EXPERIMENTAL STUDIES ON PALLADIUM ELECTRODES.

BY JAMES C. ANDREWS.

(From the Department of Physiological Chemistry, School of Medicine, University of Pennsylvania, Philadelphia.)

(Received for publication, January 31, 1924.)

INTRODUCTION.

The need of a careful scrutiny of the standards and methods employed in the determination of hydrogen ion concentrations by means of the hydrogen electrode is becoming more and more appar- ent,. Despite the ease and accuracy with which calorimetric deter- minations of pH values may be made on some biological fluids, the electrometric method must still be regarded as standard and the present well merited vogue of pH measurements is such that no detail of the standard procedure is too slight to warrant close study.

In describing the preparation of hydrogen electrodes, many investigators, particularly the authors of texts, have indicated that platinum, palladium, and iridium may be used interchange- ably as the medium for the reversible reaction between molecular and ionic hydrogen (1, 2). Platinum has been chiefly employed by the majority of investigators. Iridium and palladium have had but limited use, but the latter has been frequently recomi mended because of the ease with which it may be removed from old electrodes by anodic electrolysis in hydrochloric acid. This has been regarded as an important advantage, particularly in biological investigations where measurements in protein solutions necessitate the frequent use of newly prepared electrodes. Indeed, it is probable that the biological importance of pH determinations is chiefly responsible for the extent to which palladium has been recommended. While palladium-black electrodes have undoubt- edly been used with success by some investigators, verbal com- munications from a number of others have led to the conclusion that this metal has more often been discarded after a few trials

479

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480 Studies on Palladium Electrodes

in favor of the less erratic platinum-black. During the course of attempts t’o find in palladium a satisfactory substitute for plati- num, similar results were obtained in this laboratory and it soon became apparent that an investigation of palladium electrodes and their mechanism as compared with that of platinum was highly advisable. In this investigation, of which the results are recorded below, it was attempted to define the optimum con- dit’ions for the preparation of palladium-black electrodes and to indicate a theoretical basis for the differences observed between palladium and platinum.

Method.

Apparatus and Reagents.

All measurements were made with a Leeds and Northrup Type K potentiometer and a sensitive galvanometer. The hydrogen electrode vessels used were of the Clark t,ype as modified by Cullen (3). The small thermometers used in these cells were calibrated against a U. S. Bureau of Standards thermometer. Electrolytic hydrogen obtained from a cylinder was used after being passed successively through a train of wash bottles con- taining concentrated NaOH, sodium pyrogallate, and water.

The determinations were carried on at room temperature and were corrected for the same in all cases where temperature played an appreciable part; i.e., the correction of barometric readings, the temperature coefficients of Weston and calomel cell, etc. As will be noted below, the effect of temperature variation on the actual pH of the 0.1 N HCl used as a standard amounts to only 0.005 pH for a total change of 18°C. Assuming, therefore, that the neces- sary corrections had been made, the use of a thermostat was unnecessary. Our purpose being to investigate the electrode per se, standard 0.1 I\T HCI was used for most of the measurements. The acid was standardized through a 0.2 N NaOH solution against repeatedly crystallized acid potassium phthalate and was also checked against 0.1 N HCl prepared by the constant boiling method of Hulett and Bonner (4). The pH of this solution, according to the activity data of Noyes and Ellis (5) was taken as 1.085 at 20°C. Since their data give a value of 1.090 at 38°C. and the temperature curve may be safely assumed to be nearly a straight line, it is evident that the effect of ordinary temperature varia-

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J. C. Andrews

tions is negligible. However, in order not to subject the Weston and the calomel cells to undue temperature changes, care was taken to keep the room temperature as nearly constant as possible. In those cases in which it was desired to use buffer solutions near neutrality, KH2P04-NaOH solutions were made up from care- fully standardized materials.

The nearness with which these figures of Noyes and Ellis are approximated is, of course, partially dependent upon the values assigned to the Weston and the calomel cells. The Weston cell used as standard was compared with another cell tested by the Bureau of Standards. The saturated calomel cells used were made from mercury and calomel furnished by the Eppley laboratories. The KC1 was four times recrystallized from c. P. material. These cells proved very satisfactory. No “drifting” was in evidence and several cells showed, at different times, a maximum varia- tion among themselves of less than 0.1 millivolt. The values used for these cells were interpolated from those given by Clark (1) in appendix Table A. Using these values and correcting to the nearest 0.5”C., the data listed below were obtained. Final results are expressed only in terms of PI-I, the intermediate data being omitted for the sake of brevity.

