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Electrode Potential of Nickel
H. von Euler gave for the electrode potential of nickel against the calomel electrode and N-NSO4, 0.466 volt, and with 0.2N-NSO4, 0.472 volt; taking the calomel electrode as 0.560 volt, these data become, respectively, -0.094 and -0.088 volt. B. Neumann gave -0.022 volt for the electrode potential of nickel against N-NSO4, -0.020 volt against N-NCl2, and -0.060 volt against N-N(NO3)2. There is, indeed, a striking difference in the values reported by different observers for the electrode potential of nickel in a normal soln. of the sulphate. In addition to the values just quoted, N. T. M. Wilsmore and W. Ostwald gave -0.049 volt; W. Pfannhausen, -0.041 and -0.060 volt for nickel in N-NSO4 and N-(NH4)2Ni(SO4)2, respectively; O. Bauer, for nickel in a 1 per cent. soln. of sodium chloride, and a normal calomel electrode, -0.213 volt at the start, and -0.080 volt after 120 hrs.; A. Siemens, -0.036 volt; E. Newbery, -0.031 volt; and A. Schweitzer, -0.033 volt; N. M. Haring and E. G. van der Bosche, -0.231 volt at 25°; F. W. Kuster, -0.243 volt; W. Muthmann and F. Fraunberger, -0.323 volt; and T. Heymann and K. Jellinek, -0.268 volt at 25°. Observations were also made by S. Bodforss, K. Murata, O. Erbacher, V. Sihvonen and O. Enwald, B. B. Banerji, and K. Georgi. N. R. Dhar studied the e.m.f. of soln. of potassium chloride against nickel; C. Bedel, a soln. of sodium hydroxide, hydrofluoric acid, and sulphuric acid; E. Muller and J. Janitzky used a soln. of nickel chloride; H. N. Huntzicker and L. Kahlenberg, soln. of salts of copper, silver, and nickel; and G. B. Kistiakowsky, in hydrogen, and nitrogen. S. Makishima studied the subject. E. Newbery attributed the discrepancies to the use of electrolytic nickel, or of nickel treated electrolytically before use, for he considered that under these conditions the metal contained x per cent, of occluded hydrogen or nickel hydride. He observed that the electrode potential of nickel depends largely on the method of preparing the electrode and on its subjection to intermittent anodic action. A very slight motion of the nickel electrode suffices to raise the potential 0.07 volt, and a rapid shaking of the electrode in the soln. raised the potential 0.11 volt. A current of 10-5 amp. is sufficient to alter the cathode potential 0.1 volt; whilst the value for the anode potential remains nearly constant until the current passing is nearly a thousand times this value. Hence, in agreement with L. Colombier, it was inferred that adsorbed hydrogen (or oxygen) affects seriously the cathode potential of nickel, although K. Murata said that hydrogen has no effect on the true electrode potential of nickel except in the presence of oxygen, which should be excluded when measurements are made. K. Georgi, and P. H. Dowling studied the subject. A. Schweitzer observed that the electrode potential of a cathode, in which nickel is being deposited, becomes more and more positive as the current density increases and as the temp, falls. At 16°, with nickel powder, charged with hydrogen, the electrode potential of nickel in N-NSO4 is -0.33 volt, and in 0.1N-NSO4, -0.35 volt; whilst in N-NCl2, and 0.lN-NCl2 the potentials are respectively -0.31 and -0.36 volt - the potential of the normal calomel electrode is 0.283 volt. With sheet nickel in N-NSO4 the potential is 0.31 volt, and in 0.1N-NSO4, -0.336 volt. These results were determined in an atm. of hydrogen. The potential required to deposit nickel is higher than the values just indicated, even with the smallest current density. A. Schweitzer suggested that possibly a nickel-hydrogen alloy is formed which gives higher potentials. The potential corresponding with N+2+=N•• cannot be measured if traces of oxygen are present. The potentials in an atm. of hydrogen may therefore be too high, and those without hydrogen too low. Hence, the real value for nickel in N-NSO4 lies between E. P. Schoch's value –0.2 volt, and the above mentioned -0.33 volt. A. Smits pointed out that nickel is a very inert metal, and that the presence of hydrogen ions or of molecular hydrogen can profoundly modify the electrode so that its potential becomes the same as that of a hydrogen electrode. An atm. of hydrogen should not therefore be employed for the measurements, but preferably an atm. of nitrogen, or else the containing vessel should be completely filled with the soln. which has been previously boiled in vacuo. In this way, A. Smits obtained for the potential of the nickel electrode relative to the normal calomel electrode -0.480 volt, or with reference to the hydrogen electrode, 0.194 volt. A. Oliverio and O. Belfiori studied zinc and nickel electrodes in soln. of nitrates.
