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A polystyrene-based membrane electrode sensitive to molybdate ions
0039-9140/83/040285-03$03.00/O Pergamon Press Ltd
Talanta, Vol. 30, No. 4, pp 285-287, 1983 Printed in Great Britain
A POLYSTYRENE-BASED MEMBRANE ELECTRODE SENSITIVE TO MOLYBDATE IONS S. K. SRIVASTAVA,A. K. SHARMA and C. K. JAIN Chemistry Department, Roorkee University, Roorkee (U.P.), India (Received
17 December
1981. Revised
24 August
1982. Accepted
27 October
1982)
Summary-A polystyrene-based zirconium oxide membrane has been used to determine the concentration of molybdate ions in the range 0.5-IO-‘M and pH range 7-l 1. The response time is about 20 set and the electrode remains usable for at least 6 months. It can also be used as an indicator electrode for titrations involving molybdate ions. Univalent anions interfere more strongly than bivalent and multivalent anions.
The electrode assembly used for potential measurements was the same as that reported earlier,l and the reference solution was O.OlM sodium molybdate. All emf measurements were made at 25 f 0.1” with a Radiometer (model PHM64) millivoltmeter coupled to a Servoscribe recorder. Ceramic-junction calomel reference electrodes were used.
Inorganic gels, besides being used for separatory purposes, have been tried as membrane electrodes.‘,’ Hydrous zirconium oxide exhibits anion-exchange properties.3,4 Investigations of the salt-rejection properties of dynamically formed hydrous zirconium oxide
membranes5,6 and of those formed on a microporous support’ have been reported. We have examined the utility of this compound as a sensor material for a membrane electrode and found that a polystyrenebased membrane containing this compound shows promising selectivity for molybdate ions. The characteristics of this are reported in this paper.
EXPERIMENTAL Reagents
All reagents used were analytical-grade. The hydrous zirconium oxide (HZO) gel was prepared and characterized by the method reported by Shor et aL6 Preparation
of the membrane
Unsupported membranes of HZ0 could not be prepared. Membranes supported with a binder (polystyrene) were found to be quite stable and were therefore employed in these investigations. The following method* was used. Polystyrene granules were heated in a glass tube in an oil-bath. The molten mass was allowed to cool and then ground to yield a product of 50-mesh particle size. The minimum amount of polystyrene needed for a fairly stable membrane was determined by trial and error. Membranes were obtained by mixing polystyrene and HZ0 in 1:9 w/w ratio and heating the homogeneous mass in a die of diameter 2.5 cm in a metallurgical-specimen mounting press at 120” under a pressure of 650&7000 psi. By this method, membranes that were both mechanically and chemically fairly stable were obtained and were checked under a high-resolution microscope for any deformities or cracks. Equilibration
of membranes
and potential
RESULTS AND
DISCUSSION
The potentials recorded with this membrane in contact with molybdate ions (concentration range 0.5-10-‘M) are shown in Fig. l(a). The response is linear and practically Nernstian (slope -30 mV/pMo) over the concentration range 0.5-10-3M. The electrode response is fast, stable potentials being obtained within a few seconds, and no deviations are observed within an hour, after which a slow drift in potential occurs. The response time (static) was found to be 20 set in dilute solutions ( 10m3M) and a little more (30 set) in more concentrated solution (O.lM). Repetition of the potential measurements gave a standard deviation of 0.5 mV in the higher concentration range and 0.6 mV in the low concentration range. It can be used for 6 months in a working range of two orders of magnitude of molybdate ion activity without showing any drift. If some 50 40 30
1
measurements
Membranes thus obtained were equilibrated in O.OlM sodium molybdate for 2-3 days. The solution was changed four or five times at intervals during this period. Excess of the sodium molybdate solution was washed from the surface of the membrane. The equilibration time was determined in a preliminary investigation and it was observed that membranes equilibrated for shorter times did not give stable potentials. 285
6
5
4
3
2
I
6
5
4
3
2
-log
6 cmcn
5 4 (sodium
oa I I
3 2 molybdate)
, Ob I
oc
Fig. 1. Potential us. log concentration of medium: (a) MOO:-; (b) Cl- + MOO:-; and (c) NO, + MOO:-.
