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Polarography of metal ions in hexamethylphosphoramide
Electroanalytieal Chemistry and Interfacial Electrochemistry, 41 (1973) 113-117
113
© Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
PO LAROGRAPHY OF METAL IONS IN HEXAMETHYLPHOSPHORAMIDE
DEAN C. LUEHRS and DAVID G. LEDDY
Department of Chemistry and Chemical Engineering, Michigan Technological University, Houghton, Michigan 49931 (U.S.A.) (Received 14th May 1972; in revised form 17th July 1972)
Recently hexamethylphosphoramide (HMPA) has gained some popularity as a nonaqueous solvent. The extreme resistance to reduction, high dielectric constant, fairly high basicity, and steric bulk are a unique combination of properties 1' 2. Numerous solvates of metal salts with HMPA have been prepared 3, but little electrochemistry has been done in HMPA 1'4. An excellent study of the polarographic behavior of alkali metal ions in HMPA by Fujinaga and co-workers s appeared after most of this work was completed. In order to study the strength of interaction of metal ions with HMPA and the effect of the unique properties of HMPA upon electrochemical processes, the polarography of the halide ions and selected metal ions was carried out in HMPA. EXPERIMENTAL Purification of the 1-IMPA HMPA (Research Organic/Inorganic Chemical Corp.) was purified by fractional distillation under vacuum from calcium hydride at 60°C. This was followed by two recrystallizations of the neat liquid in a cold room at 0°; this greatly reduced the background current, especially at potentials more negative than 1 V vs. the aqueous saturated calomel electrode (SCE). In order to initiate crystallization, it was necessary to seed the HMPA with a crystal obtained by freezing HMPA with liquid nitrogen. About two-thirds of the HMPA was allowed to freeze, and the remaining liquid was decanted off. Presumably a zone refining technique could give an even purer solvent, if necessary 6. The method of distilling from sodium 5 was tried in this laboratory but was not successful. The purified solvent had a background current of 0.25 #A at -0.8 V and 1.5 #A at - 2 . 0 V. Even solvent that was degassed with nitrogen and stored under nitrogen gave a much larger background current after standing for a week and gave a brown color when silver nitrate was dissolved in it. All HMPA was used within three days after purification but was repurified if the background current became too great or if its solution with silver nitrate turned brown. Other materials The source of hydrogen ion was 70~o perchloric acid in water. The tetraethylammonium and tetrabutylammonium perchlorates used as supporting electrolytes were prepared as described before 7. Lithium perchlorate 7 and thallous per-
114
D. C. LUEHRS, D. G. LEDDY
chlorate 8 were as described before. Sodium, potassium, and cesium perchlorates were obtained by careful neutralization of the reagent grade metal carbonate with reagent grade perchloric acid, recrystallizing from water, and drying at 110°. Rubidium perchlorate 997oo (Research Organic/Inorganic Chemical Corp.) was recrystallized twice from water and dried at 110°. Cobaltous, zinc, nickel, cadmium, cupric and lead perchlorates were prepared by adding reagent grade perchloric acid to an excess of the reagent grade metal carbonate, warming to eliminate carbon dioxide, filtering, evaporating the solution until crystals of the hydrated metal perchlorate began to appear, and filtering off the hydrated metal perchlorate after the solution cooled. These hydrated metal perchlorates were carefully dehydrated by heating in an oil bath at 60° under vacuum. The source of barium ion was G. F. Smith anhydrous barium perchlorate. Analysis of the cesium, rubidium, potassium and copper perchlorates by the method of Alley and Dykes 9 indicated that the alkali metal salts were more than 997o pure, the remainder presumably being water; the copper perchlorate had 3.0 mol of water of hydration. Analysis of the other metal perchlorates by standard methods indicated the following number of moles of water of hydration: Ni, 5.7; Pb, 2.5; Ba, 1.1; Zn, 2.0; Cd, 3.8. Reagent grade lithium chloride and lithium bromide used as the source of chloride and bromide ions were dried under vacuum at 110°. Analysis of these dried salts by potentiometric titration with silver nitrate showed that the lithium chloride was more than 99~o pure, but the lithium bromide had 1.0 mol of water of hydration. Eastman tetrabutylammonium iodide, the source of iodide ion, was found to be more than 997o pure by potentiometric titration with silver nitrate. Polarographic measurements
