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Electrochemical elimination of water in acetonitrile medium
Electroanalytical Chemistry and Interracial Electrochemistry
489
Elsevier Sequoia SA., L a u s a n n e - Printed in The Netherlands
SHORT COMMUNICATION
Electrochemical elimination of water in acetonitrile medium
FRANCO MAGNO and GINO BONTEMPELLI
Istituto di Chimica Analitica, Universitd di Padova, Padova, Via Loredan, 4 (Italy)
(Received 23rd March 1972)
Several methods have been suggested for preparing pure, anhydrous acetonitrile for electrochemical purposes 1. All are satisfactory, but there can be a great difference between a freshly distilled solvent and the solution containing the supporting electrolyte. The transfer of the solvent into the cell, the use of a not perfectly anhydrous supporting electrolyte and in general any contact with the atmosphere and the surroundings can raise the water content of solutions to a level considerable higher than the minimum allowed. To overcome this many authors suggest the use of molecular sieves in the final dehydration of a solution 1-3 This note reports on a very simple method for carrying out the final dehydration of a solution in the working cell. Since water undergoes an anodic oxidation on a platinum electrode in acetonitrile medium near the anodic limit of the solvent 2, we suggest its elimination by a controlled-potential electrolysis. In this procedure hydrogen ions are produced. It is however well known that the presence of H ÷ can either deeply modify the mechanism of a reaction 4- 6 or change the potential values of electrochemical reactions. Moreover the determination of protons released in an anodic oxidation can help in elucidating a complex mechanism 7- s. We therefore suggest their elimination by a second controlled potential electrolysis. Experimental Details of the electrolytic cell, electrical apparatus, solvent and NaC104 purification have been described in a previous paper s. NaBF 4 and (n-Bu)4NBF4 were prepared by neutralizing H B F 4 with Na2CO3 and (n-Bu)4NOH, respectively. In the voltammetric tests the working electrode was a platinum sphere; in the controlled potential electrolyses a platinum gauze was employed. A silver4).1 M silver nitrate-acetonitrile reference electrode was used. Results and discussion In Fig. 1 curves (a) and (b) refer to two cyclic voltammetric experiments carried out at 25°C before and after a controlled potential electrolysis at + 2.1 V vs. Ag/0.1 M Ag +. This potential was selected because it was sufficient to oxidize water but not the C102 ions. With this working potential no colour change was observed but with higher potentials a very light yellow colour appeared due to the presence of C102 in the solution 9. J. Electroanal. Chem., 39 (1972)
490
SHORT C O M M U N I C A T I O N
{i
E-
E+
200mY
I 20/uA i0× Fig. 1. Cyclic voltammetric curves with a platinum electrode : (a, ) I 0 - 3 M H20, 0.1 M NaC10 4, CH3CN soln. ; ( b , - - ) 2 x 10 -3 M H +, 0.1 M NaCIO4, CH3CN soln. ; Scan rate 0.2 Vs-1.