SHORT
286
CO~NICA~ONS It is observed that the bivalent and multivalent anions have very low kff; values and would not
PH
Fig, 2. Variation of membrane potential with pH, for two concentrations of molybdate.
contamination occurs, it can be removed by equilibrating the membrane with 0.01M sodium molybdate for 2-3days. If this treatment fails, it is advisable
to discard the electrode and prepare a new one. The pH range in which this electrode can be used has also been determined (Fig. 2) at two molybdate concentrations and found to be 7-l 1. The sharp decrease in response slope on acidification, may be attributed to the formation of para-, tri- and tetramolybdate ions.9 The electrode also appears to respond to protons at low pH. The electrode assembly can also be used to measure molybdate ion concentration in partially nonaqueous media, up to a maximum of 25% v/v nonaqueous content. The response in 25% v/v methanol, ethanol and acetone is linear but not quite Nemstian (slopes of - 32, - 3 1 and - 33 mV/pMo respectively). This, however, does not effect the functioning of the electrode system. The response time remains almost the same. If the non-aqueous content exceeds 30% there is an increase in response time and a drift in potential. The performance of the electrode has been assessed in the presence of other ions and reported in terms of the selec-tivity coefficients (kc;), determined by the fixed interference method.‘* The values obtained at two concentrations of the interfering ions are given in Table 1. Table 1. Selectivity coefficient (krh) values for the hydrous zirconium o&e membrane electrode at 10e4 and lo-?A4 concentrations of interfering ions, as determined by the fixed interference method
interfere even if present at the same concentration as the molybdate. The selectivity coefficients for univalent anions are about 12-14. Normally a zero value of krpt, is ideal but the values obtained for univalent anions are negligibly small in comparison with the values corresponding to equal electrode response to primary and interfering ion, calculated from the expression:” aA = kk6 (a&“, where .zis the charge on the primary ion A (activity aA) and y is the charge on the interfering ion B (activity as). Thus the molybdate electrode would exhibit equal response to molybdate and a univalent interfering ion at lo-’ for kp; = lo’, and 10e4M for kfff, = 104. When mixed-solution rubs were made with c~lo~de-molybdate and nitrate-molybdate mixtures at a fixed interfering ion concentration of 10P3M and
primary (molybdate) ion concentration varied from lo-’ to 0.5M, the potentials [Fig. l(b), (c)] were the same as those in the calibration plot for molybdate [Fig. l(a)] except at concentrations < 10m3M (where this electrode system is not usable anyway). At molybdate concentrations < 10m3&f,the ratio of interfering ion to molybdate was > 1, and there was a slight increase in slope (relative to that for molybdate alone). The electrode has also been used as an end-point indicator in titrations involving molybdate ions. The titration of sodium molybdate with thorium nitrate is shown in Fig. 3. The break in the curve is quite sharp and reproducible, and corresponds to precipitation of Th(MoO,),. The standard deviation (ten replicates) of these particular titrations was 0.07 ml. The determination of molybdate ions with this membrane electrode is faster than the usual spectrophotometric procedures,” and more convenient.
0
-5 F
z
--Is
I; Q
-20
kpt A.%
Ion ClSCN ClO,NO; 2%; SOi’ AsO:PO:Fe(CN)i-
10-4A4 12 14 14 12 0.11 0.21 0.12 0.029 0.016 0.021
b
lo-2M 15 18 18 13 0.14 0.23 0.14 0.034 0.020 0.023
0
I 2 3 4 5 6 7 6 9 Volume of thorium nitrate added, ml
IO
Fig. 3. Curves for titration of (a) 20 ml of 0.005M sodium molybdate with O.OlM thorium nitrate at pH 8; (b) 20 ml of 0.002M sodium molybdate with 0.005M thorium nitrate at pH 7.
SHORT
287
COMMUNICATIONS
REFERENCES
1. W. U. Malik, S. K. Srivastava and S. Kumar, J. Electroanal. Chem., 1976, 72, 111. 2. S. K. Srivastava, R. P. Singh and S. Agarwal, J. Chem. Tech. Biotech., 1977, 27, 680. 3. K. A. Kraus and H. 0. Phillips, J. Am. Chem. Sot.,
1956, 78, 249. 4. N. J. Singh and S. N. Tandon, Talanta, 1977, 24, 459. 5. A. E. Marcinkowskv. K. A. Kraus. H. 0. Phillios. J. S. Johnson, Jr., and A:‘I. Shor, J. A&. Chem. So;., ‘1966, 88, 5744. 6. A. J. Shor, K. A. Kraus, W. T. Smith and J. S. Johnson, Jr., J. Phys. Chem., 1968, 72, 2200.
7. D. Freilich and G. B. Tonny, J. Colloid Interface Sci., 1978, 64, 362. 8. M. R. J. Wyllie and H. W. Patnode, J. Phys. Chem., 1950, 54, 204. 9. F. G. R. Gimblett, Inorganic Polymer Chemistry, p. 195.