Polarograms were obtained with the controlled-potential polarograph and techniques described before 7'~°. The DME had these characteristics at a mercury height (uncorr. for back pressure) of 65 cm: m=3.17 mg s -1, t=2.1 s with open circuit in a 0.! M solution of tetraethylammonium perchlorate in H M P A . All data are for 0.10 M tetraethylammonium perchlorate as supporting electrolyte except for thallous ion where tetrabutylammonium perchlorate was used. RESULTS AND DISCUSSION
The results obtained for polarograms of selected metal ions and halide ions in HMPA with 0.10 M tetraethylammonium perchlorate as supporting electrolyte are given in Tables 1 and 2, and E½ values for selected metal ions vs. ferrocene/ ferricinium ion are given for HMPA, water and dimethyl sulfoxide as solvents in Table 3 for a comparison of reduction potentials in the various solvents with the complication of liquid junction potentials eliminated as much as possible 11,~2. Water and dimethyl sulfoxide are chosen for comparison because water is the most important solvent and dimethyl sulfoxide is a common solvent with properties similar to those of HMPA. It can be seen that all of the metal ions are reduced at a more negative
115
POLAROGRAPHY OF METAL IONS IN HMPA TABLE 1 SUMMARY OF POLAROGRAPHIC REDUCTION WAVES IN HMPA Ion
H ÷ at at at at K+ Rb + Cs + TI + CuZ + Cd 2+ 02 Pb 2-
E~/V vs. SCE
3.0 mM 1.2 mM 0.6 mM 0.3 mM
Zn2+,Co 2+ and Ni 2+ Na +, Li + and Ba 2+
Slope/V o1 log [( ia- i )/i] vs. E
ia/nCm ~ t~,
-0.793 0.086 0.49 -0.789 0.067 0.38 -0.764 0.044 1.13 -0.752 0.046 1.36 - 2.092 0.12 0.79 - 2.043 0.092 0.89 - 1.981 0.080 0.85 -0.25 0.19 1.3 - 0.035 0.07 1.0 - 0.531 0.029 0.80 -0.950 0.083 has both a prewave and a maximum at all concentrations. Discharge starts around -0.5 V with no well-defined limiting current. waves in the region of -0.7 to -0.8 V with current not proportional to concentration no wave before -2.8 V
TABLE 2 SUMMARY OF OXIDATION WAVES IN HMPA AT THE DME OR ROTATING PLATINUM ELECTRODE (RPE) Ion
E~/V vs. SCE
CI at the DME
-0.328 0.061 1st wave +0.095 0.037 2nd wave long, drawn out oxidation wave, appears to be two waves, but not clearly distinguished -0.322 0.051 +0.046 0.035 +0.371 0.096 1st wave +0.818 0.080 2nd wave +0.351 0.067 1st wave +0.717 0.075 2nd wave +0.347 0.060 1st wave +0.504 0.050 2nd wave
Br- at the DME
I- at the DME ( 1.39 mM) I at the RPE (1.04 mM) I - at the RPE (0.104 mM) I at the RPE (0.041 mM)
( E ~ - E~ )/V
ia/nCm~t ~
0099 1st wave 0.039 2nd wave
p o t e n t i a l in H M P A t h a n in w a t e r . T h e d i f f e r e n c e is s u b s t a n t i a l for l i t h i u m , s o d i u m , silver, a n d all o f t h e m u l t i p l y c h a r g e d ions. The half-wave reduction potentials of potassium, rubidium, and cesium ions a r e in fair a g r e e m e n t w i t h t h e v a l u e s o f F u j i n a g a a n d c o - w o r k e r s 5 a n d v e r y c l o s e to t h e v a l u e s in d i m e t h y l sulfoxide. S o m e o f t h e d i f f e r e n c e f r o m t h e v a l u e s o f F u j i n a g a a n d c o - w o r k e r s m a y b e d u e to t h e d i f f e r e n c e in s u p p o r t i n g e l e c t r o l y t e .
116
D. C. L U E H R S , D. G. L E D D Y
TABLE3 HALF-WAVE REDUCTION
POTENTIALS IN HMPA, WATER AND DIMETHYL SULFOXIDE ION
vs. F E R R O C E N E / F E R R I C I N I U M Ion
E~/V in HMPA
E~/V in watera
E~/Vin dimethyl sulfoxide e
K÷
-2.606 - 2.52: -2.557 - 2.50: - 2.495 -2.47: - 0.76 -0.549 - 1.045 -0.128 c
-2.25
-2.54
-2.18
-2.49
- 2.24
- 2.46
- 0.62 -0.15 - 0.74 +0.41
- 0.97 -0.49 - 1.13 -0.13 c
Rb + Cs + T1 + C u 2+ Cd 2+ Ag +
The values used for ferrocene/ferricinium ion vs. SCE were +0.147 V in w a t e r ~, + 0 . 4 3 V in d i m e t h y l sulfoxide b, a n d + 0 . 5 1 4 V in H M P A c. a I. M. K o l t h o f f a n d F. G. T h o m a s , J. Phys. Chem., 69 (1965) 3049. b D. C. Luehrs, Ph.D. Thesis, U n i v e r s i t y of Kansas, p. 127. c ref. 10. d L. Meites, Polarographic Techniques, Interscience Publishers, N e w York, 2nd ed., 1965, A p p e n d i x B. e Ref. 13. : Ref. 5.
