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The effect of coumarin on the polarographic reduction of cadmium
EIectrwhimica Acts. 1969. Vol. 14. pp. 223 to 229. P~amon
Press. Printed in Northern It’dand
THE EFFECT OF COUMARIN ON THE POLAROGRAPHIC REDUCTION OF CADMIUM* L. K. PARTRIDGE, A. C. TANSLEY and A. S. PORTER Albright & Wilson Ltd., Oldbury Division, P.O. Box 3, Oldbury, Warley, Worcestershire, England Abstract-Cadmium deposition from solutions of varying ionic strength and coumarin concentration Under certain conditions two polarographic waves are obtained, has been studied polarographically. both corresponding to cadmium deposition. This effect has been explained by the variation with potential of the orientation of adsorbed coumarin molecules. R&nn&La deposition de cadmium a partir de solutions de force ionique et de concentration de varites a ete emdie polarographiquement coumarine. Dam certaines conditions, il y a deux vagues polarographiques qui correspondent toutes les deux a la deposition de cadmium. Cet effet a et6 explique par la dependance pour le potentiel de l’orientation des molecules de coumarine adsorb&s. Zusammeufassnng-Die Abscheidung von Kadmium aus LSsungen verschiedener Ionenstarken und Kumarinkonzentrationen wurde polarographisch untersucht. Unter gewissen Bedingungen erhllt man zwei Reduktionswellen, die beide mit der Kadmiumabscheidtmg verkntipft sind. Dieser Effekt wurde durch eine Orientierungs%nderung der Adsorbierten Kumarinmolektile mit dem Potential erklart. INTRODUCTION REDUCTION of cations at the dme usually occurs with the production of a single polarization wave and interpretation of experimental results is then relatively simple. The appearance of additionalcatalytic,1-7anomalous,7~8adsorption7~n~10orkinetic7~11-15 waves may complicate the situation. Extra waves are also formed if reduction occurs in more than one stage due to stabilization of an intermediate oxidation state by some constituent of the solution. The analysis of any such abnormal or extra waves can yield information on effects either at the electrode surface or in the bulk solution. The polarography of cadmium deposition at the dme has been studied with a variety of supporting electrolytes and is summarized by Meites.’ It is concluded that Cda+ reduction occurs in a single step with the production of a single polarographic wave. We have found that Cda+ is reduced in the presence of coumarin with the formation of either one or two waves, depending on the ionic strength of the supporting electrolyte and the concentration of coumarin. EXPERIMENTAL
TECHNIQUE
AND
RESULTS
Materials
Mercury was purified as described by VogeP and redistilled twice under reduced pressure. Coumarin was recrystallized once from water; all other reagents were of A.R. grade and were used without further purification. Interfacial tension measurements
For interfacial tension determinations a modification of the drop-weight method was employed .17 These measurements were used to calculate the adsorption isotherms * Manuscript received 9 February 1967. 223
L. K. PARTRIDGE,A. C. TANSLEYand A. S. Poxma
224
for coumarin at the mercury/sodium sulphate interface in the presence of cadmium sulphate, Fig. 1. The results are compared with similar results obtained in the absence of cadmium in Fig. 2.
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-0.5
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l?kctrocapillarycurvesfor M)l M cadmium sulpbatein @05 M sodium s~dphate
illthCpresenCe of coumarin; pH 4*6,2O”C. The number shown against each cmve is the fractional saturation with coumarin.
L
1
0.02
/
. (2)
i
a ?i I
0.01 I-
0
-0.2 to -0.4 vhlhel
i!’ I I 0.2 Froctimal
Fro.
.
I 0.4 saturotii
I
0.6
I
0.6
I.0
with caumarin
2. Adsorption isotherms for coumarin at the mercury elwtrolyte interface; pH +6,2O”C . 1, -0.1 to -0.3 V, 1 M sodium sulphate;” 2, --02 to -0.4 V, 0.01 M cadmium sulphate in 0.05 M sodium sulphate.