When the temperature was lowered the peak of the water oxidation and the anodic limit of the solvent shifted towards more positive potentials. During these electrolyses, the current dropped from about 10 mA to about
300/zA. The reduction peak in curve (b) revealed the presence of hydrogen ions as a product of the oxidation of water. A second electrolysis at - 300 mV v s . Ag/0,1 M Ag + reduced the protons to H 2 which could be removed by flowing nitrogen. The charges used in anodic and cathodic reactions were in good agreement. Consequently we believe the efficiency of the anodic current to be very close to one and the charge used a good measure of the water removed. The same results were obtained with tetrafluoroborate as supporting electroTABLE 1
Supportin 9 electrolyte
NaC104
(n-Bu)aNBF 4
NaBF4
Initial water concn. ~, [ HzO]/m mol 1-1
1.25 1.10 1.00 1.25 1.00 0.85 1.25 1.10 0.90
a Karl Fischer titration b Volumes of solns, were always 50 ml [H20]e = [ H 2 0 ] , - Q/2F
J. Electroanal. Chem., 39 (1972)
Charge in anodic reaction b,
Charge in cathodic reaction b,
QA/C
Oc/C
9.30 8.70 7.50 10.40 8.60 7.00 9.50 9.10 6.90
9.27 8.65 7.50 10.30 8.60 6.95 9.40 9.00 6.85
Final water conch, c, [H20]/m tool l l
0.29 0.21 0.22 0.19 0.11 0.13 0.28 0.17 0.20
SHORT COMMUNICATION
491
lyte. The control of the anodic potential could then be less strict since the discharge of the solvent occurs at a higher potential. It is advisable not to exceed a potential of a b o u t + 2.2 V as a light yellow colour appears at m o r e positive potentials. Since the reduction peak of H ÷ is at about - 5 0 0 m V with N a B F 4 or (n-BuhN B F 4 as supporting electrolyte, the working potential of the platinum gauze was, in this case, a b o u t - 6 0 0 m V vs. Ag/0.1 M A g ÷. Table 1 gives the results obtained with this purification method. The final water level (about 2 x 10 - 4 M) is c o m p a r a b l e with that obtained with m o r e laborious methods. Acknowledgement
This w o r k was supported by the L a b o r a t o r y of P o l a r o g r a p h y and Preparative Electrochemistry, C.N.R., Padua, Italy; this support is gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10
C. K. Mann in A. J. Bard (Ed.), Electroanalytical Chemistry, Vol. 3, Marcel Dekker, New York, 1969. J. P. Billon, J. Electroanal. Chem., 1 (1959/1960) 486. B. Kratochvil and R. Long, Anal. Chem.. 42 (1970) 43. S. Wawzonek and R. C. Duty, J. Electrochem. Soc., 108 (1961) 1135. A. H. Maki and D. H. Geske, J. Chem. Phys., 33 (1960) 825. S. H. Cadle, P. R. Tice and J. Q. Chambers, J. Phys. Chem., 71 (1967) 3517. R. C. Nelson and R. N. Adams, J. Electroanal. Chem., 16 (1968) 439. F. Magno and G. Bontempelli, J. Electroanal. Chem., in press. G. Cauquis and D. Serve, J. Electroanal. Chem., 27 (1970) App. 3. M. Fleischmann and D. Pletcher, Tetrahedron Lett., (1968) 6255.
J. Electroanal. Chem., 39 (1972)
489
Elsevier Sequoia SA., L a u s a n n e - Printed in The Netherlands
SHORT COMMUNICATION
Electrochemical elimination of water in acetonitrile medium
FRANCO MAGNO and GINO BONTEMPELLI
Istituto di Chimica Analitica, Universitd di Padova, Padova, Via Loredan, 4 (Italy)
(Received 23rd March 1972)
Several methods have been suggested for preparing pure, anhydrous acetonitrile for electrochemical purposes 1. All are satisfactory, but there can be a great difference between a freshly distilled solvent and the solution containing the supporting electrolyte. The transfer of the solvent into the cell, the use of a not perfectly anhydrous supporting electrolyte and in general any contact with the atmosphere and the surroundings can raise the water content of solutions to a level considerable higher than the minimum allowed. To overcome this many authors suggest the use of molecular sieves in the final dehydration of a solution 1-3 This note reports on a very simple method for carrying out the final dehydration of a solution in the working cell. Since water undergoes an anodic oxidation on a platinum electrode in acetonitrile medium near the anodic limit of the solvent 2, we suggest its elimination by a controlled-potential electrolysis. In this procedure hydrogen ions are produced. It is however well known that the presence of H ÷ can either deeply modify the mechanism of a reaction 4- 6 or change the potential values of electrochemical reactions. Moreover the determination of protons released in an anodic oxidation can help in elucidating a complex mechanism 7- s. We therefore suggest their elimination by a second controlled potential electrolysis. Experimental Details of the electrolytic cell, electrical apparatus, solvent and NaC104 purification have been described in a previous paper s. NaBF 4 and (n-Bu)4NBF4 were prepared by neutralizing H B F 4 with Na2CO3 and (n-Bu)4NOH, respectively. In the voltammetric tests the working electrode was a platinum sphere; in the controlled potential electrolyses a platinum gauze was employed. A silver4).1 M silver nitrate-acetonitrile reference electrode was used. Results and discussion In Fig. 1 curves (a) and (b) refer to two cyclic voltammetric experiments carried out at 25°C before and after a controlled potential electrolysis at + 2.1 V vs. Ag/0.1 M Ag +. This potential was selected because it was sufficient to oxidize water but not the C102 ions. With this working potential no colour change was observed but with higher potentials a very light yellow colour appeared due to the presence of C102 in the solution 9. J. Electroanal. Chem., 39 (1972)
490
SHORT C O M M U N I C A T I O N
{i
E-
E+
200mY
I 20/uA i0× Fig. 1. Cyclic voltammetric curves with a platinum electrode : (a, ) I 0 - 3 M H20, 0.1 M NaC10 4, CH3CN soln. ; ( b , - - ) 2 x 10 -3 M H +, 0.1 M NaCIO4, CH3CN soln. ; Scan rate 0.2 Vs-1.