Detailed Procedure.

Preliminary experiments indicated the necessity of about 10 to 15 min- utes shaking of electrode vessels containing solution and electrode in con- tact with hydrogen before equilibrium was attained. For this reason a standardized interval of 15 minutes shaking was adopted for each reading. The procedure was as follows: The electrode vessel was first rinsed and filled with the standard solution and the stopper cqntaining the electrode under examination placed in position in such a way as to leave the vessel completely filled with liquid. A portion of the solution was then expelled by means of hydrogen and the vessel was rocked for 15 minutes. ’

Immediately after the 15 minute period of shaking, the hydrogen inlet was closed, the connection into the salt bridge opened and the reading taken at once in order to minimize any error due to contact potential between the 0.1 N HC.1 and the saturated KCl. Particular care was taken to effect this junction in the same way in each case and to use the same vessel throughout each principal series of determinations. After each reading, the vessel was rinsed and refilled with a new portion of the standard solution. By thus maintaining a definitely standardized schedule for each determination, it was possible to interpret the data given below in terms of the total time required for each electrode to give equilibrium values in the 0.1 N HCl.

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Each successive pH value given represents a 15 minute period plus the few minutes required to empty and refill the electrode vessel. Only the final pH values are recorded.

The small platinum electrodes which fit the Clark-Cullen vessel were employed. These electrodes were of practically constant area throughout; no variations in current density were employed. The procedure used in plating palladium-black upon the platinum was as follows:

The principal palladium solution used contained 3 per cent PdCh. To this were added 2 cc. of concentrated HCl per 140 cc. of solution. These

concentrations were chosen in order to duplicate as nearly as possible the concentrations which had given best results in the hands of other investi- gators. A few preliminary trials showed that more concentrated solutions or more alkaline ones were impractical because of their tendency to deposit basic salts and, further, because electrodes made from freshly prepared solu- tions of higher concentration showed no superiority over those made from the 3 per cent solution. The question of alkalinity will be considered later. The palladium chloride used was a C.P. product which analyzed 97.86 per cent PdClz by the dimethylglyoxime method of Wunder and Thiiringer (6).

The cleaned platinum electrodes were rubbed with very fine emery pow- der for the purpose of removing any undissoived portion of previous deposits, washed, and placed for several hours in hot chromic acid cleaning solution. They were thenwashed with distilled water, electrolyzed cathod- ically in 10 per cent HzS04, washed again, and electrolyzed in the PdClz solution. For both the preliminary electrolysis in 10 per cent H&SO* and the plating in PdCl+ two dry cells in series, giving a total of nearly 2.5 volts, were used. The electrolysis in 10 per cent HzS04 was chiefly used as a test for perfect cleanliness, uniform evolution of hydrogen from the entire surface of the electrode being required. In the actual plating, the same distance between anode and cathode was preserved in each case as closely as possible. It was desired to introduce no variations in amount of current passing and in current density except those caused by actualvariations in the composition of the plating solutions. After plating, the electrodes were washed in distilled water, and, except as otherwise stated, used at once. In washing, a fairly strong stream of water from the wash bottle was played on the electrode in order that any loose portions of the deposit might be dislodged. Any visual evidence of uneven distribution or any bright spots on the electrode caused its rejection. The presence of bright spots, showing where the deposit had flaked off, was especially common in the heavier platings. Rejected or used electrodes were cleaned by anodic electrolysis in HCl and then put through the above described cleaning process before replating.