Uv absorption Coumarin concentrations were determined by uv absorption at 280 nm.1’ The uv absorption spectra, between 220 and 350 nm, were determined for coumarin in sodium sulphate and cadmium sulphate solutions of unit molar ionic strength. No difference in shape of the spectrum could be detected, Fig. 3.
The effect of coumarin on the polarographic reduction of cadmium
225
I.0 -
0.6-
0.4-
0,2-
I
I
I
I
I
I
I
I
220
240
260
280
300
320
340
Wavelength,
nm
FIG. 3. Uv absorption spectra for coumarin in solutions of unitmolar ionic strength; pH 4.6,2O”C. 0, sodium sulphate; x , cadmium sulphate. Curves adjusted to coincide at 280 MI.
Polarography Polarograms were obtained with a Metrohm Polarecord and are reproduced by drawing smooth curves through the mid-points of the current oscillations caused by the dme. Dissolved oxygen and carbon dioxide were removed by bubbling oxygenfree nitrogen through the solutions for at least 10 min. Cadmium was found to deposit at the dme from 1 M sodium sulphate solution with the formation of a single wave at a half-wave potential of -040 f O$K2v(nhe), Fig. 4. This value compares well with those listed by Meites’ for Cd8+ reduction from inorganic salt solutions. I
Vbhel
FIG. 4. Polarographic wave for cadmium depositing from 0.01 M cadmium sulphate in 1 M sodium sulphate; pH 4.6, 20°C. Curve drawn through mid-points of current oscillations.
226
L. K. PARTRIDGE,A. C. TANSLEY and A. S. PORTER
The effect of coumarin is shown in Figs. 5 and 6. At low ionic strengths two waves are obtained; the first occurs at the potential for cadmium deposition from a coumarin-free solution and the second at a more negative potential.
0
-0-Z
-0.4
-0.6
Vhlhe)
FIG.5. Polarographicwaves for O-01 M cadmium sulphate in sodium sulphate saturated with coumarin; pH 4-6, 20°C. 1, 1 M sodium sulphate, 2, O-2 M sodium sulphate, 3, 0.1 M sodium sulphate, 4, 0.05 M sodium sulphate, 5, No sodium sulphate.
Vb-lhe)
FIG. 6. Polarographic waves for 0.01 M cadmium sulphate in the presence of coumarin; pH 4.6, 20°C. The number shown against each curve is the fractionalsaturation withcoumarin. The reduction wave of coumarin in sodium sulphate solution is shown in Fig. 7 and compared with the reduction waves for cadmium from cadmium sulphate solutions of
The effect of coumarin on the polarographic
/
reduction of cadmium
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coumarin solutions; pH 4*6, 20°C. - - -, cadmium sulphate, ionic strength OG4M; - - -, cadmium sulphate, ionic strength 0.04 M, saturated with coumarin; sodium sulphate, ionic strength 0.04 M; - *- *- . -, sodium sulphate, ionic strength’004 M, saturated with coumarin. FIG. 7. Polarographic
waves for cadmium sulphate and
the same ionic strength. Reduction of coumarin occurs at potentials more negative than the two waves in the presence of cadmium. DISCUSSION The shapes of the waves in Figs. 5 and 6 are unlike those produced by any mechanism reported in the literature and we note below some further reasons for rejecting the usual explanations for complex waves. The two waves formed during the polarographic reduction of Cd2+ from solutions containing coumarin cannot be explained on the basis of a two-stage reduction mechanism because this would give stepwise waves such as are obtained for copper.’ Catalytic waves for Cos+ and Cos+ reduction in the presence of certain thioproteins in NH,Cl/NH,OH solutions have been reported.ls*ls At constant metal-ion concentration, a plot of catalytic wave height against protein concentration resembles an adsorption isotherm. Such an explanation is not possible for the Cdr+-coumarin system since a similar plot of our results, Fig. 8, resembles a desorption, rather than adsorption, isotherm. The “anomalous” wave interpretation is usually applied to polarographic peaks that do not correspond to any possible electrochemical reaction. In this investigation the first wave occurs at the same potential as cadmium deposition from coumarin-free electrolytes of the same ionic strength. Adsorption waves can be produced if either the organic additive or its reduction product is strongly adsorbed at the mercury surface. Since the wave under investigation is not formed in cadmium-free coumarin solutions, Fig. 7, this explanation is improbable. Also, coumarin is reduced at far more negative potentials than the potential range of the first peak, Fig. 7, so that the effects of reduction products may be neglected.