When the temperature was lowered the peak of the water oxidation and the anodic limit of the solvent shifted towards more positive potentials. During these electrolyses, the current dropped from about 10 mA to about
300/zA. The reduction peak in curve (b) revealed the presence of hydrogen ions as a product of the oxidation of water. A second electrolysis at - 300 mV v s . Ag/0,1 M Ag + reduced the protons to H 2 which could be removed by flowing nitrogen. The charges used in anodic and cathodic reactions were in good agreement. Consequently we believe the efficiency of the anodic current to be very close to one and the charge used a good measure of the water removed. The same results were obtained with tetrafluoroborate as supporting electroTABLE 1
Supportin 9 electrolyte
NaC104
(n-Bu)aNBF 4
NaBF4
Initial water concn. ~, [ HzO]/m mol 1-1
1.25 1.10 1.00 1.25 1.00 0.85 1.25 1.10 0.90
a Karl Fischer titration b Volumes of solns, were always 50 ml [H20]e = [ H 2 0 ] , - Q/2F
J. Electroanal. Chem., 39 (1972)
Charge in anodic reaction b,
Charge in cathodic reaction b,
QA/C
Oc/C
9.30 8.70 7.50 10.40 8.60 7.00 9.50 9.10 6.90
9.27 8.65 7.50 10.30 8.60 6.95 9.40 9.00 6.85
Final water conch, c, [H20]/m tool l l
0.29 0.21 0.22 0.19 0.11 0.13 0.28 0.17 0.20
SHORT COMMUNICATION
491
lyte. The control of the anodic potential could then be less strict since the discharge of the solvent occurs at a higher potential. It is advisable not to exceed a potential of a b o u t + 2.2 V as a light yellow colour appears at m o r e positive potentials. Since the reduction peak of H ÷ is at about - 5 0 0 m V with N a B F 4 or (n-BuhN B F 4 as supporting electrolyte, the working potential of the platinum gauze was, in this case, a b o u t - 6 0 0 m V vs. Ag/0.1 M A g ÷. Table 1 gives the results obtained with this purification method. The final water level (about 2 x 10 - 4 M) is c o m p a r a b l e with that obtained with m o r e laborious methods. Acknowledgement
This w o r k was supported by the L a b o r a t o r y of P o l a r o g r a p h y and Preparative Electrochemistry, C.N.R., Padua, Italy; this support is gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10
C. K. Mann in A. J. Bard (Ed.), Electroanalytical Chemistry, Vol. 3, Marcel Dekker, New York, 1969. J. P. Billon, J. Electroanal. Chem., 1 (1959/1960) 486. B. Kratochvil and R. Long, Anal. Chem.. 42 (1970) 43. S. Wawzonek and R. C. Duty, J. Electrochem. Soc., 108 (1961) 1135. A. H. Maki and D. H. Geske, J. Chem. Phys., 33 (1960) 825. S. H. Cadle, P. R. Tice and J. Q. Chambers, J. Phys. Chem., 71 (1967) 3517. R. C. Nelson and R. N. Adams, J. Electroanal. Chem., 16 (1968) 439. F. Magno and G. Bontempelli, J. Electroanal. Chem., in press. G. Cauquis and D. Serve, J. Electroanal. Chem., 27 (1970) App. 3. M. Fleischmann and D. Pletcher, Tetrahedron Lett., (1968) 6255.
J. Electroanal. Chem., 39 (1972)