Sodium, lithium and barium ions are not reduced before the cathodic limit of - 2 . 8 V. Cadmium ion was the only ion reduced reversibly in this study. Its reduction potential was at a much more negative value than in water but slightly more positive than in dimethyl sulfoxide. The reduction wave of thallous ion was better in tetrabutylammonium perchlorate than in tetraethylammonium perchlorate, as in the work of Fujinaga and co-workers 5, but was still extremely drawn out. At concentrations lower than 1 m M a prewave predominated. Cupric ion also had a prewave and in addition had a maximum at concentrations over 1 raM. The reduction wave of lead ion was so distorted by a prewave and m a x i m u m at all concentrations that it was impossible to analyze. Zinc, nickel and cobalt ions all gave reduction waves in the neighborhood of the reduction wave of hydrogen ion from perchloric acid. The current was not proportional to the concentration for zinc, nickel, cobalt or hydrogen ion. The reduction of oxygen appeared to be very similar to the reduction of oxygen in dimethyl sulfoxide, where superoxide ion is the product 13. Chloride, bromide and iodide ion each gave a two-step oxidation wave at the DME. This appeared to be similar to the behavior in acetone, where HgX 3 is the product of the first oxidation step and HgX z the product of the second step 14. Iodide ion also gave a two-step oxidation wave at the rotating platinum electrode. Here the first step must be oxidation to tri-iodide ion and the second step oxidation to iodine a3. F r o m the half-wave potentials of the oxidation waves, the stability constant of tri-iodide ion in H M P A can be calculated to be 1 0 9 " ° . Although this value is not highly accurate because the waves were not entirely reversible, it is the best available.
POLAROGRAPHY OF METAL IONS IN HMPA
117
The precision of the half-wave potentials was + 5 mY, and the accuracy is estimated to be at least as good as _+ 10 mV. ACKNOWLEDGEMENT
Some of the metal perchlorates were analyzed by Carol A. Waslawski who was supported by the Michigan Technological University College Work-Study Program. SUMMARY
The polarographic behavior of oxygen, halide ions and selected metal ions in HMPA was investigated. Solvation of metal ions was found to be stronger than in water but generally similar to that in dimethyl sulfoxide. REFERENCES I 2 3 4 5 6 7 8 9 10 11 12 13 14 15
H. Normant, Angew. Chem., Int. Ed., 6 (1967) 1046. V. Gutmann, Coordination Chemistry in Non-Aqueous Solutions, Springer-Verlag, Vienna, 1968, p. 159. M. W. G. Bolster and L. Groeneveld, Reel. Tray. Chim., 90 (1971) and references therein. J. E. Dubois, P. C. Lacaze and A. M. De Ficquelmont, C. R, Ser. C, 262(1966) 181,"249. K. Izutsu, S. Sakura and T. Fujinaga, Bull. Chem. Soc. Jap., 45 (1972) 445. G. J. Sloan, Anal. Chem., 38 (1966) 1805. D. C. Luehrs, R. T. Iwamoto and J. Kleinberg, lnorg. Chem., 5 (1966) 201. D. C. Luehrs, J. lnor O. Nucl. Chem., 31 (1969) 3517. B. J. Alley and H. W. H. Dykes, Anal. Chem., 36 (1964) 1124. D. C. Luehrs, J. lnorg. Nucl. Chem., 34 (1972) 791. D. C. Luehrs, R. W. Nicholas and ~D. A. Hamm, J. Electroanal. Chem., 29 (1971) 417. I. M. Kolthoff and M. K. Chantooni, J. Amer. Chem. Soc., 93 (1971) 7104. J. N. Butler, J. Electroanal. Chem., 14 (1967) 89. J. F. Coetzee and W. Siao, lnorg. Chem., 2 (1963) 14. A. J. Parker and R. Alexander, J. Amer. Chem. Soc., 90 (1968) 3313.
