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Effect of protic solvents on the polarographic reduction of the alkali metal ions in hexamethylphosphoric triamide
Ekctrochimm Pergamon
.&to. Vol. 25, PP. 1247-1250.
Press
Ltd. 1980. Printed
in Great Britain
EFFECT OF PROTIC SOLVENTS ON THE POLAROGRAPHIC REDUCTION OF THE ALKALI METAL IONS IN HEXAMETHYLPHOSPHORIC TRIAMIDE* SACHIKO
Department
SAmmAt, HEUNG
of Chemistry,
LEEI and TAITIRO FUJINAGA
LARK
Faculty of Science, University (Received
of Kyoto, Sakyo-ku, Kyoto 606, Japan
26 November 1979)
Abstract-Sodium and lithium ions are not reduced at the dropping mercury electrode in hexamethylphosperchlorate while rubidium and caesium phoric triamide containing 0.05 mol dm- 3 tetraethylammonium ions show diffusion controlled, l-electron reduction waves. Thereduction of potassium ion is partially kinetic. When increasing amount of water is added to the solution, reduction waves of sodium and lithium begin to appear, and the. kinetic contribution of potassium wave gradually disappears to give a diffusion controlled wave. The diffusion controlled waves ofrubidium and caesium ions are essentially unaffected. The addition of methanol, ethanol, I-buthanol and dimethyl sulfoxide also has similar enhancing effects, which is attributed to the acceleration of the desolvation process by protons. These effects of protic solvent areapparently similar to that of the cation size of the supporting electrolytes, but of different @in, as indicated by the different behavior of the electrocapillary curves.
INTRODUCTION
Effects of water on the reduction of metal ions in aprotic solutions are very complicated, being influenced by the nature of the solvent, solute, electrode and by many other factors. There are a number of reports concerning the effects of water on the redox reactions of organic compounds[ l] as well as the effects of water on the behaviour of solid electrodes, e.g. platinum, gold, or amalgam electrodes[2]. However, the effects of water on the reduction of metal ions have not been studied systematically. The polarographic reduction of various metal ions in hexamethylphosphoric triamide (HMPA) has been reported previously[3]. The present paper deals with the effects of water and several protic solvents on the reduction of the alkali metal ions in HMPA solutions.
three-electrode polarograph, Type PB-DP. The measurements were carried out at 25°C f O.l”C. All the potential were referred to a freshly-prepared Ag-O.1 M AgC104 (HMPA) electrode, which had a potential of + 0.36 V us aq. see in 0.05 M EthNCIO,HMPA. A platinum wire served as the auxiliary electrode. Two dropping mercury electrodes were used: under a mercury head of 62 cm their m-values at open circuit were (A) 1.72 mgs-’ and (B) 1.76mg 5-l and the drop time at -2.6V were 1.40s and 1.45 s, respectively. The purification of commercially obtained HMPA was described previously[3]. Methanol (MeOH), ethanol (EtOH), and l-butanol (BuOH) were of G. R. grade and used without further purification. G. R. grade dimethyl sulfoxide (DMSO) was purified by distilling it under a reduced pressure.
EXPERIMENTAL
I
I
I
I
-2.6
-2.8
-3.0
I
All solutions of the metal ions were prepared from the anhydrous perchlorates. The supporting electrdlytcs were tetraethylammonium perchlorate (Et,NCIO,), tetrapropylammonium perchlorate (Pr,NCIO,), tetrabutylammonium perchlorate (Bu,NClO,) and tetrahexylammonium perchlorate (Hex.+NClO& Each solution was 0.05 M (M = moldm-‘). The dc polarograms were recorded with a Yanagimoto
-2.0
* Electrochemical Studies in Hexamethyfphosphoric triamide, Part 10: Part 9 ; S. Satura, R. Nakata and T. Fujinaga, Bull. Chem. Sot. Jpn. $3, 545 (1980). t Present address : Department of Chemistry, Hamamatsu University School of Medicine, Hamamatsu, 431-31 Japan, : Fresent address : Department of Chemistry, College of Liberal Arts and Sciences, Kyunpook National University, Taegu, 635 Korea. E *.