E. Newbery gave -0.32 volt for the electrode potential of nickel in N-NSO4; -0.34 volt in N-N(NO3)2; and -0.32 volt in N-NCl2, and in N-(NH4)2N(SO4)2. He found that the electrode potential of nickel in 0.5N-NSO4 assumes three different values under different conditions, namely, -0.25 and -0.65 volt, with an intermediate value -0.53 volt at 25°. He explained this result by assuming that solid nickel contains two allotropes with normal electrode potentials approximating -0.5 and -0.0 volt respectively when referred to the normal hydrogen electrode; an equilibrium mixture of the two allotropes, at ordinary temp., has a potential of -0.17 volt. S. Triandafil studied the effect of temp. A. M. Hasebrink observed that the potential of nickel rubbed with emery in an indifferent atm. falls at first, then recovers partially, and, after repeated rubbings, the potential becomes constant at a value which is lower than the initial one.
S. J. French and L. Kahlenberg found that the potential of nickel in N-KCl in hydrogen becomes more basic, reaching a maximum and then falling, and likewise also in nitrogen; in oxygen the potential becomes less basic, reaching a maximum and then falling off in bubbling gas, but in a quiescent state the potential becomes more basic. L. Kahlenberg and J. V. Steinle observed that the single potential of nickel in 0.5N-Na3AsO4 is 0.216 volt; in 0.5N-K3AsO4, 0.241 volt; and a N-KCl sat. with arsenic trioxide, 0.228 volt. G. W. Smith and L. H. Reyerson studied the electrokinetic potential; G. Tammann and E. Jenckel, the potential in soln. of potassium hydroxide under press.; P. Bechtereff, the potential of nickel in fused sodium hydroxide; and O. Scarpa, the resistance at the contact surface of electrode and electrolyte.
M. H. Jacobi studied the cell N: dil. HNO3: soln. KCl: Zn (Cu, Cd, Sn, Ag, or N); J. Regnauld amalgamated and unamalgamated zinc with a soln. of zinc sulphate, nitrate, or chloride and the corresponding nickel salt and nickel; A. Schweitzer gave -0.596 volt for the e.m.f. of the cell N: 0.5N-NCl2: N-KCl: Hg2Cl2: Hg; and C. von Neumann, the e.m.f. of the nickel-carbon cell with nitric acid, aqua regia, or sulphuric acid as exciting liquid. C. H. Prescott and M. J. Kelly studied the cell with a nickel wire anode, and an oxidized silver foil with a deposit of caesium. N. A. Furman and G. W. Low studied the use of nickel tungsten electrodes in the electrometric titration of strong acids and bases. P. Jolibois found that a nickel salt soln. and distilled water in separate vessels connected by a U-tube, with a platinum electrode, furnished nickel hydroxide.
J. Tafel observed that the potential of a nickel cathode goes on increasing for hours, and he considered that his results do not favour the hypothesis that the varying polarization is due to the varying thickness of a layer of gas on the electrode, but rather does the cathode surface have different catalytic effects on the process of forming hydrogen gas. E. P. Schoch gave -0.48 volt for the true equilibrium potential of nickel in N-NSO4. This value was attained in about 12 hrs. with an electrode of commercial nickel immersed in a soln. of nickel sulphate which had been boiled for some minutes, and allowed to cool in contact with air. The soln. is neutral to litmus. The potential is diminished by air or oxygen; it is increased by hydrogen, and lowered by a slight acidity of the soln. Occluded hydrogen increases the potential so that the electrodeposition of nickel does not take place until the potential is 0.2 to 0.3 volt higher than the equilibrium value, and hydrogen simultaneously appears on the electrode. The equilibrium potential of finely- divided nickel in N-NSO4 was taken to be -0.52 volt.
