118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE CATHODIC BEHAVIOUR OF ALLOYS. PART I. IRON-NICKEL ALLOYS. BY S. GLASSTONE, M.Sc., Ph.D. ( A Paper vend before THE FARADAY SOCIETY, Monday, November 1 2 f 4 1923, SIR ROBERT ROBERTSON, PRESIDENT, K.B.E., F.R.S., itz the Ckir.) ReceivedJuGy I 3th, I 9 2 3. Up to the present very little work appears to have been done on the measurement of polarisation voltage at electrodes composed of metallic alloys, and therefore a systematic investigation in this field has been undertaken with the hope of throwing some light on the problem of over- voltage and allied phenomena.In working with alloys that are prepared from a fused mixture of the metals, the structure and no doubt the pro- perties of the alloy as an electrode will depend upon the method of cooling the melt ; further, the surface skin will probably be in a different condition from the rest of the alloy. In order to avoid difficulties introduced in this way, it is proposed to deposit the alloys electrolytically from solutions containing salts of both of the metals in varying proportions; by this method a series of mixtures should be obtained varying in composition from that of one pure metal to that of the other. The object of this work is to investigate the cathodic behaviour of these alloys, as well as the electrolytically deposited pure metals for the sake of comparison, along three lines : (I) to measure the cathodic potential during deposition from a solution of mixed salts, ( 2 ) to determine the potential required for hydrogen liberation in sulphuric acid and sodium hydroxide solutions, Le., ‘‘ over- voltage,” and (3) to investigate the action of depolarisers.Schlotter has shown that the reduction efficiency of any particular cathode in potassium chlorate solution is greatly influenced by depositing upon it traces of another metal in amounts insufficient to cover the surface of the cathode completely. Thus the system electrode metal-deposited metal-hydrogen might be more or less efficient for the purposes of reduction than the original electrode system before metal deposition. Since an alloy at which hydrogen is being evolved as a result of electrolysis, resembles to a very great extent the electrode system used by Schlotter, it appeared that a systematic investigation of the cathodic behaviour in the presence of de- polarisers would be of considerable interest.In the present paper the results obtained with a series of iron-nickel alloys are described and discussed. These alloys were deposited from solutions containing varying proportions of ferrous and nickel sulphates ; the total concentration of metal was always two equivalents per litre, and in addition contained free sulphuric acid to the extent of 0.05 N. The cathode consisted of a small sheet of lead which had been completely Zeitsch. Ehkirochem., 1921, 27, 394. 574PART I. IRON-NICKEL ALLOYS 0.85 0.68 0-60 0'50 0'39 5 75 0'75 0.79 078 0.74 0'63 0'72 0'70 0*63 0'55 0.65 0.62 0.52 0'48 0.51 0'55 0'50 0'39 0'39 v44 0'44 waxed over with the exception of an area of one sq.cm. which was left clean and bright ; the anode was of carbon, which was thought to be pre- ferable to platinum, since the latter might have dissolved and been de- posited on the cathode. With a current density of 0.045 amps./cm.2 smooth and bright adherent deposits were obtained in about one and a half hours; the presence of ferric salt, however, caused the deposit to be dark and powdery. The electrolytic bath was kept at room temperature (about 18') and was of sufficiently large volume to leave its composition practi- cally unchanged as the result of a comparatively small cathodic deposition.I n order to determine the composition of the alloy, which was only slowly soluble in dilute sulphuric acid, the deposition was made under identical conditions except that a hard gas carbon electrode replaced the one of lead; the alloy was dissolved off the carbon by means of a mixture of dilute nitric and hydrochloric acids, and the ratio of nickel to iron deter- mined as follows. Ammonium chloride and hydroxide were added to the solution and the mixture boiled and filtered ; the filtrate was analysed for nickel by the volumetric method using potassium cyanide whilst the ferric hydroxide precipitate was dissolved in dilute sulphuric acid, reduced and estimated by means of standard permanganate solution. In addition to deposition from mixed solutions, pure iron and nickel were also deposited electrolytically in order to have a complete series varying from 100 per cent.of one metal to IOO per cent. of the other. In the following table (I.) are given the compositions of the various solutions and cathodic deposits, together with the numbers which will be applied to them throughout the rest of this paper. TABLE I. No. I. 11. 