25/l o---B
-2.2
-2.4
V
Fig. 1. Polarograms of 1 mM alkali metal ions in 0.05 M Et,NCIO,-HMPA at 25.0% (1) Cs+; (2) Rb+ ; (3) K’ and (4) Na’, Li+ and the residual current. DME A was used.
1247
SA(.HIKO
1248
SAKURA,
HBIING
LARK
Len ANU TA~TIRO FIIJINAGA
RESULTS AND DISCUSSION
The polarograms of the alkali metal ions in 0.05 M Et,NClO,-HMPA solution areshown in Fig. 1. In this solution, caesium and rubidium ions were reduced at around - 2.3 V, whereas potassium ion was reduced at ca. - 2.4 V, its limiting current being about one-half of those of the first two. Sodium and lithium ions gave no waves before the final rise. The limiting currents of caesium and rubidium ions were diffusion-controlled and that of potassium was controlled by a preceding reaction[3]. Figure 2 shows the changes in the limiting currents of the reductions of the alkali metal ions in 0.05 M EtqNCIO-HMPA 1Oml solution when water was added to the solution. On addition of water, the diffusion currents of caesium and rubidium were little affected while the limiting current of potassium increased rapidly to reach an almost constant value; a reduction wave of sodium ion began to appear, and a well-developed wave was observed when sufficient water was added. A similar effect was observed with lithium but to a lesser extent. As seen in Fig. 3, where the relationship between the square root of the mercury height (h1j2) and the limiting current divided by the depolarizer concentration (ii/c) is shown in the cases of the reductions of rubidium and potassium, the reduction of rubidium ion was diffusion controlled both in the presence and in the absence of water. The kinetic contribution of the limiting current of potassium ions gradually disappeared with the addition of water and became diffusion controlled, when the limiting current reached the constant value (cf Fig. 2). A similar transition from kinetic to diffusion controlled current was observed in the case of sodium.
I
I
I
I
I
0.4 -
(4)
0.2-J/
o0
.
0.1
15)
-
I
,
I
I
0.2
0.3
0.4
0.5
+I&
Fig. 2. Effect of water on the reduction of 1 mM alkali metal ions in 0.05 M Et,NC!IO,-HMPA at 25.0% (1) Cs+; (2) Rb l ; (3) K + ; (4) Na + and IS) Li +. The limitina currents were tieasu&d at 1 i.6 V and h = 62cm, except 6~ Li*, whose
limiting currents were measured at - 2.9 V, h = 70 cm. The dme used A for 1,2,3 and B for 4,5. Corrected for the residual current and the volume change.
Fig. 3. Dependence of ii/c on h’f2 in 0.05 M Et,NCIO,HMPA (dme A). Experimental condition was the same with
Figs. 1 and (2). (1) 0 Rb+, xHIO=O; (2) l Rb+, xHIO=0.327,; (3) 0 K+, x,,, = 0; (4) q K*, xHIO= 0.046, ; and (5) m K+, xHZo= 0.327,.
This transition should reflect the acceleration of the rate determining step prior to the electron transfer. In the present systems the alkali salts and the supporting electrolytes are completely dissociated as has been demonstrated by the conductivity measurements[4], and no ion-association is taking place. HMPA is a very basic solvent and it strongly solvates cations, especially those of smaller sizes. It seems quite reasonable that the rate-determining step of the preceding reaction is the desolvation process, which is accelerated by the presence of water. The reduction of sodium ion in Et4NC104 was accelerated also by the other protic solvents (Fig. 4). Although DMSO is usually classified to the aprotic solvent, sometimes it is also classified to the protic solvent, because its autoprotolysis constant pK, is about 17.3 and methylsulfinylmethanide ion (CH,SOCH; ; often abbreviated as dimsyl and so on) exists stably as the complexes with the sodium or potassium ions[5]. The effect of DtiSO on the limiting currents of the akali metal ions (Fig. 5) was very much alike to that of water. Again the limiting current of potassium ion lost its kinetic character on the addition of DMSO just as in the case of water in Fig. 2. Therefore, the accelerating effect of these solvents is most probably attributable to their proton donating nature. As has been reported[3] the size of the cation of the supporting electrolyte has a similar acceleration effect in the absence of protic solvent. As shown in Table 1, the larger the size of the supporting electrolyte cations, the faster is the desolvation. These two effects - the effect of water in a given supporting electrolyte and the
Effect of protic Table Metal ion Supporting Et,N +
1. The reduction
solvents
on polarographic
of metal ions in HMPA
reduction
(in the absence
1249
of water, at 25.O”C) CS+
Rb+
Li+
Na+
K+
N.R.*
N.R.