The question as to the optimum depth of plating was early brought to the writer’s attention by the complete failure of elec- trodes due to too heavy deposits. Few investigators have empha- sized the advisability of using fairly light deposits of any metal

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J. C. Andrews 483

and the almost universal criterion of proper thickness for platinum deposits has also been generally used for palladium. This is probably one cause for much of the lack.of success with palladium electrodes. It has been found that if palladium deposits are carried to the point, of the rich, velvet black which is by most workers regarded as the sign of a good platinum deposit, it is almost impossible to prevent flaking. The electrode is thus ruined or is, at best, extremely sluggish. In some of the following experi- ments this is shown’by the use of three different.depths of plating. The minimum thickness which gave good results was deposited by only 15 second electrolysis under the conditions described. In some series, therefore, the electrodes were plated 15, 30, and 45 seconds. The plating of 15 seconds which was finally adopted as the standard depth, was so thin that its color was only light grayish. The platinum surface was barely covered.

EXPERIMENTAL.

Variations in Acidity.

The data in Table I show the results of increased acidity of the stock palladium solution. Each successive solution used was made by .addition of concentrated HCl to a 15 cc. sample of fresh stock solution. With each solution 15, 30, and 45 second platings were made.

The portion of Table I showing the behavior of electrodes made directly from stock PdClz solution was selected as typical from a large amount of such data. The data in the “15 second elec- trode” column represent good average results obtained from such electrodes and may be taken as indicating the best results that could be consistently obtained with any form of palladium elec- trode used. The sluggishness of the 30 and 45 second electrodes is plainly evident. The remaining data in Table I show, barring minor eriaticisms, the detrimental effect of increased acidity of the plating, solution. The very erratic behavior of electrodes made in the most acid solution is probably due to the fact that the poorer quality of the deposit is temporarily offset by the charging of the electrode with hydrogen during plating. This effect will be discussed later in more detail.

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484 Studies on Palladium Electrodes

Variations in Dilution.

In Table II are shown the results of diluting the stock solution. It would seem from the above that although dilution of the

PdCL solution with a proportionate increase in the time of elec- TABLE I.

Effect of Zncreasing Acidity of the Plating Solution. Each Successive pH Represents a 80 Minute Interval and a New Reading on the Same Electrode.

15 sec. plating. 30 sec. plating. I

45 8ec. plating.

Stock PdC& solution.

PH PH PH

1.040 1.016 1.008 1.068 1.028 1.015 1.073 1.052 1.029 1.079 1.059 1.032 1.082 1.065 1.043 1.080 1.067 1.048

15 cc. stock PdC& + 1 cc. concentrated HCl.

. 1.010 1.002 1.003 1.028 1.009 1.007 1.045 1.019 1.012

1.067 1.035 1.072 1.057 1.041

15 cc. stock PdClz + 2 cc. concentrated HCl.

1.015 1.008 0.96 1.019 1.026 1.014 1.035 1.028 1.052 1.032 1.035 1.036 1.047 1.046

15 cc. stock PdClz + 3 cc. concentrated HCl.

1.061 1.003 0.92 1.052 1.029 0.98 1.066 1.034 1.042 1.056 1.033 1.016 1.048 1.028 1.031

trolysis would be expected to yield about the same results, the actual data obtained are more erratic and show less tendency to

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J. C. Andrews 485

reach the equilibrium value than when a more concentrated solu- tion is used. The “15 second” electrodes obtained were extremely thin and were probably incapable of absorbing sufficient hydrogen to give a reliable electrode.

E$ect of Acid Immersion.

During the whole of these experiments, it was often observed that when an electrode which had been used for several determina- tions was allowed to stand overnight in the standard 0.1 N HCI,

TABLE II.

Effect of Dilution.of the Plating Solution. Successive Readings Have the Same Significance as in Table I.

15 sec. plating. I

30 sec. plating. I

45 sec. plating.

1 volume stock PdCh + 1 volume HzO.

PH PH PH

1.028 1.043 1.027 1.060 1.059 1.030 1.075 1.071 1.043 1.057 1.081 1.052 1.066 1.084 1.067

1 volume stock PdCL + 2 volumes HzO.

1.031 1.013 1.060 1.047 1.059 1.059 1.065 1.066 1.052 1.068

1.005 1.041 1.059

1.065

the next day’s readings invariably showed absurdly low values. For example, a reading of practically equilibrium value would be reduced to less than pH 1 in most cases and subsequent readings showed no very marked increase.