Butterworths, London, 1963. 10. G. G. Guilbault, Ion-Selective Electrode Review, 1979, 1, 139. 11. G. E. Baiulescu and V. V. Cogofret, Applications of Ion-Selective
Membrane Electrodes
in Organic Analysis,
p. 17. Hot-wood, Chichester, 1977. 12. Z. Marczenko, Spectrophotometric Determination Elements. Hot-wood, Chichester, 1976.
of
Talanta, Vol. 30, No. 4, pp 285-287, 1983 Printed in Great Britain
A POLYSTYRENE-BASED MEMBRANE ELECTRODE SENSITIVE TO MOLYBDATE IONS S. K. SRIVASTAVA,A. K. SHARMA and C. K. JAIN Chemistry Department, Roorkee University, Roorkee (U.P.), India (Received
17 December
1981. Revised
24 August
1982. Accepted
27 October
1982)
Summary-A polystyrene-based zirconium oxide membrane has been used to determine the concentration of molybdate ions in the range 0.5-IO-‘M and pH range 7-l 1. The response time is about 20 set and the electrode remains usable for at least 6 months. It can also be used as an indicator electrode for titrations involving molybdate ions. Univalent anions interfere more strongly than bivalent and multivalent anions.
The electrode assembly used for potential measurements was the same as that reported earlier,l and the reference solution was O.OlM sodium molybdate. All emf measurements were made at 25 f 0.1” with a Radiometer (model PHM64) millivoltmeter coupled to a Servoscribe recorder. Ceramic-junction calomel reference electrodes were used.
Inorganic gels, besides being used for separatory purposes, have been tried as membrane electrodes.‘,’ Hydrous zirconium oxide exhibits anion-exchange properties.3,4 Investigations of the salt-rejection properties of dynamically formed hydrous zirconium oxide
membranes5,6 and of those formed on a microporous support’ have been reported. We have examined the utility of this compound as a sensor material for a membrane electrode and found that a polystyrenebased membrane containing this compound shows promising selectivity for molybdate ions. The characteristics of this are reported in this paper.
EXPERIMENTAL Reagents
All reagents used were analytical-grade. The hydrous zirconium oxide (HZO) gel was prepared and characterized by the method reported by Shor et aL6 Preparation
of the membrane
Unsupported membranes of HZ0 could not be prepared. Membranes supported with a binder (polystyrene) were found to be quite stable and were therefore employed in these investigations. The following method* was used. Polystyrene granules were heated in a glass tube in an oil-bath. The molten mass was allowed to cool and then ground to yield a product of 50-mesh particle size. The minimum amount of polystyrene needed for a fairly stable membrane was determined by trial and error. Membranes were obtained by mixing polystyrene and HZ0 in 1:9 w/w ratio and heating the homogeneous mass in a die of diameter 2.5 cm in a metallurgical-specimen mounting press at 120” under a pressure of 650&7000 psi. By this method, membranes that were both mechanically and chemically fairly stable were obtained and were checked under a high-resolution microscope for any deformities or cracks. Equilibration
of membranes
and potential
RESULTS AND
DISCUSSION
The potentials recorded with this membrane in contact with molybdate ions (concentration range 0.5-10-‘M) are shown in Fig. l(a). The response is linear and practically Nernstian (slope -30 mV/pMo) over the concentration range 0.5-10-3M. The electrode response is fast, stable potentials being obtained within a few seconds, and no deviations are observed within an hour, after which a slow drift in potential occurs. The response time (static) was found to be 20 set in dilute solutions ( 10m3M) and a little more (30 set) in more concentrated solution (O.lM). Repetition of the potential measurements gave a standard deviation of 0.5 mV in the higher concentration range and 0.6 mV in the low concentration range. It can be used for 6 months in a working range of two orders of magnitude of molybdate ion activity without showing any drift. If some 50 40 30
1
measurements
Membranes thus obtained were equilibrated in O.OlM sodium molybdate for 2-3 days. The solution was changed four or five times at intervals during this period. Excess of the sodium molybdate solution was washed from the surface of the membrane. The equilibration time was determined in a preliminary investigation and it was observed that membranes equilibrated for shorter times did not give stable potentials. 285
6
5
4
3
2
I
6
5
4
3
2
-log
6 cmcn
5 4 (sodium
oa I I
3 2 molybdate)
, Ob I
oc
Fig. 1. Potential us. log concentration of medium: (a) MOO:-; (b) Cl- + MOO:-; and (c) NO, + MOO:-.