Bu4N+
N.R.
Hcx,N+
N.R.
Kinetic control Almost diffusion control
Kinetic control Diffusion control Diffusion control
electrolyte Diffusion control Diffusion control Diffusion control
Diffusion control Diffusion control Diffusion control
Supporting electrolyte in 0.05 M. * : N.R. denotes that the metal ion is “not reduced”in this solution.
I.0 4
0
(A: PO& sdventsl
0. I
0.2
0.3
0.4
0.5
Fig. 4. Effectof the protic solventson the limitingcurrentsof reduction of 2.016mM Naf in (dme B) at 25.O”C. (1) water; EtOH and (5) BuOH. Corrected the volume
0.05 M EtbNCIOI-HMPA (3) MeOH; (4) for the residual current and change.
(2) DMSO;
Fig. 6. Effect of water on the reduction of 1 mM sodium ion in various supporting electrolytes in HMPA (dme A) at 25.O”C. Experimentalcondition was the same with Figs. 1 and 2. (1) Et,NClO, ; (2) Pr,NClO,; (3) Bu,NClO, and (4) Hex,NClO,. Supporting electrolyte concentration, 0.05 M.
effect of the cation size in the absence of water-can be superimposed. For example, the dependence of the limiting current of sodium ion on water content in Bu4NC10, potassium
(Fig. 6) is much the same as that of ion in Et,NClO, (cfTable 1 and Fig. 2).
Interestingly, however, the electrocapillary curve remained unchanged when water was added to a given supporting electrolyte solution, whereas the electrocapillary curves in the different tetraalkylammonium salt solutions were quite different[3]. These facts indicate that two effects, though apparently similar, are essentially different ; the acceleration effect of the cation size is related to the change of the interfacial structure but that of protic solvents is not.
0
0.04
0 OS
0 I*
0 16
0
ao
0 24
XOMM Fig. 5. Effect of DMSO on the reduction of 1 mM alkali metal ions in 0.05 M Et&NClO*-HMPA (dme B) at 25.O”C. Experimental condition was the same with Fig. 4. (1) Cs+ ; (2)
Rb+ ; (3) K+ and (4) Na+.
Acknowledgement-The authors should like to express their thanks to Prof. K. Izutsu of Shinshu University and Prof. Gen. P. Satb of Sophia University for their helpful suggestion and discussion. This work is supported in part by the Grantin-Aid for Scientific Research from the Ministry of Education.
1250
SACHIKO SAKURA, HEUNG LARK LEE AND TAITIRO FUJINAGA
REFERENCES
1. For example, T. Fujinaga, K. Izutsu and K. Takaoka, J. electmanal. Chem. 12,203 (1966). 2. J. Courtot-Coupez and M. L’Her, Bull. Sot. Chim. Fr. 1631 (1970). 3. K. Izutsu, S. Sakura and T. Fujinaga, BuIl. Chem. Sot.
JP.
45, 445 (1972); ibid., 46, 493 (1973); S. Sakura,
J.
electroanal.chml. 80, 315 (1977).
4. T. Fujinaga, K. Izutsu and S. Sakura, Nippon Kagaku Kaishi 191 (1973). 5. I. M. Koltboff, T. Il. Reddy, Inorg. Chem. 1, 189 (1962); Chem. Engng News 44, 48 (1966).