A short series of experiments was made to show the effect of immersion in acids of different concentrations. Four electrodes were made up by the “standard” procedure (15 second electroly- sis in 3 per cent PdCl?, etc.). Each was used until practically equilibrium values were obtained and then placed overnight in the solution indicated. The next readings were taken immediately after thorough washing the next morning, the period of immer-

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486 Studies on Palladium Electrodes

sion having been the same in all cases. Table III contains the results obtained both before and after immersion.

The effect of acid is plainly evident, even though the electrode which had been placed in 0.1 N HCl yielded a higher value than was frequently found in similar cases. The experiment with the chromic acid solution was included since it is well known that chromic acid cleaning solution has no detrimental effect on plati- num electrodes. It is evident from the results on electrode A that after several hours in water, an electrode can temporarily recover in about normal time. The effect of alkali will be dis- cussed later.

TABLE III.

Effect of Immersion of Electrode in Acid. Successive Readings at Intervals of 20 Minutes.

Electrode.. . . . . . A * B c D -

Original PH PH PH. PH

readings. 1.063 1.073 1.066 1.054 1.072 1.082 1.073 1.059 1.072 1.079 1.081 1.06% 1.078 1.080 1.077 1.073

Immersion overnight in 0.1 N HCl Con&r&d Chromic acid

distilled water. cleaning solution.

Readings after 1.030 1.020 0.87 0.85 immersion. 1.050 1.029 0.98 0.79

1.067 1.034 0.92 1.074 1.038

On the other hand, various other experiments showed that the best palladium electrodes do suffer deterioration on standing for longer periods in water. 3 days of immersion in water before use not only made the attainment of any particular pII below the equilibrium value require three to four times as long as in the case of a fresh electrode, but actually prevented the attainment of the equilibrium value within any reasonable length of time. As will be showp later, this is undoubtedly due to a slow permanent, change, rather than to any temporary effect of the water in removing hydrogen from the electrode. A slight temporary sluggishness, on the other hand, might be accounted for by a loss of the hydrogen with which the electrode had become partially charged during plating and previous use.

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J. C. Andrews 487

To determine whether or not any beneficial effect resulted from further cathodic electrolysis after plating, several comparisons were made. “Standard” electrodes were used, both directly and after periods of electrolysis of varying lengths of time in solutions of acidity varying from 0.1 N to 10 per cent H804. Detailed quotation of results is unnecessary since in no case did either any definite improvement or harm result from this electrolysis on first using the electrodes. Evidently the electrode becomes fairly well saturated during plating and any further treatment with acid merely aids in hastening its final deterioration.

TABLE IV.

Progress of Deterioration of Electrodes. Successive Readings Have the Same i 4ig wijkance as in Preceding Tables.

1st day. 2nd day. I 3rd day.

Electrode. Electrode. I Wectrode.

A

PH

1.035 1.060 1.064 1.074 1.072 1.074

B

PH

1.050 1.055 1.058 1.076 1.071 1.073

A

PH

1.026 1.043 1.051 1.060 1.068 1.064 1.065

B A B

PH PH PH

0.995 0.955 1.006 1.033 1.017 1.038 1.034 1.043 1.049 1.045 1.056 1.057 1.059 1.004 1.066 1.048 0.99 1.057

Both electrolyeed in 10 per cent HzSOa for 1 min.

1.066 1.062 1.023 1.037 1.006 1.000

That cathodic electrolysis following plating does contribute to the final deterioration of an electrode is shown by the data in Table IV. Two “standard” electrodes were made: A was used directly in 0.1 N HCI while B was first electrolyzed for 1 minute as the cathode in 10 per cent H&SO*. Both were then used for 3 successive days and over the 2 intervening nights were kept in distilled water, under hydrogen. After the 3rd day’s measure- ments were made and the deterioration of the electrodes became very marked, both were electrolyzed for 1 minute, each in 10 per cent HzS04, and immediately used again. The results are highly interesting.

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488 Studies on Palladium Electrodes

The progressive deterioration is quite easily apparent, particu- larly in the case of electrode B, while the resaturation of both electrodes, on the 3rd day, by electrolysis in 10 per cent HeSO4 produced a temporary rise only as long as the effect of the hydro- gen lasted. The rapid drop which followed at once clearly shows that the electrodes were no longer capable of acting reversibly.