113
© Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
PO LAROGRAPHY OF METAL IONS IN HEXAMETHYLPHOSPHORAMIDE
DEAN C. LUEHRS and DAVID G. LEDDY
Department of Chemistry and Chemical Engineering, Michigan Technological University, Houghton, Michigan 49931 (U.S.A.) (Received 14th May 1972; in revised form 17th July 1972)
Recently hexamethylphosphoramide (HMPA) has gained some popularity as a nonaqueous solvent. The extreme resistance to reduction, high dielectric constant, fairly high basicity, and steric bulk are a unique combination of properties 1' 2. Numerous solvates of metal salts with HMPA have been prepared 3, but little electrochemistry has been done in HMPA 1'4. An excellent study of the polarographic behavior of alkali metal ions in HMPA by Fujinaga and co-workers s appeared after most of this work was completed. In order to study the strength of interaction of metal ions with HMPA and the effect of the unique properties of HMPA upon electrochemical processes, the polarography of the halide ions and selected metal ions was carried out in HMPA. EXPERIMENTAL Purification of the 1-IMPA HMPA (Research Organic/Inorganic Chemical Corp.) was purified by fractional distillation under vacuum from calcium hydride at 60°C. This was followed by two recrystallizations of the neat liquid in a cold room at 0°; this greatly reduced the background current, especially at potentials more negative than 1 V vs. the aqueous saturated calomel electrode (SCE). In order to initiate crystallization, it was necessary to seed the HMPA with a crystal obtained by freezing HMPA with liquid nitrogen. About two-thirds of the HMPA was allowed to freeze, and the remaining liquid was decanted off. Presumably a zone refining technique could give an even purer solvent, if necessary 6. The method of distilling from sodium 5 was tried in this laboratory but was not successful. The purified solvent had a background current of 0.25 #A at -0.8 V and 1.5 #A at - 2 . 0 V. Even solvent that was degassed with nitrogen and stored under nitrogen gave a much larger background current after standing for a week and gave a brown color when silver nitrate was dissolved in it. All HMPA was used within three days after purification but was repurified if the background current became too great or if its solution with silver nitrate turned brown. Other materials The source of hydrogen ion was 70~o perchloric acid in water. The tetraethylammonium and tetrabutylammonium perchlorates used as supporting electrolytes were prepared as described before 7. Lithium perchlorate 7 and thallous per-
114
D. C. LUEHRS, D. G. LEDDY
chlorate 8 were as described before. Sodium, potassium, and cesium perchlorates were obtained by careful neutralization of the reagent grade metal carbonate with reagent grade perchloric acid, recrystallizing from water, and drying at 110°. Rubidium perchlorate 997oo (Research Organic/Inorganic Chemical Corp.) was recrystallized twice from water and dried at 110°. Cobaltous, zinc, nickel, cadmium, cupric and lead perchlorates were prepared by adding reagent grade perchloric acid to an excess of the reagent grade metal carbonate, warming to eliminate carbon dioxide, filtering, evaporating the solution until crystals of the hydrated metal perchlorate began to appear, and filtering off the hydrated metal perchlorate after the solution cooled. These hydrated metal perchlorates were carefully dehydrated by heating in an oil bath at 60° under vacuum. The source of barium ion was G. F. Smith anhydrous barium perchlorate. Analysis of the cesium, rubidium, potassium and copper perchlorates by the method of Alley and Dykes 9 indicated that the alkali metal salts were more than 997o pure, the remainder presumably being water; the copper perchlorate had 3.0 mol of water of hydration. Analysis of the other metal perchlorates by standard methods indicated the following number of moles of water of hydration: Ni, 5.7; Pb, 2.5; Ba, 1.1; Zn, 2.0; Cd, 3.8. Reagent grade lithium chloride and lithium bromide used as the source of chloride and bromide ions were dried under vacuum at 110°. Analysis of these dried salts by potentiometric titration with silver nitrate showed that the lithium chloride was more than 99~o pure, but the lithium bromide had 1.0 mol of water of hydration. Eastman tetrabutylammonium iodide, the source of iodide ion, was found to be more than 997o pure by potentiometric titration with silver nitrate. Polarographic measurements
