 23, 323. 6. R. A. Llenado, Anal. Chem., 1975, 47, 2243. 7. G. Gran, Analyst, 1952, 77, 661.
Pergamon Rss, 1978.Rmtedin GreatBritain.
SHORT COMMUNICATIONS STUDIES S. K.
WITH INORGANIC
ION-EXCHANGE
MEMBRANES
SRIVASTAVA*, A. K. JAIN, SUSHMA AGRAWAL
and RAJ Chemistry Department, University, Roorkee, India
PAL
SINGH
(Received 18 May 1977. Accepted 18 October 1977) Summary-The pyridinium molybdoarsenate membrane shows a response to pyridinium ions and can be used to determine the concentration of these ions in the range 10d3-1M. The potentials generated across the membrane are reproducible and the response time is less than 1 min. There is no interference from certain inorganic and organic ions. The electrode can be used in the pH range 3-6 as well as in non-aqueous medium. Small additions of cetyltrimethylammonium bromide cause large shifts in the membrane potentials. A membrane, after being treated with this surfactant, shows a wider range of response to pyridinium ions. Precipitation titration of pyridinium nitrate has been monitored by using this membrane electrode.
The wide range of ion-selective electrodes may be extended by the use of membranes of new chemically and thermally stable inorganic ion-exchangers. We have earlier described the selectivity and performance of some inorganic ionexchange membranes. iV2This paper reports investigations with molybdoarsenate membranes, which are found to be highly selective for pyridinium ion over a fairly wide concentration range. EXPERIMENTAL
Reagents
Ammonium molybdate and arsenic pentoxide were reagent grade chemicals. All other reagents used were of analytical grade. Solutions were prepared in doubly distilled water. The pyridinium nitrate used in the preparation of the heteropoly salt was obtained as a white crystalline compound, by mixing nitric acid and distilled pyridine in equimolar quantities. It was crystallized from water and alcohol.
the electrodes were washed with distilled water to prevent cross-contamination. Procedure
The electrode assembly consisted of the membrane cemented between the two arms of a U-tube which formed the two compartments of a cell. The reference solution was O.lM pyridinium nitrate. Saturated calomel electrodes were used as reference electrodes. The emf readings were taken when they became steady. All measurements were made at a constant temperature of 30”. RESULTS
AND
DlSCUSSION
The potentials observed with the pyridinium molybdoarsenate membrane interposed between solutions of pyridinium nitrate are shown in Fig. l(a). The plots show the variation of electrode potential us. log concentration (or activity) of pyridinium ion. Activity coefficients in dilute solutions were calculated by the extended form of the Debye-Hiickel equation, while at appreciable concenPreparation of pyridinium molybdoarsen,ate trations the complete Hilckel equation3 was used. The Ammonium molybdate was dissolved in sodium hydroxvalues of the membrane potentials are smaller than the ide and the solution was heated to remove ammonia. It expected Nernstian value. The deviation from Nernstian was then cooled, and acidified with nitric acid, and arsenic behaviour may be due either to co-ion transfer or H* or pentoxide was added with stirring. Pyridinium nitrate was OH- competition with the electrolyte counter-ions.4 In then added and the precipitate formed was left overnight. spite of deviations from Nernstian behaviour, these memIt was then washed with distilled water and dried at 90”. branes are suitable for determining the pyridinium ion conThis compound was analysed (spectrophotometrically for centration from 1 to 10m3M [Fig. l(u)]. pyridine and molybdenum) and the values obtained for The precision was determined, the potential being repyridine, molybdenum and arsenic compare well with the peatedly measured in test solutions containing pyridinium theoretical values calculated on the basis of the formula ions in the range 10-3-10-1M. The results show a stan(C,HsNH),Mo,,AsO,,. Found; MO 54.4x, Py 11.4x, As dard deviation of 0.52 mV in the higher concentration 3.7%: calculated; MO 54.67x, Py 11.40~, As 3.56%. Therrange and 0.65 mV in the low concentration range. Thus mogravimetric analysis confirmed that the salt does not good reproducibility is shown over a working range of contain water. 3 orders of magnitude of pyridinium ion activity. The response time (static) is about 1 min in dilute soluPreparation of the membranes , tions and a little less in concentrated solutions. The potenThe membranes were prepared by mixing 0.65 g of the tials remain constant for 2-3 hr, after which a drift of heteropoly acid salt with 0.35 g of “Araldite”. The mixture 0.1 mV/min takes place. These membranes can be used for was spread thinly (about 0.5 mm) on filter paper and left 2 months, after which a decrease in potential of 4-8 mV/ to dry. The hardened membrane was cut into a circular decade is observed. This does not affect the response of disc 2.5 cm in diameter. It was then equilibrated with O.lM the membrane and the range of validity remains almost pyridinium nitrate for 34 days. Between measurements the same. The drift in potential is due to a shift in Ec. The pH-dependence of the electrode has been investigated in the range l-7; pH-values higher than this could * Present address: Indian Cooperation Mission, Kathnot be used owing to depolymerization of the heteropoly mandu, Nepal. 157