That this deterioration was at least partially independent of the pH of the solution in which the electrode was used is indicated by two facts: (1) that electrodes immersed 2 or 3 days in water before use are correspondingly sluggish; and (2) that electrodes used in solutions of higher pH than 0.1 N HCl during successive days deteriorate, although relatively less rapidly. For example, an electrode used for 3 successive days in a phosphate buffer of pH 6.8 gave as its maximum reading for each day, pH 6.802, 6.783, and 6.772. The bad effects of electrolysis in 10 per cent HaSO4 before use were illustrat,ed in a number of cases. For example, two electrodes were prepared exactly as were those of Table IV but were preserved in distilled water and used 1 day after prepara- tion instead of immediately. The results were comparable to those of the 2nd day in Table IV, but the unfavorable effect of the electrolysis of one electrode in 10 per cent HZSO, was some- what more marked.

E$ect of Alkali Immersion.

A series of trials was also made with palladium electrodes in which the effect of immersion in alkali of various concentra- tions was studied. ‘Standard” electrodes were prepared as described above and were immersed for varying periods of time in NaOH solutions of 0.1 to 1.3 N. The first readings obtained in 0.1 N HCI were highly promising-almost equilibrium values were obtained at once-but these figures quickly dropped to values as low as pH 1.0 and below. In all cases, the results obtained were too erratic to be worth quoting. Even values obtained on 0.1 and 0.01 N NaOH solutions and on nearly neutral buffer solutions after such immersion, showed the same lack of agreement. Varia- tions in the method of preparation of the electrodes, the use, before and after alkali immersion, of cathodic electrolysis, etc., effected no improvement. Altogether, the net result of these experiments was such as to throw considerable doubt on any deter- minations of high pH values with palladium electrodes.

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J. C. Andrews 48’3

In this connection the results obtained by Hammett ‘(7) in the course of some studies on platinum-black electrodes are of decided interest. He found in the case of platinum-black electrodes which had had considerable exposure to hydrogen, a high degree of sensitivity towards minute amounts of oxygen when used in solu- tions of high pH. He says in part: “Indeed the limiting factor upon the use of electrodes is the sensitiveness to oxygen which finally becomes so great that no reasonable precautions can give correct results. It is perhaps not realized how rapidly oxygen can diffuse through an unlubricated ground joint even against a slight pressure of hydrogen.” That the deterioration of the palladium electrodes used in acid solution in this laboratory is not due to increasing oxygen sensitivity is evidenced by the fact that no improvement was experienced on exposing the electrodes to atmospheric oxygen. This procedure Hammett found effec- tive in reducing the sensitivity of old electrodes. Moreover, the rate and extent of deterioration under acid conditions as shown by data in this paper are far greater than that ascribed by Hammett to oxygen sensitivity under like conditions. However, it seems very probable that such a mechanism is responsible for some of the results obtained in this laboratory on using alkaline solutions.

It should also be noted that this sensitivity towards oxygen may have considerable bearing on the results obtained from pH measure- ments in solutions containing oxyhemoglobin. While maximum oxygen sensitivity occurs under conditions far more alkaline than those prevailing in the usual biological range, still, even in 0.1 N

HCI the effect is perceptible and may well be of importance near neutrality.

One important factor in the use of hydrogen electrodes, particu- larly of palladium, which probably does not always receive the minute care it deserves, is the matter of contamination of the electrode with mercury. The use of mercury connections in the glass tube into which the electrode is sealed is very common and the frequent insertion and withdrawing of the copper wire lead may easily result in the spilling of small drops of mercury. This matter was first brought forcibly to the writer’s attention when using an electrode in which the platinum wire had been imperfectly sealed into the glass. Very small amounts of mercury crept through and amalgamated with the palladium, completely ruining the elec-

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490 Studies on Palladium Electrodes

trode.. In spite of the fact that metals of the platinum groups are not regarded as readily forming amalgams and that the only inorganic treatise available which mentioned palladium amalgams stated that they were only formed slowly and with difficulty, it has been repeatedly noticed that palladium-black electrodes amalgamate almost instantly when brought in contact with mer- cury. This amalgam, an easily flowing liquid, may be removed by anodic electrolysis in either dilute HCI or H&304 with the differ- ence that in the former case both mercury and palladium are removed, leaving a clean platinum surface, while in the latter, the mercury is removed as the sulfate leaving an electrode which is blacker than before amalgamation and which will not again amal- gamate-at least under the same conditions. When used as a hydrogen electrode in 0.1 N HCl against a calomel cellit gave only about 40 per cent of the voltage which should be obtained from a good electrode.