L. K. PAR-E,
A. C. TAN~LEYand A. S. PORTER
IO.0 -
“0 x 7.5 a < 5.0-
2.5 -
0
I 0.2 kdonal
0.4 wtumtico
0.6
06
with counorin
FIG. 8. Height of first polarographic wave of 0.01 M cadmium sulphate solution in the presence of coumarin, as a function of the concentration of coumarin; pH 4.6, 20°C.
The occurrence of a kinetic current depends on the existence of an equilibrium in which both participants are reducible at the mercury surface. Any possible equilibrium in this investigation would have to involve both coumarin and Cds+, since the two-wave effect occurs only when both these species are present. We have found no references to CdB+-coumarin complexes and our own measurements of the uv absorption of coumarin in the presence of cadmium sulphate, Fig. 3, show no evidence of interaction. Some approximate measurements of solubility were made in solutions of various sulphates at 25°C and an apparent ionic strength of 1.0 M with the following results. Coumarin solubility mM Cation Na+, K+, NH*+ Mg”f, Mns+, Coa+, Ni”, Zns+ Cd= C++, Al%
10410.7 10.9-12.0 11.8 12.2-124
The result for cadmium is in line with those for the other bivalent ions; an increase in solubility with increasing cation valency is expected since the true ionic strength decreases with increased ion-pairing. There is no evidence of complex formation. The shape of the first wave, Fig. 5, suggests that the deposition process starts normally but suddenly ceases and then starts again as the potential of the mercury becomes more negative. This can be explained by the change in orientation of adsorbed coumarin from parallel to perpendicular as the cathode is made more negative.1’ If the deposition potential of a cation is more positive than the reorientation potential of coumarin, the cation starts to deposit through the planar film without being inhibited, since there is no component of the dipole moment normal to the surface.
The effect of coumarin on the polarographicreduction of cadmium
229
As the potential is made more negative the coumarin molecules stand on end and deposition ceases until the potential becomes sufficiently cathodic to overcome the inhibitive effect of the dipole layer or to desorb the coumarin. This increase in potential can be calcu1ated.l’ The most positive deposition potential of cadmium from the electrolytes used in this investigation is approximately -0.25 V(nhe), Figs. 4-6. At this potential coumarin is adsorbed, from sodium sulphate electrolytes saturated with coumarin, with the perpendicular orientation, Fig. 2, and the fist peak would not be expected. However, Cdsf interferes with the adsorption of coumarin on mercury and reorientation occurs at a more negative potential. At -0.25 V(nhe), therefore, in the presence of Cd”, coumarin maintains the planar orientation. The two-wave effect for metal deposition in the presence of coumarin should apply to any metal which deposits at potentials more positive than the reorientation potential of coumarin and does not prevent the adsorption or reorientation of coumarin. Only Pbs+ and Tl+, of the metals we have studied, deposit at a suitable potential, but these ions are specifically adsorbed on mercury surfaces and have been found to displace adsorbed coumarin completely.80 REFERENCES 1. P. DELAHAYand G. L. STWL, J. Am. them. SC-W. 74, 3500(1952). 2. E. F. ARLEMANNand D. M. H. KERN, J. Am. them. Sot. X,3058 (1953). 3. M. G. JOHNSONand R. J. ROBINSON,Adyt. Gem. 24,366 (1952). 4. H. A. LATINENand W. A. ZIEQLBR, J. Am. them. Sot. 75,3045 (1953). 5. I. M. KOLTHOFF and E. P. PARRY,J. Am. &em. Sot. 73,5315 (1951).