158
SHORT
COMMUNICATIONS
salt. The useful pH-range for the electrode IS 3-6. A drift in potential, at low pH-values, is observed at all concentrations of pyridinium ions from 1 to lo-‘M. The performance of the electrode has been assessed in the presence of various interfering ions in terms of the selectivity coefficient, K,,, defined by the following equations. For an electrode responding to the primary ion i of activity ai and charge z, in the presence of an interfering ion j of activity a, and charge y, the potential E is E = constant + (2.303 RT/F)log[a, + I(,,(ajy’p]
(I)
The potential in the presence of the primary ion i at activity ai is E’ = constant + (2.303 RT/zF) loga, From equations (1) and (2) K,, = (IO(=‘)zr/2 303 RT _
(2)
~J~;‘Y/~,
(3)
In the case of inorganic cations and anions as interfering ions, the selectivity coefficients, K, have been evaluated from cell potentials with 10e4M pyridinium ion and with 10m4M pyridinium + lo-*M interfering ion. The results are reported in Table 1. Selectivity coefficients of the organic ions picolinium, lutidinium and collidinium have been evaluated from the potentials of solutions having O.OlM concentration of the interfering cation along with varying concentrations of pyridinium ion, and the data are given in Table 2. The electrode preferentially responds to the prtmary ion if K,j < 1. The K, values of various univalent and bivalent inorganic ions (Table 1) show that these ions do not interfere. The organic cations picolinium, lutidinium and collidinium do not interfere (Table 2) unless the pyridinium ion concentration is too low. The response of the membrane electrode in a partially non-aqueous medium has been investigated. The potential us. activity plots (not shown here) are linear in the concentration range 10e3-1M in 10% alcohol or acetone solutions, and in 25% alcohol or acetone are linear down to 10m4M. The slopes of the curves are higher than those obtained in aqueous solution (Table 3). Thus the membrane shows a better response in 25% alcohol solution and this can be attributed to the low colloidal dispersion of molybdo and tungstoarsenates in aqueous media.’ This dispersion effect vanishes in non-aqueous media of low dielectric constant. The response time rises to 4 min and the drift in potential increases considerably in solutions containing 35% or more of alcohol. The electrode exhibits interesting behaviour in the presence of surfactant cations. A small addition of the cationic surfactant cetyltrimethylammonium bromide (CTAB) causes a shift in the membrane potential (in the presence
Table 2. Selectivity coefficient (K,,) values for organic cations (O.OlM) at varying pyridinium ion concentrations Concentration of pyridinium ion, M
0.20 0.02 0.001
K, a-Picolinium
Lutidinium
Collidinium
-0 -0 0.185
-0 -0 0.161
-0 -0 0.183
Table 3. Effect of solvent composition on calibration curves Solvent composition, via%
Slope dE/dpPy+ 40.0 33.0 34.0 40.0 45.0 40.0 42.0
Water Methanol 10% Methanol 25% Ethanol 10% Ethanol 25% Acetone 10% Acetone 25%
of low concentrations of pyridinium ion) to a more negative value and extends the range of linear response. The effect can be made permanent by treating the membrane with the surfactant solution (10e4M) for 1 hr. There is a slight change in the potential from that observed with the untreated electrode, and also a slight change in the slope of the response curve (Fig. lb). A similar effect has been reported6 for the calcium-selective membrane electrode, which is affected by the presence of an anionic surfactant, but becomes immune to the effect after conditioning for about an hour. The effect is attributed to initial adsorption of surfactant at the interface and subsequent extraction into the membrane phase. In the present case the ionexchanger in the membrane is negatively charged and thus (a) 50l-
_ (a)
Activity
30
Concentration
(b)
P
Table 1. Selectivity coefficients (Kij) values for various interfering ions (lo-‘M) in the presence of pyridinium ion (lO-+M) Interfering ion Na+ K+ Rb+ NH: Ag+ Tl+ Sra + Ca’+ PO:AsO:MOO:-
0 /
Ki, 7.50 4.90 2.64 4.56 10.0 3.48 1.65 2.19 1.20 2.41 3.52
x x x x X x x x x x x
10-a 10-a 1O-3 1O-3 10-a 10-a 1O-3 1O-3 10-s 10-a 1O-3
(0 )
w
-4.0
tb)
. log
cone,
mole
1-l
Fig. 1. (a) Plots of electrode potential VS.log of concentration of pyridinium ions. (b) Plot of electrode potential us. log of pyridinium ion concentration, in the presence of 2 x 10-4M CTAB.