S. Glasstone found the cathode potential of nickel in N-NSO4, at 15°, for current densities D amp. ×l0-4 per sq. cm., to be:
The values at 95° were:
C. G. Fink and C. M. Decroly found that the contact potentials of nickel against sulphuric acid of the following percentage compositions are:
E. Vigouroux observed that with nickel electrodes in N-Ag2SO4, the e.m.f. varies from -0.0021 to +0.0168 volt; and A. Krupkowsky gave -0.490 to -0.506 volt for the e.m.f. of nickel in N-NH4NO3 containing 1 per cent, each of nickel and cobalt nitrates. The potentials of nickel and of platinum in 140 c.c. Of 0.1N-soln. of nickel nitrate containing 0.4, 0.6, 0.8, and 1.0 c.c. of a 3 per cent. soln. of bromine water were measured by A. Smits. As shown in Fig., the potential of the nickel anode rises immediately after the addition of bromine water, attains a maximum, and then sinks rapidly. The potential of platinum also rises at the beginning, but soon attains a state of equilibrium. Nickel is therefore greatly disturbed by the corrosive action of bromine, but this disturbance is soon compensated by the activating influence of the bromine ions thus formed. The potential of nickel also falls in the presence of hypochlorite, and less so in the presence of potassium permanganate. M. Ballay examined the influence of oxidizing agents on the electrodeposition of nickel, and found that with 0.175 grm. per litre of potassium permanganate, the current efficiency was 97 per cent, without a perceptible evolution of hydrogen, but with more permanganate, the efficiency falls to 93 per cent., and the deposit is brittle. With an acidity of pH=6.8, the deposit is brittle, and the efficiency is 96.2 per cent.; with pH=6.1, the efficiency is 97.8 per cent.; and with pH=5.5 to 4.4, the efficiency is 95.4 to 94.9 per cent, and the deposit is good - vide supra.
According to A. Smits and C. A. L. de Bruyn, in the case of a metal like nickel which is violently disturbed by soln. containing oxygen by depositing the metal on the cathode in air, potentials can be obtained for very small current densities which are always somewhat less negative than the equilibrium potential; and this will be the case so long as the rate of cathode deposition is so small that corrosion can appreciably affect the deposited metal. For very small current densities, the potential of nickel during cathodic deposition remains less negative than the equilibrium potential; only at higher current densities does the potential become more negative, and only then does the deposition occur. The equilibrium potential is -0.48 volt, and even when the current density is 200×10-6 amp. per sq. cm., the potential is less negative than the equilibrium potential. With a soln. of a nickel halide instead of the nitrate or sulphate, the potential of nickel passes more quickly through the value of the equilibrium potential. So long as the rate of deposition is small, the deposit can be appreciably affected by dissolved oxygen - halogen ions exert a marked catalytic influence tending to annul the disturbance. At greater current densities, the disturbing influence of dissolved oxygen will become relatively smaller, and the positive catalytic influence relatively decrease, so that if the current density is allowed to increase, the nickel potential must soon pass through the value of the equilibrium potential, and it will exhibit cathodic polarization which increases strongly at first, and afterwards more slowly.
In the electrolysis of a soln. of a metal salt, say nickel sulphate, between soluble electrodes of the same metal, nickel, the concentration of the nickel ions in the film of soln. near the cathode decreases, and increases near the anode. As a result, the cathode becomes more negative, and the anode more positive; in other words, the electrodes are polarized. The static or equilibrium potential of the metal is the single electrode potential when no current is flowing, and the dynamic potential is the single potential when the current is flowing at a specified current density; the difference between the two potentials is the polarization at the specified current density. If the dynamic potential is more negative than the static potential, the polarization is cathodic, and if less negative, anodic. The relations between the static and dynamic potentials of nickel in a soln. of N-NSO4, 0.25N-NH4Cl, and 0.25N-H3BO3 are shown in Fig. due to W. Blum, and the general treatment here is also due to him. The static potential of nickel is here assumed to be -0.26 volt, i.e., 0.03 volt more negative than the normal potential of nickel -0.23 volt. The conc. of the N••-ion in this soln. is about 0.1N. The curve for a typical cathode current density-potential curve for nickel deposition is shown along with a typical current density-potential curve for anode polarization with active nickel. The anodic or cathodic polarization at any specified current density is equal to the difference between the potentials of the points corresponding with the equilibrium potential and the current density on appropriate curves.