Polarograms were obtained with the controlled-potential polarograph and techniques described before 7'~°. The DME had these characteristics at a mercury height (uncorr. for back pressure) of 65 cm: m=3.17 mg s -1, t=2.1 s with open circuit in a 0.! M solution of tetraethylammonium perchlorate in H M P A . All data are for 0.10 M tetraethylammonium perchlorate as supporting electrolyte except for thallous ion where tetrabutylammonium perchlorate was used. RESULTS AND DISCUSSION
The results obtained for polarograms of selected metal ions and halide ions in HMPA with 0.10 M tetraethylammonium perchlorate as supporting electrolyte are given in Tables 1 and 2, and E½ values for selected metal ions vs. ferrocene/ ferricinium ion are given for HMPA, water and dimethyl sulfoxide as solvents in Table 3 for a comparison of reduction potentials in the various solvents with the complication of liquid junction potentials eliminated as much as possible 11,~2. Water and dimethyl sulfoxide are chosen for comparison because water is the most important solvent and dimethyl sulfoxide is a common solvent with properties similar to those of HMPA. It can be seen that all of the metal ions are reduced at a more negative
115
POLAROGRAPHY OF METAL IONS IN HMPA TABLE 1 SUMMARY OF POLAROGRAPHIC REDUCTION WAVES IN HMPA Ion
H ÷ at at at at K+ Rb + Cs + TI + CuZ + Cd 2+ 02 Pb 2-
E~/V vs. SCE
3.0 mM 1.2 mM 0.6 mM 0.3 mM
Zn2+,Co 2+ and Ni 2+ Na +, Li + and Ba 2+
Slope/V o1 log [( ia- i )/i] vs. E
ia/nCm ~ t~,
-0.793 0.086 0.49 -0.789 0.067 0.38 -0.764 0.044 1.13 -0.752 0.046 1.36 - 2.092 0.12 0.79 - 2.043 0.092 0.89 - 1.981 0.080 0.85 -0.25 0.19 1.3 - 0.035 0.07 1.0 - 0.531 0.029 0.80 -0.950 0.083 has both a prewave and a maximum at all concentrations. Discharge starts around -0.5 V with no well-defined limiting current. waves in the region of -0.7 to -0.8 V with current not proportional to concentration no wave before -2.8 V
TABLE 2 SUMMARY OF OXIDATION WAVES IN HMPA AT THE DME OR ROTATING PLATINUM ELECTRODE (RPE) Ion
E~/V vs. SCE
CI at the DME
-0.328 0.061 1st wave +0.095 0.037 2nd wave long, drawn out oxidation wave, appears to be two waves, but not clearly distinguished -0.322 0.051 +0.046 0.035 +0.371 0.096 1st wave +0.818 0.080 2nd wave +0.351 0.067 1st wave +0.717 0.075 2nd wave +0.347 0.060 1st wave +0.504 0.050 2nd wave
Br- at the DME
I- at the DME ( 1.39 mM) I at the RPE (1.04 mM) I - at the RPE (0.104 mM) I at the RPE (0.041 mM)
( E ~ - E~ )/V
ia/nCm~t ~
0099 1st wave 0.039 2nd wave
p o t e n t i a l in H M P A t h a n in w a t e r . T h e d i f f e r e n c e is s u b s t a n t i a l for l i t h i u m , s o d i u m , silver, a n d all o f t h e m u l t i p l y c h a r g e d ions. The half-wave reduction potentials of potassium, rubidium, and cesium ions a r e in fair a g r e e m e n t w i t h t h e v a l u e s o f F u j i n a g a a n d c o - w o r k e r s 5 a n d v e r y c l o s e to t h e v a l u e s in d i m e t h y l sulfoxide. S o m e o f t h e d i f f e r e n c e f r o m t h e v a l u e s o f F u j i n a g a a n d c o - w o r k e r s m a y b e d u e to t h e d i f f e r e n c e in s u p p o r t i n g e l e c t r o l y t e .
116
D. C. L U E H R S , D. G. L E D D Y
TABLE3 HALF-WAVE REDUCTION
POTENTIALS IN HMPA, WATER AND DIMETHYL SULFOXIDE ION
vs. F E R R O C E N E / F E R R I C I N I U M Ion
E~/V in HMPA
E~/V in watera
E~/Vin dimethyl sulfoxide e
K÷
-2.606 - 2.52: -2.557 - 2.50: - 2.495 -2.47: - 0.76 -0.549 - 1.045 -0.128 c
-2.25
-2.54
-2.18
-2.49
- 2.24
- 2.46
- 0.62 -0.15 - 0.74 +0.41
- 0.97 -0.49 - 1.13 -0.13 c
Rb + Cs + T1 + C u 2+ Cd 2+ Ag +
The values used for ferrocene/ferricinium ion vs. SCE were +0.147 V in w a t e r ~, + 0 . 4 3 V in d i m e t h y l sulfoxide b, a n d + 0 . 5 1 4 V in H M P A c. a I. M. K o l t h o f f a n d F. G. T h o m a s , J. Phys. Chem., 69 (1965) 3049. b D. C. Luehrs, Ph.D. Thesis, U n i v e r s i t y of Kansas, p. 127. c ref. 10. d L. Meites, Polarographic Techniques, Interscience Publishers, N e w York, 2nd ed., 1965, A p p e n d i x B. e Ref. 13. : Ref. 5.