A recent communication by Bovie and Hughes (8) calls atten- tion to another source of trouble resulting from the introduction of mercury into the hydrogen electrode vessel. They find that the decomposition of mercurous chloride into mercuric chloride and free mercury, results in a gradual diffusion of the former through the salt bridge to the hydrogen electrode vessel. As a result, the electrode is “poisoned” and rendered useless. The authors recommend that the KC1 solution of the salt bridge should at times be tested with sodium sulfide solution for the presence of mercury.

Such tests applied to salt bridge solutions used in this laboratory have thus far given negative results. This may be ascribed to the fact that the calomel employed in the standard half-cell already contained a considerable proportion of finely divided metallic mercury which undoubtedly tended to reverse the action referred to above.

Causes of Electrode Deterioration.

The mechanics of the absorption of hydrogen in palladium have been the subject of much investigation, but only those papers having direct bearing upon the question of hydrogen electrode mechanism will be considered here. Holt, Edgar, and Firth (9), in studying the sorption of hydrogen in palladium, concluded that

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an active form of palladium, which is evidently the result of a metastable condition of the metal, is necessary in order that hydro- gen be adsorb^ed.

Andrew and Holt (10) studied further the sorption of hydrogen by palladium and concluded that additional evidence had been found to point to the dimorphic nature of the metal. They also stated that the relative stability of the two forms depends on the temperature. When existing separately, the change from the amorphous to the.crystalline form is very slow. That the metal exists in two different st’ates, depending on temperature and mode of treatment, is also indicated by consideration of the heating and cooling curves of palladium in hydrogen.

It seems very probable that the behavior of palladium elec- trodes recorded in this paper can be most simply explained by the dimorphic nature of the metal emphasized by the investigators quoted above. In the determination of hydrogen ions only the amorphous form of palladium would be expected to yield a thermo- dynamically reversible electrode. This amorphous form, the more active of the two, exists in metastable equilibrium with the stable crystalline variety and changes to the latter, particularly when the two are in contact. It seems reasonable, therefore, to explain the behavior of palladium electrodes by the supposition that this change to the stable form takes place slowly at all times and quite rapidly under favorable conditions. High hydrogen ion concentrations evidently constitute a favorable condition. Aside from the possibility of some catalytic action of unknown mechanism, it is conceivable that the more active form may well have a solution tension sufficiently higher than the crystalline to permit of an ionic exchange with H+.

Pd (amorphous) + 2H+ -+ Hz + Pd++ Pd++ + Hz -+ 2H+ + Pd (crystalline)

Such a mechanism would explain the particularly rapid dete- rioration of palladium electrodes in solutions of high acidity. Experimental evidence dealing with this point will be discussed below.

In this connection, the studies of Sieverts and Peters (11) on the catalytic oxidation of hypophosphorus acid by means of various forms of palladium-black are pertinent. They found that

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492 Studies on Palladium Electrodes

palladium-black is very active if prepared by reduction with for- mic acid, hypophosphorus acid, or carbon monoxide, that palla- dium sponge is less active after heating, and that’ wire and foil are inactive even if previously saturated cathodically with hydro- gen. They also found that the activity of palladium-black, made by electrolytic deposition on platinum or copper, drops very quickly and that this drop is hastened by addition of further quantities of acid. The drop obtained, even without addition of acid, is undoubtedly due to the increasing acidity which results from the formation of the more highly ionized phosphorus and phosphoric acids. The results of Sieverts and Peters throw particular suspi- cion upon the palladium-black made by electrolytic deposition. It seems probable that this method of preparation results in a product which contains an unusually large proportion of crystal- line palladium.

A rough measure of the proportions of the two forms present in various samples of palladium-black was effected by Firth (12) who determined the sorption of hydrogen by various samples in such a way that the results indicated the relative proportions of the two forms. Firth does not seem to have worked with electrolytic palladium-black, but it is interesting to note that the product formed by reducing PdClz with Hz contains much less of the amorphous phase than does that formed by reducing PdO with Hz. Here again the action of acid seems to come into play. Ahogether, he concludes that the mode of preparation bears a decided influence on the proportion of the two forms present.