The main factors tending to counteract the changes in concentration about the electrodes, and to decrease the polarization are ionization, migration, diffusion, and convection currents whether produced by changes of temp, or sp. gr. resulting from electrolysis, by stirring due to the escape of gases or external agitation. The polarization may be due not only to a change in metal-ion concentration of the adjacent soln., but also to a change in the soln. press, of the cathode. For pure metals, such an effect is usually very slight. However, in alloy deposition, or when hydrogen is discharged simultaneously with a metal, the composition and solution pressure of the cathode may change during electrolysis, and thus may cause an effect upon the polarization. N. A. Isgarischeff and H. Ravikovitsch investigated the effect of neutral salts on the cathodic polarization of nickel; S. Triandafil, the effect of acidity. G. Athanasiu examined the effect of illuminating the electrodes; G. W. Smith and L. H. Reyerson, agitation of the electrolyte. W. J. Muller and K. Konopicky, and 0. Scarpa and E. Denina studied the subject.
The potentials at the electrodes during electrolysis determine the anode and cathode efficiencies. In accord with Faraday's law, the total electrode efficiency is always 100 per cent, when all the products of electrolysis, or changes in valency, are considered - where, in electrodeposition, the valency of the metal ions is reduced to zero, the valency of metals in the free state, the cathode efficiency is determined by the proportion of current used in the discharge of the metal. It is sometimes erroneously assumed that the tendency for the discharge of hydrogen from any metal salt soln. will depend on the relative positions of hydrogen and metal in the electrochemical series; actually, the respective concentrations of H•-ions and metal-ions in a given soln. are not likely to be normal, or even equivalent to one another. In nickel-plating, the NSO4-content of the electrolyte may be about 0.1N, but the H•-ion content will usually be less, e.g., 10-6N, or pH=6. As a first approximation it can be assumed that the initial tendencies for the discharge of hydrogen and metal ions will depend on their respective static potentials in that soln. The actual proportions of metal and hydrogen discharged at any current density will be in accord with the respective current density-potential curves for metal deposition and for hydrogen discharge on that metal in that soln. W. Blum's curve for hydrogen evolution on nickel in the electrolysis of the given soln., and the curve for actual nickel deposition, are shown in Fig. At any given current density the ordinate of A is equal to the sum of the ordinates of G and D. The metal cathode efficiency at that current density is equal to the ratio of the corresponding ordinates of D and A. Under these conditions, the nickel cathode efficiency will increase as the current density is increased, but can never reach 100 per cent. If, however, as in acid copper soln., the static potential of the metal is much more positive than that of hydrogen in that soln., the cathode efficiency at low current densities will be 100 per cent., and will decrease as the current density is increased.
E. Newbery gave -0.32 volt for the electrode potential of nickel in N-NSO4; -0.34 volt in N-N(NO3)2; and -0.32 volt in N-NCl2, and in N-(NH4)2N(SO4)2. He found that the electrode potential of nickel in 0.5N-NSO4 assumes three different values under different conditions, namely, -0.25 and -0.65 volt, with an intermediate value -0.53 volt at 25°. He explained this result by assuming that solid nickel contains two allotropes with normal electrode potentials approximating -0.5 and -0.0 volt respectively when referred to the normal hydrogen electrode; an equilibrium mixture of the two allotropes, at ordinary temp., has a potential of -0.17 volt. S. Triandafil studied the effect of temp. A. M. Hasebrink observed that the potential of nickel rubbed with emery in an indifferent atm. falls at first, then recovers partially, and, after repeated rubbings, the potential becomes constant at a value which is lower than the initial one.
S. J. French and L. Kahlenberg found that the potential of nickel in N-KCl in hydrogen becomes more basic, reaching a maximum and then falling, and likewise also in nitrogen; in oxygen the potential becomes less basic, reaching a maximum and then falling off in bubbling gas, but in a quiescent state the potential becomes more basic. L. Kahlenberg and J. V. Steinle observed that the single potential of nickel in 0.5N-Na3AsO4 is 0.216 volt; in 0.5N-K3AsO4, 0.241 volt; and a N-KCl sat. with arsenic trioxide, 0.228 volt. G. W. Smith and L. H. Reyerson studied the electrokinetic potential; G. Tammann and E. Jenckel, the potential in soln. of potassium hydroxide under press.; P. Bechtereff, the potential of nickel in fused sodium hydroxide; and O. Scarpa, the resistance at the contact surface of electrode and electrolyte.