6. I. M. KOLTHOFF,W. E. HARRUand G. MATSUYAMA,J. Am. them. SC. 66,1782 (1944). Polarographic Techniques. Interscience, New York (1955). 8. L. MEITES,J. Am. chem.‘Soc. 75,3809 (1953). 9. 0. H. MLJLLER,Trans. electrochem. Sot. 87,441 (1945).
7. L. MEW, 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
R. BRDICKA,Z. Elektrochem. 48,278 (1942). R. BRDICKAand K. WIFSNER,Naturwissenschnfrn 31,247,391 (1943). P. DELAMY, J. Am. them. Sot. 73,4944 (1951); 74,3506,6315 (1952). D. M. H. KERN, J. Am. them. Sot. 75,2473 (1953). I. M. KOLTHOFP and E. P. PARRY,J. Am. them. Sot. 73,3718 (1951). K. WIESNER,Z. Elektrockem. 49, 164 (1943). A. I. VOOEL, A Text Book of Quantitative Inorganic Analysis. Longmans, London (1951). L. K. PARTRIDGE, A. C. TANSUY and A. S. PORTER,Electrochim. Acta 11,517 (1966). R. BRDICKA,Colln. Czech. them. Common. Engl. Edn 11,614 (1939). E. JIJRKA,Colfn. Czech. them. Commun. Engl. Ea% 11,243 (1939). L. K. PARTRIDGE, A. C. TANSLEYand A. S. PORYXR,Electrachim. Acta 12,1573 (1967).
Press. Printed in Northern It’dand
THE EFFECT OF COUMARIN ON THE POLAROGRAPHIC REDUCTION OF CADMIUM* L. K. PARTRIDGE, A. C. TANSLEY and A. S. PORTER Albright & Wilson Ltd., Oldbury Division, P.O. Box 3, Oldbury, Warley, Worcestershire, England Abstract-Cadmium deposition from solutions of varying ionic strength and coumarin concentration Under certain conditions two polarographic waves are obtained, has been studied polarographically. both corresponding to cadmium deposition. This effect has been explained by the variation with potential of the orientation of adsorbed coumarin molecules. R&nn&La deposition de cadmium a partir de solutions de force ionique et de concentration de varites a ete emdie polarographiquement coumarine. Dam certaines conditions, il y a deux vagues polarographiques qui correspondent toutes les deux a la deposition de cadmium. Cet effet a et6 explique par la dependance pour le potentiel de l’orientation des molecules de coumarine adsorb&s. Zusammeufassnng-Die Abscheidung von Kadmium aus LSsungen verschiedener Ionenstarken und Kumarinkonzentrationen wurde polarographisch untersucht. Unter gewissen Bedingungen erhllt man zwei Reduktionswellen, die beide mit der Kadmiumabscheidtmg verkntipft sind. Dieser Effekt wurde durch eine Orientierungs%nderung der Adsorbierten Kumarinmolektile mit dem Potential erklart. INTRODUCTION REDUCTION of cations at the dme usually occurs with the production of a single polarization wave and interpretation of experimental results is then relatively simple. The appearance of additionalcatalytic,1-7anomalous,7~8adsorption7~n~10orkinetic7~11-15 waves may complicate the situation. Extra waves are also formed if reduction occurs in more than one stage due to stabilization of an intermediate oxidation state by some constituent of the solution. The analysis of any such abnormal or extra waves can yield information on effects either at the electrode surface or in the bulk solution. The polarography of cadmium deposition at the dme has been studied with a variety of supporting electrolytes and is summarized by Meites.’ It is concluded that Cda+ reduction occurs in a single step with the production of a single polarographic wave. We have found that Cda+ is reduced in the presence of coumarin with the formation of either one or two waves, depending on the ionic strength of the supporting electrolyte and the concentration of coumarin. EXPERIMENTAL
TECHNIQUE
AND
RESULTS
Materials
Mercury was purified as described by VogeP and redistilled twice under reduced pressure. Coumarin was recrystallized once from water; all other reagents were of A.R. grade and were used without further purification. Interfacial tension measurements
For interfacial tension determinations a modification of the drop-weight method was employed .17 These measurements were used to calculate the adsorption isotherms * Manuscript received 9 February 1967. 223
L. K. PARTRIDGE,A. C. TANSLEYand A. S. Poxma
224
for coumarin at the mercury/sodium sulphate interface in the presence of cadmium sulphate, Fig. 1. The results are compared with similar results obtained in the absence of cadmium in Fig. 2.