.&to. Vol. 25, PP. 1247-1250.
Press
Ltd. 1980. Printed
in Great Britain
EFFECT OF PROTIC SOLVENTS ON THE POLAROGRAPHIC REDUCTION OF THE ALKALI METAL IONS IN HEXAMETHYLPHOSPHORIC TRIAMIDE* SACHIKO
Department
SAmmAt, HEUNG
of Chemistry,
LEEI and TAITIRO FUJINAGA
LARK
Faculty of Science, University (Received
of Kyoto, Sakyo-ku, Kyoto 606, Japan
26 November 1979)
Abstract-Sodium and lithium ions are not reduced at the dropping mercury electrode in hexamethylphosperchlorate while rubidium and caesium phoric triamide containing 0.05 mol dm- 3 tetraethylammonium ions show diffusion controlled, l-electron reduction waves. Thereduction of potassium ion is partially kinetic. When increasing amount of water is added to the solution, reduction waves of sodium and lithium begin to appear, and the. kinetic contribution of potassium wave gradually disappears to give a diffusion controlled wave. The diffusion controlled waves ofrubidium and caesium ions are essentially unaffected. The addition of methanol, ethanol, I-buthanol and dimethyl sulfoxide also has similar enhancing effects, which is attributed to the acceleration of the desolvation process by protons. These effects of protic solvent areapparently similar to that of the cation size of the supporting electrolytes, but of different @in, as indicated by the different behavior of the electrocapillary curves.
INTRODUCTION
Effects of water on the reduction of metal ions in aprotic solutions are very complicated, being influenced by the nature of the solvent, solute, electrode and by many other factors. There are a number of reports concerning the effects of water on the redox reactions of organic compounds[ l] as well as the effects of water on the behaviour of solid electrodes, e.g. platinum, gold, or amalgam electrodes[2]. However, the effects of water on the reduction of metal ions have not been studied systematically. The polarographic reduction of various metal ions in hexamethylphosphoric triamide (HMPA) has been reported previously[3]. The present paper deals with the effects of water and several protic solvents on the reduction of the alkali metal ions in HMPA solutions.
three-electrode polarograph, Type PB-DP. The measurements were carried out at 25°C f O.l”C. All the potential were referred to a freshly-prepared Ag-O.1 M AgC104 (HMPA) electrode, which had a potential of + 0.36 V us aq. see in 0.05 M EthNCIO,HMPA. A platinum wire served as the auxiliary electrode. Two dropping mercury electrodes were used: under a mercury head of 62 cm their m-values at open circuit were (A) 1.72 mgs-’ and (B) 1.76mg 5-l and the drop time at -2.6V were 1.40s and 1.45 s, respectively. The purification of commercially obtained HMPA was described previously[3]. Methanol (MeOH), ethanol (EtOH), and l-butanol (BuOH) were of G. R. grade and used without further purification. G. R. grade dimethyl sulfoxide (DMSO) was purified by distilling it under a reduced pressure.
EXPERIMENTAL
I
I
I
I
-2.6
-2.8
-3.0
I
All solutions of the metal ions were prepared from the anhydrous perchlorates. The supporting electrdlytcs were tetraethylammonium perchlorate (Et,NCIO,), tetrapropylammonium perchlorate (Pr,NCIO,), tetrabutylammonium perchlorate (Bu,NClO,) and tetrahexylammonium perchlorate (Hex.+NClO& Each solution was 0.05 M (M = moldm-‘). The dc polarograms were recorded with a Yanagimoto
-2.0
* Electrochemical Studies in Hexamethyfphosphoric triamide, Part 10: Part 9 ; S. Satura, R. Nakata and T. Fujinaga, Bull. Chem. Sot. Jpn. $3, 545 (1980). t Present address : Department of Chemistry, Hamamatsu University School of Medicine, Hamamatsu, 431-31 Japan, : Fresent address : Department of Chemistry, College of Liberal Arts and Sciences, Kyunpook National University, Taegu, 635 Korea. E *.