M. H. Jacobi studied the cell N: dil. HNO3: soln. KCl: Zn (Cu, Cd, Sn, Ag, or N); J. Regnauld amalgamated and unamalgamated zinc with a soln. of zinc sulphate, nitrate, or chloride and the corresponding nickel salt and nickel; A. Schweitzer gave -0.596 volt for the e.m.f. of the cell N: 0.5N-NCl2: N-KCl: Hg2Cl2: Hg; and C. von Neumann, the e.m.f. of the nickel-carbon cell with nitric acid, aqua regia, or sulphuric acid as exciting liquid. C. H. Prescott and M. J. Kelly studied the cell with a nickel wire anode, and an oxidized silver foil with a deposit of caesium. N. A. Furman and G. W. Low studied the use of nickel tungsten electrodes in the electrometric titration of strong acids and bases. P. Jolibois found that a nickel salt soln. and distilled water in separate vessels connected by a U-tube, with a platinum electrode, furnished nickel hydroxide.
J. Tafel observed that the potential of a nickel cathode goes on increasing for hours, and he considered that his results do not favour the hypothesis that the varying polarization is due to the varying thickness of a layer of gas on the electrode, but rather does the cathode surface have different catalytic effects on the process of forming hydrogen gas. E. P. Schoch gave -0.48 volt for the true equilibrium potential of nickel in N-NSO4. This value was attained in about 12 hrs. with an electrode of commercial nickel immersed in a soln. of nickel sulphate which had been boiled for some minutes, and allowed to cool in contact with air. The soln. is neutral to litmus. The potential is diminished by air or oxygen; it is increased by hydrogen, and lowered by a slight acidity of the soln. Occluded hydrogen increases the potential so that the electrodeposition of nickel does not take place until the potential is 0.2 to 0.3 volt higher than the equilibrium value, and hydrogen simultaneously appears on the electrode. The equilibrium potential of finely- divided nickel in N-NSO4 was taken to be -0.52 volt.
S. Glasstone found the cathode potential of nickel in N-NSO4, at 15°, for current densities D amp. ×l0-4 per sq. cm., to be:
| D | 0.2N-H2SO4 | pH=2.8 | pH=4.0 | pH=5.0 | pH=6.0 |
| 0.14 | -0.14 | -0.18 | -0.21 | -0.25 | -0.26 |
| 0.42 | -0.21 | -0.29 | -0.28 | -0.35 | -0.39 |
| 0.84 | -0.30 | -0.40 | -0.39 | -0.46 | -0.50 |
| 2.80 | -0.43 | -0.51 | -0.52 | -0.58 | -0.58 |
| 5.60 | -0.52 | -0.58 | -0.59 | -0.61 | -0.62 |
| 12.00 | -0.59 | -0.61 | -0.61 | -0.63 | -0.46 |
| 40.00 | -0.61 | -0.64 | -0.64 | -0.65 | - |
| 80.00 | -0.62 | -0.65 | -0.65 | -0.66 | - |
The values at 95° were:
| D | pH=2.8 | pH=4 | pH=5 | pH=6 | pH=7 |
| 2.8 | -0.16 | -0.19 | -0.23 | -0.28 | -0.25 |
| 5.6 | -0.18 | -0.23 | -0.25 | -0.30 | -0.31 |
| 12.0 | -0.23 | -0.28 | -0.27 | -0.30 | -0.37 |
| 40.0 | -0.29 | -0.30 | -0.30 | -0.31 | -0.44 |
| 160.0 | -0.31 | -0.31 | -0.31 | -0.31 | -0.47 |
C. G. Fink and C. M. Decroly found that the contact potentials of nickel against sulphuric acid of the following percentage compositions are:
| N-H2SO4 | 0.45 | 2.40 | 5.14 | 9.92 | 17.54 | 32.03 | 54.69 per cent. |
| Potential | -0.420 | -0.598 | -0.587 | -0.548 | -0.446 | -0.471 | -0.480 volt |
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| The Effect of Bromine Water on the Electrode Potential of Nickel and Platinum. |
According to A. Smits and C. A. L. de Bruyn, in the case of a metal like nickel which is violently disturbed by soln. containing oxygen by depositing the metal on the cathode in air, potentials can be obtained for very small current densities which are always somewhat less negative than the equilibrium potential; and this will be the case so long as the rate of cathode deposition is so small that corrosion can appreciably affect the deposited metal. For very small current densities, the potential of nickel during cathodic deposition remains less negative than the equilibrium potential; only at higher current densities does the potential become more negative, and only then does the deposition occur. The equilibrium potential is -0.48 volt, and even when the current density is 200×10-6 amp. per sq. cm., the potential is less negative than the equilibrium