111. IV. V. VI. VII. Atomic Ratio Fe/Ni. Solut1an. r m p i t . Fe only IOO per cent. Fe 4.0 5'1 1'5 2.3 0.67 1.2 0.25 0'47 0'020 0.14 Ni only 100 per cent. Ni Potentitzd during Catlrodic Dtposition.-Whilst the alloys were being deposited, the potential of the cathode at various current densities was measured against that of a normal calomel electrode, using saturated potassium chloride solution as intermediate liquid.The jet of the tube connecting the experimental electrode with the standard electrode pressed tightly against the surface of the former. The results obtained are given in Table 11. TABLE 11. C.D. 0'12 0.06 0.03 0.015 0.005 I. 1.11 0'95 0.82 0.70 0.67 11. 1 111. I IV. 1 v. I VI. i I-1-1- VII. 0'94 0.88 0'73 0'60 0.66 ~ All current densities are given in amps. per sq. cm. and potentials in volts on the Sutton, '' Volumetric Analysis,'' 8th ed., p. 252. hydrogen scale. The negative sign has been omitted throughout.s 76 THE CATHODIC BEHAVIOUR OF ALLOYS Fig. I , Curve A shows the relationship of the concentration of the nickel- iron solution to that of the deposited alloy, whilst Curve B gives the earresponding deposition potentials at a current density of 0.12 amps.per sq. cm. The deviations from the latter curve are believed to be due to the inability to reproduce the same conditions exactly in every case, since the potentials measured by the l' direct " method depend to some extent upon the state of the electrode surface (see communication to the Chemical Society entitled : l1 Intermittent Current Electrolysis, Part II."). Potentiah for HydrogeB EvoCuiion.-The lead electrodes which had been coated with alloy at a C.D. of o-oqg amps./cm.s, were used as cathodes in an electrolytic cell containing either N-sodium hydroxide or N-sulphuric acid, the anode being of platinum foil in every case. The potent$ of the cathode was measured during polarisation at various current densities, the standard electrode being Hg/HgO N-NaOH in the alkaline solution, and - 1'3 - 1'1 - 1.0 - 0.9 - 0.8 - 0.7 P) 2 e U (D Y d B 2: ." o 10 20 30 40 50 60 70 80 go 100 Atomic per cent of Ni in solid.FIG. I. Hg/HgSO, N-H2S04 in the acid solution. Connection between the test and standard electrodes was made by means of a tube ending in a fine jet ; both this and the connecting vessel contained the same liquid as was used in the electrolytic cell and in the standard electrode. The method of measurement was to put on the polarising current at a C.D. of 0.12 amps./cm.Z for five minutes and then to measure the potential at this and at other current densities at one minute intervals. Owing to the un- certainty as to the correct method of measuring overvoltage, the measured potentials of the cathode with current flowing are recorded in Table 111.; the values obtained may include a potential due to the resistance of a gas film at the surface of the electrode.PART I. Eiectrolyte : N-NaOH. C.D. 1 I. I 11. -I I-- 0'12 1-57 1'21 006 0.03 1-04 Electrolyte : N-H,SO,. 0.12 0.71 0'36 0.06 0.67 0.30 0.015 0.60 0.25 0.03 1 0'63 1 0'28 IRONNICKEL ALLOYS 5 7 7 TABLE 111. 111. I -08 1-05 1.03 1-01 0.36 0.30 0'28 0.26 IV. 1'12 1-07 1.04 1'00 0'37 0'30 0.28 0'26 V. 1.11 1.08 1.04 1'02 0.41 0'37 0'34 0.29 VI. 1 VII. 1-22 1.38 1-08 1.30 s o 4 1-27 1.18 I 1-34 Negative signs have been omitted throughout. Fig. 2, Curve A gives the potentials for hydrogen evolution in sodium hydroxide solution at a current density of 0-1 z amps. per sq. cm. as a function of the composition of the electrode material, whilst Curve B gives the cor- responding potentials with N-sulphuric acid as electrolyte.The remarks made concerning deviations from Curve B in Fig. I apply here too. - 1.6 B z -1'5 I z c -1.4 .- - 0-8 - 0.7 - 0'6 - 03 - 0.4 - 0.3 d I 2 z C ." o 10 20 30 40 50 60 70 80 go 100 Atomic per cent. of Ni in electrode. FIQ. 2. Cufhdic Behviwr in fh Presence of DcpoZan>ers.