25/l o---B
-2.2
-2.4
V
Fig. 1. Polarograms of 1 mM alkali metal ions in 0.05 M Et,NCIO,-HMPA at 25.0% (1) Cs+; (2) Rb+ ; (3) K’ and (4) Na’, Li+ and the residual current. DME A was used.
1247
SA(.HIKO
1248
SAKURA,
HBIING
LARK
Len ANU TA~TIRO FIIJINAGA
RESULTS AND DISCUSSION
The polarograms of the alkali metal ions in 0.05 M Et,NClO,-HMPA solution areshown in Fig. 1. In this solution, caesium and rubidium ions were reduced at around - 2.3 V, whereas potassium ion was reduced at ca. - 2.4 V, its limiting current being about one-half of those of the first two. Sodium and lithium ions gave no waves before the final rise. The limiting currents of caesium and rubidium ions were diffusion-controlled and that of potassium was controlled by a preceding reaction[3]. Figure 2 shows the changes in the limiting currents of the reductions of the alkali metal ions in 0.05 M EtqNCIO-HMPA 1Oml solution when water was added to the solution. On addition of water, the diffusion currents of caesium and rubidium were little affected while the limiting current of potassium increased rapidly to reach an almost constant value; a reduction wave of sodium ion began to appear, and a well-developed wave was observed when sufficient water was added. A similar effect was observed with lithium but to a lesser extent. As seen in Fig. 3, where the relationship between the square root of the mercury height (h1j2) and the limiting current divided by the depolarizer concentration (ii/c) is shown in the cases of the reductions of rubidium and potassium, the reduction of rubidium ion was diffusion controlled both in the presence and in the absence of water. The kinetic contribution of the limiting current of potassium ions gradually disappeared with the addition of water and became diffusion controlled, when the limiting current reached the constant value (cf Fig. 2). A similar transition from kinetic to diffusion controlled current was observed in the case of sodium.
I
I
I
I
I
0.4 -
(4)
0.2-J/
o0
.
0.1
15)
-
I
,
I
I
0.2
0.3
0.4
0.5
+I&
Fig. 2. Effect of water on the reduction of 1 mM alkali metal ions in 0.05 M Et,NC!IO,-HMPA at 25.0% (1) Cs+; (2) Rb l ; (3) K + ; (4) Na + and IS) Li +. The limitina currents were tieasu&d at 1 i.6 V and h = 62cm, except 6~ Li*, whose
limiting currents were measured at - 2.9 V, h = 70 cm. The dme used A for 1,2,3 and B for 4,5. Corrected for the residual current and the volume change.
Fig. 3. Dependence of ii/c on h’f2 in 0.05 M Et,NCIO,HMPA (dme A). Experimental condition was the same with
Figs. 1 and (2). (1) 0 Rb+, xHIO=O; (2) l Rb+, xHIO=0.327,; (3) 0 K+, x,,, = 0; (4) q K*, xHIO= 0.046, ; and (5) m K+, xHZo= 0.327,.
This transition should reflect the acceleration of the rate determining step prior to the electron transfer. In the present systems the alkali salts and the supporting electrolytes are completely dissociated as has been demonstrated by the conductivity measurements[4], and no ion-association is taking place. HMPA is a very basic solvent and it strongly solvates cations, especially those of smaller sizes. It seems quite reasonable that the rate-determining step of the preceding reaction is the desolvation process, which is accelerated by the presence of water. The reduction of sodium ion in Et4NC104 was accelerated also by the other protic solvents (Fig. 4). Although DMSO is usually classified to the aprotic solvent, sometimes it is also classified to the protic solvent, because its autoprotolysis constant pK, is about 17.3 and methylsulfinylmethanide ion (CH,SOCH; ; often abbreviated as dimsyl and so on) exists stably as the complexes with the sodium or potassium ions[5]. The effect of DtiSO on the limiting currents of the akali metal ions (Fig. 5) was very much alike to that of water. Again the limiting current of potassium ion lost its kinetic character on the addition of DMSO just as in the case of water in Fig. 2. Therefore, the accelerating effect of these solvents is most probably attributable to their proton donating nature. As has been reported[3] the size of the cation of the supporting electrolyte has a similar acceleration effect in the absence of protic solvent. As shown in Table 1, the larger the size of the supporting electrolyte cations, the faster is the desolvation. These two effects - the effect of water in a given supporting electrolyte and the
Effect of protic Table Metal ion Supporting Et,N +
1. The reduction
solvents
on polarographic
of metal ions in HMPA
reduction
(in the absence
1249
of water, at 25.O”C) CS+
Rb+
Li+
Na+
K+
N.R.*
N.R.