potential. With a soln. of a nickel halide instead of the nitrate or sulphate, the potential of nickel passes more quickly through the value of the equilibrium potential. So long as the rate of deposition is small, the deposit can be appreciably affected by dissolved oxygen - halogen ions exert a marked catalytic influence tending to annul the disturbance. At greater current densities, the disturbing influence of dissolved oxygen will become relatively smaller, and the positive catalytic influence relatively decrease, so that if the current density is allowed to increase, the nickel potential must soon pass through the value of the equilibrium potential, and it will exhibit cathodic polarization which increases strongly at first, and afterwards more slowly.
 |
| Typical Electrode Potentials during Nickel Deposition. |
The main factors tending to counteract the changes in concentration about the electrodes, and to decrease the polarization are ionization, migration, diffusion, and convection currents whether produced by changes of temp, or sp. gr. resulting from electrolysis, by stirring due to the escape of gases or external agitation. The polarization may be due not only to a change in metal-ion concentration of the adjacent soln., but also to a change in the soln. press, of the cathode. For pure metals, such an effect is usually very slight. However, in alloy deposition, or when hydrogen is discharged simultaneously with a metal, the composition and solution pressure of the cathode may change during electrolysis, and thus may cause an effect upon the polarization. N. A. Isgarischeff and H. Ravikovitsch investigated the effect of neutral salts on the cathodic polarization of nickel; S. Triandafil, the effect of acidity. G. Athanasiu examined the effect of illuminating the electrodes; G. W. Smith and L. H. Reyerson, agitation of the electrolyte. W. J. Muller and K. Konopicky, and 0. Scarpa and E. Denina studied the subject.
The potentials at the electrodes during electrolysis determine the anode and cathode efficiencies. In accord with Faraday's law, the total electrode efficiency is always 100 per cent, when all the products of electrolysis, or changes in valency, are considered - where, in electrodeposition, the valency of the metal ions is reduced to zero, the valency of metals in the free state, the cathode efficiency is determined by the proportion of current used in the discharge of the metal. It is sometimes erroneously assumed that the tendency for the discharge of hydrogen from any metal salt soln. will depend on the relative positions of hydrogen and metal in the electrochemical series; actually, the respective concentrations of H•-ions and metal-ions in a given soln. are not likely to be normal, or even equivalent to one another. In nickel-plating, the NSO4-content of the electrolyte may be about 0.1N, but the H•-ion content will usually be less, e.g., 10-6N, or pH=6. As a first approximation it can be assumed that the initial tendencies for the discharge of hydrogen and metal ions will depend on their respective static potentials in that soln. The actual proportions of metal and hydrogen discharged at any current density will be in accord with the respective current density-potential curves for metal deposition and for hydrogen discharge on that metal in that soln. W. Blum's curve for hydrogen evolution on nickel in the electrolysis of the given soln., and the curve for actual nickel deposition, are shown in Fig. At any given current density the ordinate of A is equal to the sum of the ordinates of G and D. The metal cathode efficiency at that current density is equal to the ratio of the corresponding ordinates of D and A. Under these conditions, the nickel cathode efficiency will increase as the current density is increased, but can never reach 100 per cent. If, however, as in acid copper soln., the static potential of the metal is much more positive than that of hydrogen in that soln., the cathode efficiency at low current densities will be 100 per cent., and will decrease as the current density is increased.
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