--In these experi- A. Saturated aqueous solution of potassium chloride and chlorate. I3. Mixture of IOO C.C. zN-NaOH (aqueous), 100 C.C. alcohol, and C. Equal volumes of zN-NaOH and alcohol saturated with nitro- D. Mixture of 30 C.C. 6N-H,S04 and 7 0 C.C. alcohol saturated with ments four different solutions were used I o grams benzaldehyde. benzene. ni trobenzene.5 78 THE CATHODIC BEHAVIOUR OF ALLOYS In the first place the efficiency of the various electrodes for reduction pro- cesses was determined. The experiments were carried out in a small piece of apparatus resembling the Hofmann apparatus for the electrolysis of water, in which the volume of hydrogen liberated at the cathode could be measured from time to time.By measuring the volume of gas liberated at the same time in the same electrolyte without the depolariser, the effici- ency of the reduction process could be determined at a particular current density. The anode used in these experiments consisted of a short platinum wire fused into glass, and diffusion between the anodic and cathodic portions of the apparatus was obviated by the introduction of a plug of asbestos.It is possible that the various electrodes may exert different catalytic effects on the substances produced by the reduction process; no attempt has yet been made to investigate this point further, but the fact that the potentials recorded during the process of electrolytic reduction (see below) were all of the same order of magnitude indicates that for a given de- polariser and electrolyte, the reduction products were always the same. In general the reduction eficiency of an electrode was greater at the beginning of the experiment, when the electrode was fresh from its de- position which always occurred with simultaneous hydrogen evolution. The rate of reduction soon settled down to a constant value, and it was during this time that the readings, from which the figures in Table IV.were cal- culated, were taken. This alteration in the reduction efficiency corresponds with the observations by R Russl that the efficiency of an electrode is increased by previous polarisation, but that this increase falls off after use. Since both electrolytic iron and nickel are known to contain sorbed hydrogen, it is possible that the latter acts as a positive catalyst for the reduction process, but its gradual removal by the depolariser causes the catalytic effect to disappear. In the table below are recorded the reduction efficiencies of the vari- ous electrodes in the presence of different depolarisers at definite current densities. TABLE IV. Dopol. A B C D C.D. 0.03 0015 0.03 I. 1 11. 1 111. I IV. I-I-I- 86 78 Practically 100 per cent.for all C.D. 1 v. j VI. 1 VXI. l-I-I----- per cent.lPer centlper cent, 1 Nqooapprecitye reduction 44 79 I 73 I 58 '8 up to 0'15. In the second portion of the investigation of the effect of depolarisers, the potentials of the electrodes were measured in the presence of the former, at various current densities. The electrolytic cell was fitted up in the usual way with a normal calomel electrode as standard; saturated potassium chloride solution was used as intermediate liquid. The polaris- ing current was put on for two minutes at a C.D. of 0.12 amps./cm.%, and then the potentials wkre measured at one minute intervals. I t was found that a very slight movement of the jet of the connecting tube of the standard electrode caused a large change in the measured potential, consequently the results obtained cannot be regarded a.s definite but the general slope 1 Zeitsch.physikal. Chcm., goo,^, 641.PART I. IRON-NICKEL ALLOYS 579 of the current density-potential curve, as well as its actual position is of some interest. The curves are represented in Figs. 3 to 6 below, and are self-explanatory. Discussion ofResuZts.