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360 45
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.-.--_-.-e-4-~._
..
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I
I
I
-0.3
-0.5
I -07
I -0.9
Wnlnd
PKt. 1.
l?kctrocapillarycurvesfor M)l M cadmium sulpbatein @05 M sodium s~dphate
illthCpresenCe of coumarin; pH 4*6,2O”C. The number shown against each cmve is the fractional saturation with coumarin.
L
1
0.02
/
. (2)
i
a ?i I
0.01 I-
0
-0.2 to -0.4 vhlhel
i!’ I I 0.2 Froctimal
Fro.
.
I 0.4 saturotii
I
0.6
I
0.6
I.0
with caumarin
2. Adsorption isotherms for coumarin at the mercury elwtrolyte interface; pH +6,2O”C . 1, -0.1 to -0.3 V, 1 M sodium sulphate;” 2, --02 to -0.4 V, 0.01 M cadmium sulphate in 0.05 M sodium sulphate.
Uv absorption Coumarin concentrations were determined by uv absorption at 280 nm.1’ The uv absorption spectra, between 220 and 350 nm, were determined for coumarin in sodium sulphate and cadmium sulphate solutions of unit molar ionic strength. No difference in shape of the spectrum could be detected, Fig. 3.
The effect of coumarin on the polarographic reduction of cadmium
225
I.0 -
0.6-
0.4-
0,2-
I
I
I
I
I
I
I
I
220
240
260
280
300
320
340
Wavelength,
nm
FIG. 3. Uv absorption spectra for coumarin in solutions of unitmolar ionic strength; pH 4.6,2O”C. 0, sodium sulphate; x , cadmium sulphate. Curves adjusted to coincide at 280 MI.
Polarography Polarograms were obtained with a Metrohm Polarecord and are reproduced by drawing smooth curves through the mid-points of the current oscillations caused by the dme. Dissolved oxygen and carbon dioxide were removed by bubbling oxygenfree nitrogen through the solutions for at least 10 min. Cadmium was found to deposit at the dme from 1 M sodium sulphate solution with the formation of a single wave at a half-wave potential of -040 f O$K2v(nhe), Fig. 4. This value compares well with those listed by Meites’ for Cd8+ reduction from inorganic salt solutions. I
Vbhel
FIG. 4. Polarographic wave for cadmium depositing from 0.01 M cadmium sulphate in 1 M sodium sulphate; pH 4.6, 20°C. Curve drawn through mid-points of current oscillations.
226
L. K. PARTRIDGE,A. C. TANSLEY and A. S. PORTER
The effect of coumarin is shown in Figs. 5 and 6. At low ionic strengths two waves are obtained; the first occurs at the potential for cadmium deposition from a coumarin-free solution and the second at a more negative potential.