In view of the mechanism suggested above, by which amorphous palladium is changed to crystalline, it seemed pertinent to com- pare the solution tensions of the two forms against a series of three palladium solutions. Pure PdCL solutions were made and their concentration checked by the method of Wunder and Thiiringer (6). An electrode of bright, carefully cleaned palladium wire and one of thickly plated palladium-black were used in each of the solutions. Voltages were measured in conjunction with a satu- rated calomel cell and the solution tension of the metal (KP~) was calculated by means of the usual formula:

Epd = 0.0287 log 2

applying to a divalent metal at 17°C.

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J. C. Andrews 493

These values were calculated on the assumption of complete ionization and no attempt was made to do more than to obtain a comparison of the two forms of palladium. For this reason no steps were taken to determine the cause of the progressive drop in values for K of both forms of palladium, which is of far too great magnitude to be accounted for by the degree of ionization of PdC12, but for which the contact potential between the saturated KC1 solution and the dilute PdClz solution is probably responsible. However, the higher solution tension of the less stable form is very evident at all concentrations used. A number of measure- ments were taken in each case, the above figures for K represent- ing the average of all values obtained. In no case did these values vary by more than 0.3 X 10 with the same exponent as that of the average value. An older determination of Kpd by Neumann (13) gives the value of 4.0 X 10-36.

TABLE V.

Solution Tensions of Amorphous and Crystalline Palladium.

c Electrode.

mozs per 1.

0.02805 0.02805 0.002805 0.002805 0.0002805 0.0002805

Crystalline. Black. Crystalline. Black Crystalline. Black.

Epd

volts 0.9512 0.8508 1.0006 0.9283 1.0184 0.9776

Kpd

2.0 x 10-36 6.4 X lo-= 3.9 x 10-38 1.3 x 10-35 9.2 x 10-40 2.4 X 1O-38

SUMMARY.

1. Palladium electrodes for hydrogen ion determinations are much less reliable than platinum electrodes because of the lack of permanence of the former which results from a more or less rapid change of amorphous to crystalline palladium.

2. This change is positively catalyzed by high hydrogen ion concentrations, the mechanism being probably an electronic exchange facilitated by the higher solution tension of amorphous palladium.

3. The use of palladium electrodes in connection with solutions of high pH gave very erratic results.

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4. Optimum conditions for the preparation of palladium elec- trodes were determined.

5. The relative solution tensions of the two forms of palladium were determined.

BIBLIOGRAPHY.

1. Clark, W. M., The determination of hydrogen ions, Baltimore, 2nd edition, 1922.

2. Michaelis, L., Die Wasserstoffionenkonzentration, Berlin, 1914. 3. Cullen, G. E., J. Biol. Chem., 1922, lii, 521. 4. Hulett, G. A., and Bonner, W. D., J. Am. Chem. Sot., 1909, xxxi, 390. 5. Noyes, A. A., and Ellis, J. H., J. Am. Chem. Sot., 1917, xxxix, 2532. 6. Wunder, M., and Thiiringer, V., 2. anal. Chem., 1913, lii, 101. 7. Hammett, L. P., Experimental studies on the hydrogen electrode,

Thesis, Columbia University, New York, 1922. 8. Bovie, W. T., and Hughes, W.S., J. Am. Chem. Xoc., 1923, xiv, 1904. 9. Holt, A., Edgar, E. C., and Firth, J. B., 2. physik. Chem., 1913, Ixxxij

513. 10. Andrew, J. H., and Holt, A., Proc. Roy. Xoc., London, Series A, 1913-14,

lxxxix, 170. 11. Sieverts, A., and Peters, E., 2. physik. Chem., 1916, xci, 199. 12. Firth, J. B., J. Chem. SOL, 1921, cxix, 1120. 13. Neumann, B., 2. physik. Chem., 1894, xiv, 193. by guest on M

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James C. AndrewsPALLADIUM ELECTRODES

EXPERIMENTAL STUDIES ON

1924, 59:479-494.J. Biol. Chem. 

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