-An examination of Tables I. and 11. and Fig. I shows that much lower potentials are required for the deposition of mixtures of iron and nickel than for either of the pure metals, and that the deposits contain relatively more iron than do the solutions from which they were deposited. That the mixtures should be deposited at less negative po- tentials is to be expected; iron and nickel probably form a continuous series of mixed crystals in which the free energy of each metal is less than it is in the pure state.The solution pressure will thus be diminished and so a less negative potential will be required for the deposition of the alloy than for either of the pure metals. We should expect that when the liquid solution contains excess of iron that the solid solution deposited should contain relatively more nickel, since the concentration of nickel in the solid is much less than that of iron and so its deposition potential would be depressed to a very much greater extent. Similarly when the solution contains a large proportion of nickel then the solid should contain relatively more iron, but since the deposition of nickel takes place at less negative potentials than that of iron, we should expect this increase to be less marked than in the previous case where the solution contains iron in excess.The actual results obtained (Table I.) are, however, not in agreement with this theory, and so it is evident that other factors, in addition to those mentioned, are operative; two suggestions are put forward which may account for the anomalous behaviour. In general the cathodic polarisation which results when a metal is being deposited may be attributed to the slowness of the reaction Me" + 2 0 + Me. I f we regard nickel atoms or ions as a positive catalyst for the reaction Fe" + 2 @ + Fey and iron as either an inert substance or as a negative catalyst for the reaction Ni" + 2 0 3 Ni, then it is to be expected that the deposits will contain relatively more iron than do the solutions.Foerster has suggested that the retardation of the process Me" + 2 0 3 Me is due to the hydrogen, which is deposited simultaneously, acting as a negative catalyst ; if the pre- sence of small amounts of nickel in the iron causes a considerable diminu- tion in the amount of sorbed hydrogen during electrodeposition, then the reduction in the amount of the negative catalyst will enable the process Fe" + 2 0 4 Fe to proceed at a much more rapid rate. If on the other hand we assume that iron has practically no effect on the sorption of hydrogen by electrolytic nickel, the speed of the process Ni" + 2 0 3 Ni will be unaltered and we should expect all the deposits to contain a relative excess of iron over that in the solution.Kremann and his co-workers a have shown that at increased temperatures the electrolytic deposits contain more nickel than do those deposited at lower temperatures. At high temperatures the sorption of hydrogen by electrolytic iron is diminished to a very great extents and consequently the further effect of any dissolved nickel can only be relatively small; hence at increased temperatures there will be a tendency for the deposits to contain relatively more nickel. I t appears that an investigation of the amounts of hydrogen sorbed by the electrolytically deposited mixtures of iron and nickel at various temperatures Zeitsch. Elektrochem, 1916, 22, 96. See Foerster, Elektrochemle Waseriger LGsungen, 1922, p. 378. Mmratsh.9 1913,341 I757 ; 19% 359 731.THE CATHODIC BEHAVIOUR OF ALLOYS would be of some interest, and it is hoped to be able to undertake this work at a later date.According to the views of Tammannl on the properties of metallic solid solutions we should expect the more noble metal, nickel, to exert a " protective action '' on the iron, making the latter more electronegative and so more easily deposited. I t is possible that this factor may influence the relative amounts of iron and nickel in the cathodic deposit, but it will hardly explain the fact that solutions containing very little nickel give de- posits containing relatively more iron. Perhaps the most striking results of the experiments described above are those shown in Table III. and Fig. 2 : it is seen that hydrogen evolution takes place at nickel-iron alloy electrodes at much lower potentials than at either pure iron or nickel.Since the cause of overvoltage is still not definitely known, it is not yet possible to offer a satisfactory explanation for this interesting result; it can only be said that the presence of iron and nickel together, prevents the accumulation of the hydrogen charge which is the immediate, if not the ultimate, cause of overvoltage. Further investiga- tion with other alloys may throw more light on the problem. Ridea12 has shown that a parallelism exists between hydrogen overvoltage and the catalytic activity of a metal for reduction processes involving gaseous hydrogen, and although this view