SHORT
286
CO~NICA~ONS It is observed that the bivalent and multivalent anions have very low kff; values and would not
PH
Fig, 2. Variation of membrane potential with pH, for two concentrations of molybdate.
contamination occurs, it can be removed by equilibrating the membrane with 0.01M sodium molybdate for 2-3days. If this treatment fails, it is advisable
to discard the electrode and prepare a new one. The pH range in which this electrode can be used has also been determined (Fig. 2) at two molybdate concentrations and found to be 7-l 1. The sharp decrease in response slope on acidification, may be attributed to the formation of para-, tri- and tetramolybdate ions.9 The electrode also appears to respond to protons at low pH. The electrode assembly can also be used to measure molybdate ion concentration in partially nonaqueous media, up to a maximum of 25% v/v nonaqueous content. The response in 25% v/v methanol, ethanol and acetone is linear but not quite Nemstian (slopes of - 32, - 3 1 and - 33 mV/pMo respectively). This, however, does not effect the functioning of the electrode system. The response time remains almost the same. If the non-aqueous content exceeds 30% there is an increase in response time and a drift in potential. The performance of the electrode has been assessed in the presence of other ions and reported in terms of the selec-tivity coefficients (kc;), determined by the fixed interference method.‘* The values obtained at two concentrations of the interfering ions are given in Table 1. Table 1. Selectivity coefficient (krh) values for the hydrous zirconium o&e membrane electrode at 10e4 and lo-?A4 concentrations of interfering ions, as determined by the fixed interference method
interfere even if present at the same concentration as the molybdate. The selectivity coefficients for univalent anions are about 12-14. Normally a zero value of krpt, is ideal but the values obtained for univalent anions are negligibly small in comparison with the values corresponding to equal electrode response to primary and interfering ion, calculated from the expression:” aA = kk6 (a&“, where .zis the charge on the primary ion A (activity aA) and y is the charge on the interfering ion B (activity as). Thus the molybdate electrode would exhibit equal response to molybdate and a univalent interfering ion at lo-’ for kp; = lo’, and 10e4M for kfff, = 104. When mixed-solution rubs were made with c~lo~de-molybdate and nitrate-molybdate mixtures at a fixed interfering ion concentration of 10P3M and
primary (molybdate) ion concentration varied from lo-’ to 0.5M, the potentials [Fig. l(b), (c)] were the same as those in the calibration plot for molybdate [Fig. l(a)] except at concentrations < 10m3M (where this electrode system is not usable anyway). At molybdate concentrations < 10m3&f,the ratio of interfering ion to molybdate was > 1, and there was a slight increase in slope (relative to that for molybdate alone). The electrode has also been used as an end-point indicator in titrations involving molybdate ions. The titration of sodium molybdate with thorium nitrate is shown in Fig. 3. The break in the curve is quite sharp and reproducible, and corresponds to precipitation of Th(MoO,),. The standard deviation (ten replicates) of these particular titrations was 0.07 ml. The determination of molybdate ions with this membrane electrode is faster than the usual spectrophotometric procedures,” and more convenient.
0
-5 F
z
--Is
I; Q
-20
kpt A.%
Ion ClSCN ClO,NO; 2%; SOi’ AsO:PO:Fe(CN)i-
10-4A4 12 14 14 12 0.11 0.21 0.12 0.029 0.016 0.021
b
lo-2M 15 18 18 13 0.14 0.23 0.14 0.034 0.020 0.023
0
I 2 3 4 5 6 7 6 9 Volume of thorium nitrate added, ml
IO
Fig. 3. Curves for titration of (a) 20 ml of 0.005M sodium molybdate with O.OlM thorium nitrate at pH 8; (b) 20 ml of 0.002M sodium molybdate with 0.005M thorium nitrate at pH 7.
SHORT
287
COMMUNICATIONS
REFERENCES
1. W. U. Malik, S. K. Srivastava and S. Kumar, J. Electroanal. Chem., 1976, 72, 111. 2. S. K. Srivastava, R. P. Singh and S. Agarwal, J. Chem. Tech. Biotech., 1977, 27, 680. 3. K. A. Kraus and H. 0. Phillips, J. Am. Chem. Sot.,
1956, 78, 249. 4. N. J. Singh and S. N. Tandon, Talanta, 1977, 24, 459. 5. A. E. Marcinkowskv. K. A. Kraus. H. 0. Phillios. J. S. Johnson, Jr., and A:‘I. Shor, J. A&. Chem. So;., ‘1966, 88, 5744. 6. A. J. Shor, K. A. Kraus, W. T. Smith and J. S. Johnson, Jr., J. Phys. Chem., 1968, 72, 2200.
7. D. Freilich and G. B. Tonny, J. Colloid Interface Sci., 1978, 64, 362. 8. M. R. J. Wyllie and H. W. Patnode, J. Phys. Chem., 1950, 54, 204. 9. F. G. R. Gimblett, Inorganic Polymer Chemistry, p. 195.
Butterworths, London, 1963. 10. G. G. Guilbault, Ion-Selective Electrode Review, 1979, 1, 139. 11. G. E. Baiulescu and V. V. Cogofret, Applications of Ion-Selective
Membrane Electrodes
in Organic Analysis,
p. 17. Hot-wood, Chichester, 1977. 12. Z. Marczenko, Spectrophotometric Determination Elements. Hot-wood, Chichester, 1976.
of