Bu4N+
N.R.
Hcx,N+
N.R.
Kinetic control Almost diffusion control
Kinetic control Diffusion control Diffusion control
electrolyte Diffusion control Diffusion control Diffusion control
Diffusion control Diffusion control Diffusion control
Supporting electrolyte in 0.05 M. * : N.R. denotes that the metal ion is “not reduced”in this solution.
I.0 4
0
(A: PO& sdventsl
0. I
0.2
0.3
0.4
0.5
Fig. 4. Effectof the protic solventson the limitingcurrentsof reduction of 2.016mM Naf in (dme B) at 25.O”C. (1) water; EtOH and (5) BuOH. Corrected the volume
0.05 M EtbNCIOI-HMPA (3) MeOH; (4) for the residual current and change.
(2) DMSO;
Fig. 6. Effect of water on the reduction of 1 mM sodium ion in various supporting electrolytes in HMPA (dme A) at 25.O”C. Experimentalcondition was the same with Figs. 1 and 2. (1) Et,NClO, ; (2) Pr,NClO,; (3) Bu,NClO, and (4) Hex,NClO,. Supporting electrolyte concentration, 0.05 M.
effect of the cation size in the absence of water-can be superimposed. For example, the dependence of the limiting current of sodium ion on water content in Bu4NC10, potassium
(Fig. 6) is much the same as that of ion in Et,NClO, (cfTable 1 and Fig. 2).
Interestingly, however, the electrocapillary curve remained unchanged when water was added to a given supporting electrolyte solution, whereas the electrocapillary curves in the different tetraalkylammonium salt solutions were quite different[3]. These facts indicate that two effects, though apparently similar, are essentially different ; the acceleration effect of the cation size is related to the change of the interfacial structure but that of protic solvents is not.
0
0.04
0 OS
0 I*
0 16
0
ao
0 24
XOMM Fig. 5. Effect of DMSO on the reduction of 1 mM alkali metal ions in 0.05 M Et&NClO*-HMPA (dme B) at 25.O”C. Experimental condition was the same with Fig. 4. (1) Cs+ ; (2)
Rb+ ; (3) K+ and (4) Na+.
Acknowledgement-The authors should like to express their thanks to Prof. K. Izutsu of Shinshu University and Prof. Gen. P. Satb of Sophia University for their helpful suggestion and discussion. This work is supported in part by the Grantin-Aid for Scientific Research from the Ministry of Education.
1250
SACHIKO SAKURA, HEUNG LARK LEE AND TAITIRO FUJINAGA
REFERENCES
1. For example, T. Fujinaga, K. Izutsu and K. Takaoka, J. electmanal. Chem. 12,203 (1966). 2. J. Courtot-Coupez and M. L’Her, Bull. Sot. Chim. Fr. 1631 (1970). 3. K. Izutsu, S. Sakura and T. Fujinaga, BuIl. Chem. Sot.
JP.
45, 445 (1972); ibid., 46, 493 (1973); S. Sakura,
J.
electroanal.chml. 80, 315 (1977).
4. T. Fujinaga, K. Izutsu and S. Sakura, Nippon Kagaku Kaishi 191 (1973). 5. I. M. Koltboff, T. Il. Reddy, Inorg. Chem. 1, 189 (1962); Chem. Engng News 44, 48 (1966).