SHORT
159
COMMUNICATIONS
60
56 54 52 50 46 46
0
36
L:
36
5 1.0 2.0
3.0
4.0
volume
5.0
6.0
of ocld
7.0
6.0
added.
> -
9.0
there is a possibility of exchange of surfactant cations with the pyridinium ions at the membrane. However, it is thought that in this case only the adsorption at the interface takes place and that there is no uptake of surfactant cations by the membrane phase. This view is supported by the fact that the behaviour of the treated membrane towards surfactant cations at various concentrations is erratic. Also, in an earlier paper5 on the ion-exchange properties of some heteropoly salts we observed that the crystal lattice of such compounds is not sufficiently open to permit the exchange of very large cations such as the cetyltrimethylammonium ion. Membranes treated with a more concentrated solution of the surfactant ion (> lo- 3bf) gave noisy signals and non-reproducible potentials, probably because of formation of micelles and their binding at the interface. This electrode has also been used as an end-point indicator in potentiometric titrations in aqueous and alcoholic media. The titration of aqueous O.OlM pyridinium nitrate with 1Zmolybdophosphoric acid (0.037N) and the same titration in 25% ethanol (pyridinium nitrate 0.005M and molybdophosphoric acid 0.027N) is illustrated in Fig. 2. There should be three breaks corresponding to the three stages of precipitation: ---+PyH,PMo,,Odo
Py+ + PyH2PMo,204,,--+Py2HPMo1~0~0 Py’ + Py2HPMo12040 --+ Py,PMo,zO,,,
I
34 (0)
32
ml
Fig. 2. (a) Titration of 25 ml of O.OlM pyridinium nitrate acid in aqueous solution, with molybdophosphoric (0.037N). (b) Titration of 25 ml of 0.005M pyridinium nitrate in 25% ethanol solution, with molybdophosphoric acid (0.027N).
Py+ + H3PMo,Z0,,
0
‘\ i
+ H+ + H+ + H+
The breaks in the titration curve (in aqueous media) are not very sharp. A Gran plot’ of the data was constructed for both titrations, lO’(V, + V)lOEF/*3RT us. V ml of the acid added (V, = initial solution volume) being plotted. Breaks in the Gran plot (Fig. 3) are quite sharp and easily detected, but the first two breaks do not correspond to
26 24 (tl)
2000
.
.
.
Volume
.
of ood added,
ml
Fig. 3. (a) Gran plot of the titration data for aqueous medium. (b) Gran plot of the titration data for ethanolic medium. stoichiometric ratios and it is only for the third step, that is, the complete precipitation of the salt as pyridinium molybdophosphate, that the correct stoichiometric ratio is realized. This, however, does not affect the utility of this electrode as an indicator electrode in precipitation titrations involving pyridinium ions. Acknowledgement-The financial aid received from the U.G.C. India for carrying out this work is gratefully acknowledged.
REFERENCES
1. W. U. Malik, S. K. Srivastava, V. M. Bhandari and S. Kumar, J. Colloid Interface Sci., 1974, 1, 47. 2. W. U. Malik, 9. K. Srivastava, P. Razdan and S. Kumar, J. Electroanal. Chem., 1976, 72, 111. 3. S. Glasstone, Electrochemistry of Solutions, p. 145. Van Nostrand, New York, 1965. 4. G. J. Moody and J. D. R. Thomas, Talanta, 1972, 19, 623. 5. W. U. Malik, S. K. Srivastava and S. Kumar, ibid., 1976, 23, 323. 6. R. A. Llenado, Anal. Chem., 1975, 47, 2243. 7. G. Gran, Analyst, 1952, 77, 661.