0
-0-Z
-0.4
-0.6
Vhlhe)
FIG.5. Polarographicwaves for O-01 M cadmium sulphate in sodium sulphate saturated with coumarin; pH 4-6, 20°C. 1, 1 M sodium sulphate, 2, O-2 M sodium sulphate, 3, 0.1 M sodium sulphate, 4, 0.05 M sodium sulphate, 5, No sodium sulphate.
Vb-lhe)
FIG. 6. Polarographic waves for 0.01 M cadmium sulphate in the presence of coumarin; pH 4.6, 20°C. The number shown against each curve is the fractionalsaturation withcoumarin. The reduction wave of coumarin in sodium sulphate solution is shown in Fig. 7 and compared with the reduction waves for cadmium from cadmium sulphate solutions of
The effect of coumarin on the polarographic
/
reduction of cadmium
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227
----I_----,’
lO,O-
::
/
/’
,/’
“0
7-5-
,’ 8’
z v 5.0 -
-0.4
-0.6
-0.6
- I-O
-1.2
-I
4
-16
N-he)
coumarin solutions; pH 4*6, 20°C. - - -, cadmium sulphate, ionic strength OG4M; - - -, cadmium sulphate, ionic strength 0.04 M, saturated with coumarin; sodium sulphate, ionic strength 0.04 M; - *- *- . -, sodium sulphate, ionic strength’004 M, saturated with coumarin. FIG. 7. Polarographic
waves for cadmium sulphate and
the same ionic strength. Reduction of coumarin occurs at potentials more negative than the two waves in the presence of cadmium. DISCUSSION The shapes of the waves in Figs. 5 and 6 are unlike those produced by any mechanism reported in the literature and we note below some further reasons for rejecting the usual explanations for complex waves. The two waves formed during the polarographic reduction of Cd2+ from solutions containing coumarin cannot be explained on the basis of a two-stage reduction mechanism because this would give stepwise waves such as are obtained for copper.’ Catalytic waves for Cos+ and Cos+ reduction in the presence of certain thioproteins in NH,Cl/NH,OH solutions have been reported.ls*ls At constant metal-ion concentration, a plot of catalytic wave height against protein concentration resembles an adsorption isotherm. Such an explanation is not possible for the Cdr+-coumarin system since a similar plot of our results, Fig. 8, resembles a desorption, rather than adsorption, isotherm. The “anomalous” wave interpretation is usually applied to polarographic peaks that do not correspond to any possible electrochemical reaction. In this investigation the first wave occurs at the same potential as cadmium deposition from coumarin-free electrolytes of the same ionic strength. Adsorption waves can be produced if either the organic additive or its reduction product is strongly adsorbed at the mercury surface. Since the wave under investigation is not formed in cadmium-free coumarin solutions, Fig. 7, this explanation is improbable. Also, coumarin is reduced at far more negative potentials than the potential range of the first peak, Fig. 7, so that the effects of reduction products may be neglected.
L. K. PAR-E,
A. C. TAN~LEYand A. S. PORTER
IO.0 -
“0 x 7.5 a < 5.0-
2.5 -
0
I 0.2 kdonal
0.4 wtumtico
0.6
06
with counorin
FIG. 8. Height of first polarographic wave of 0.01 M cadmium sulphate solution in the presence of coumarin, as a function of the concentration of coumarin; pH 4.6, 20°C.
The occurrence of a kinetic current depends on the existence of an equilibrium in which both participants are reducible at the mercury surface. Any possible equilibrium in this investigation would have to involve both coumarin and Cds+, since the two-wave effect occurs only when both these species are present. We have found no references to CdB+-coumarin complexes and our own measurements of the uv absorption of coumarin in the presence of cadmium sulphate, Fig. 3, show no evidence of interaction. Some approximate measurements of solubility were made in solutions of various sulphates at 25°C and an apparent ionic strength of 1.0 M with the following results. Coumarin solubility mM Cation Na+, K+, NH*+ Mg”f, Mns+, Coa+, Ni”, Zns+ Cd= C++, Al%
10410.7 10.9-12.0 11.8 12.2-124
The result for cadmium is in line with those for the other bivalent ions; an increase in solubility with increasing cation valency is expected since the true ionic strength decreases with increased ion-pairing. There is no evidence of complex formation. The shape of the first wave, Fig. 5, suggests that the deposition process starts normally but suddenly ceases and then starts again as the potential of the mercury becomes more negative. This can be explained by the change in orientation of adsorbed coumarin from parallel to perpendicular as the cathode is made more negative.1’ If the deposition potential of a cation is more positive than the reorientation potential of coumarin, the cation starts to deposit through the planar film without being inhibited, since there is no component of the dipole moment normal to the surface.