Sodium, lithium and barium ions are not reduced before the cathodic limit of - 2 . 8 V. Cadmium ion was the only ion reduced reversibly in this study. Its reduction potential was at a much more negative value than in water but slightly more positive than in dimethyl sulfoxide. The reduction wave of thallous ion was better in tetrabutylammonium perchlorate than in tetraethylammonium perchlorate, as in the work of Fujinaga and co-workers 5, but was still extremely drawn out. At concentrations lower than 1 m M a prewave predominated. Cupric ion also had a prewave and in addition had a maximum at concentrations over 1 raM. The reduction wave of lead ion was so distorted by a prewave and m a x i m u m at all concentrations that it was impossible to analyze. Zinc, nickel and cobalt ions all gave reduction waves in the neighborhood of the reduction wave of hydrogen ion from perchloric acid. The current was not proportional to the concentration for zinc, nickel, cobalt or hydrogen ion. The reduction of oxygen appeared to be very similar to the reduction of oxygen in dimethyl sulfoxide, where superoxide ion is the product 13. Chloride, bromide and iodide ion each gave a two-step oxidation wave at the DME. This appeared to be similar to the behavior in acetone, where HgX 3 is the product of the first oxidation step and HgX z the product of the second step 14. Iodide ion also gave a two-step oxidation wave at the rotating platinum electrode. Here the first step must be oxidation to tri-iodide ion and the second step oxidation to iodine a3. F r o m the half-wave potentials of the oxidation waves, the stability constant of tri-iodide ion in H M P A can be calculated to be 1 0 9 " ° . Although this value is not highly accurate because the waves were not entirely reversible, it is the best available.
POLAROGRAPHY OF METAL IONS IN HMPA
117
The precision of the half-wave potentials was + 5 mY, and the accuracy is estimated to be at least as good as _+ 10 mV. ACKNOWLEDGEMENT
Some of the metal perchlorates were analyzed by Carol A. Waslawski who was supported by the Michigan Technological University College Work-Study Program. SUMMARY
The polarographic behavior of oxygen, halide ions and selected metal ions in HMPA was investigated. Solvation of metal ions was found to be stronger than in water but generally similar to that in dimethyl sulfoxide. REFERENCES I 2 3 4 5 6 7 8 9 10 11 12 13 14 15
H. Normant, Angew. Chem., Int. Ed., 6 (1967) 1046. V. Gutmann, Coordination Chemistry in Non-Aqueous Solutions, Springer-Verlag, Vienna, 1968, p. 159. M. W. G. Bolster and L. Groeneveld, Reel. Tray. Chim., 90 (1971) and references therein. J. E. Dubois, P. C. Lacaze and A. M. De Ficquelmont, C. R, Ser. C, 262(1966) 181,"249. K. Izutsu, S. Sakura and T. Fujinaga, Bull. Chem. Soc. Jap., 45 (1972) 445. G. J. Sloan, Anal. Chem., 38 (1966) 1805. D. C. Luehrs, R. T. Iwamoto and J. Kleinberg, lnorg. Chem., 5 (1966) 201. D. C. Luehrs, J. lnor O. Nucl. Chem., 31 (1969) 3517. B. J. Alley and H. W. H. Dykes, Anal. Chem., 36 (1964) 1124. D. C. Luehrs, J. lnorg. Nucl. Chem., 34 (1972) 791. D. C. Luehrs, R. W. Nicholas and ~D. A. Hamm, J. Electroanal. Chem., 29 (1971) 417. I. M. Kolthoff and M. K. Chantooni, J. Amer. Chem. Soc., 93 (1971) 7104. J. N. Butler, J. Electroanal. Chem., 14 (1967) 89. J. F. Coetzee and W. Siao, lnorg. Chem., 2 (1963) 14. A. J. Parker and R. Alexander, J. Amer. Chem. Soc., 90 (1968) 3313.