has been criticised in the First Report of the Committee on Contact Cataly~is,~ it certainly appears to be a fact that the low overvoltage metals are the best catalysts.In view of the results described above, an examination of the catalytic activity of finely divided solid solutions of iron and nickel would be of great interest. The results obtained in the presence of depolarisers show clearly that the reduction efficiency of a particular electrode is not altogether dependent upon the hydrogen overvoltage, but that the electrode itself may exert a catalytic effect. In the case of the reduction of potassium chlorate, either iron is a positive catalyst or nickel is a negative catalyst, since reduction occurs to some extent at electrodes II., 111. and IV., although the over- voltage at these is much lower than at a pure nickel electrode where no reduction takes place.With alkaline benzaldehyde as depolariser, it is obvious that nickel is a positive catalyst, since the percentage of hydrogen available for reduction purposes at this electrode is greater than at one of iron which h a a higher overvoltage; further the alloys with their low over- voltage are as efficient as iron, due no doubt to the catalytic effect of the nickel. For the reduction of alkaline nitrobenzene iron is a powerful positive catalyst, since the alloys are very much better for reduction purposes than is nickel with its much higher overvoltage. All the electrodes appear to be equally efficient in the reduction of acid nitrobenzene, since all the hydrogen produced at current densities up to 0.15 amps./cm.2 is available for reduction purposes ; differences between the various electrodes are, however, indicated by the curves in Fig 6.Since the current density-potential curves cannot be regarded as exactly reproducible, it will only be necessary to discuss them briefly. The position and slope of these curves depends upon three factors : (i) the rate of the reaction 2H +- H,, (ii) the rate of the reduction process, and (iii) diffusion of the depolariser, if the liquid is not vigorously stirred. When factor (i) is very large, that is the metal has a low overvoltage, or (ii) small, 1 Cf. Zeitsch. w g . Chrm., 1919, 107, I et seq. a J. Amer. Chem. Sac., 1920,*, 94. *See Bancroft, J. Ind. Eng. Chem., 1922, 14,644.PART I. IRON-NICKEL ALLOYS 581 then as a rule very little reduction will take place, and the curve obtained will be the normal curve for hydrogen evolution. If however (i) is small, and (ii) is fairly large, then either all or part of the hydrogen will be available for reduction purposes and the cathode potential will be lowered ; the greater the value of (ii) the lower will be the potential for a given 0'12 - d 0.015 I I I - 0.9 - 1'0 - 1'1 - 1'2 volt Cathode potential.FIG. 3.-Depolariser A. current density. At high current densities when the rate of reduction is great, the concentration of depolariser in the region of the electrode will be considerably diminished, since fresh depolariser is not able to diffuse in as fast as it is removed; in this case the potential will rise greatly with increasing current density. In the case of alkaline nitrobenzene as depolariser (Fig. 5 ) the cathodic potentials of iron and of the alloys - 1'0 - 1'1 - 1'2 volt Cathode potential. FIG. 4.-Depolariser B. increase much more rapidly with increasing current density than do those of nickel, at which electrode the amount of reduction is comparatively small. When several electrodes have the same overvoltage, that is factor (i) is constant, and the rate of the reduction reaction (ii) is also constant, then factor (iii) must also be constant, and the P.D.-C.D. Curves should all5 82 THE CATHODIC BEHAVIOUR OF ALLOYS 0'12 C1 E 2 a 2 0.06 g 0'02 0.015 - = - - 0'12 =. '1- E k 2 0-06 4 0.03 o*org - - - I 1 IDISCUSSION 583 ComZusWn. An examination of the cathodic behaviour of iron-nickel alloys from several standpoints has brought to light a number of new facts concerning their electrode deposition, overvoltage and action in the presence of de- polarisers. Suggestions have been offered to account for some of the observed facts, but a fuller discussion of their significance in connection with the theories of overvoltage, delayed cathodic deposition and of cathodic reduction, must be left until further information is available. University College, Exeter.