The effect of coumarin on the polarographicreduction of cadmium
229
As the potential is made more negative the coumarin molecules stand on end and deposition ceases until the potential becomes sufficiently cathodic to overcome the inhibitive effect of the dipole layer or to desorb the coumarin. This increase in potential can be calcu1ated.l’ The most positive deposition potential of cadmium from the electrolytes used in this investigation is approximately -0.25 V(nhe), Figs. 4-6. At this potential coumarin is adsorbed, from sodium sulphate electrolytes saturated with coumarin, with the perpendicular orientation, Fig. 2, and the fist peak would not be expected. However, Cdsf interferes with the adsorption of coumarin on mercury and reorientation occurs at a more negative potential. At -0.25 V(nhe), therefore, in the presence of Cd”, coumarin maintains the planar orientation. The two-wave effect for metal deposition in the presence of coumarin should apply to any metal which deposits at potentials more positive than the reorientation potential of coumarin and does not prevent the adsorption or reorientation of coumarin. Only Pbs+ and Tl+, of the metals we have studied, deposit at a suitable potential, but these ions are specifically adsorbed on mercury surfaces and have been found to displace adsorbed coumarin completely.80 REFERENCES 1. P. DELAHAYand G. L. STWL, J. Am. them. SC-W. 74, 3500(1952). 2. E. F. ARLEMANNand D. M. H. KERN, J. Am. them. Sot. X,3058 (1953). 3. M. G. JOHNSONand R. J. ROBINSON,Adyt. Gem. 24,366 (1952). 4. H. A. LATINENand W. A. ZIEQLBR, J. Am. them. Sot. 75,3045 (1953). 5. I. M. KOLTHOFF and E. P. PARRY,J. Am. &em. Sot. 73,5315 (1951).
6. I. M. KOLTHOFF,W. E. HARRUand G. MATSUYAMA,J. Am. them. SC. 66,1782 (1944). Polarographic Techniques. Interscience, New York (1955). 8. L. MEITES,J. Am. chem.‘Soc. 75,3809 (1953). 9. 0. H. MLJLLER,Trans. electrochem. Sot. 87,441 (1945).
7. L. MEW, 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
R. BRDICKA,Z. Elektrochem. 48,278 (1942). R. BRDICKAand K. WIFSNER,Naturwissenschnfrn 31,247,391 (1943). P. DELAMY, J. Am. them. Sot. 73,4944 (1951); 74,3506,6315 (1952). D. M. H. KERN, J. Am. them. Sot. 75,2473 (1953). I. M. KOLTHOFP and E. P. PARRY,J. Am. them. Sot. 73,3718 (1951). K. WIESNER,Z. Elektrockem. 49, 164 (1943). A. I. VOOEL, A Text Book of Quantitative Inorganic Analysis. Longmans, London (1951). L. K. PARTRIDGE, A. C. TANSUY and A. S. PORTER,Electrochim. Acta 11,517 (1966). R. BRDICKA,Colln. Czech. them. Common. Engl. Edn 11,614 (1939). E. JIJRKA,Colfn. Czech. them. Commun. Engl. Ea% 11,243 (1939). L. K. PARTRIDGE, A. C. TANSLEYand A. S. PORYXR,Electrachim. Acta 12,1573 (1967).