T H E PREPARATIOK OF T H E HYDROGES ELECTRODE AXD T H E REMOVAL O F A COhIMON SOURCE O F TROUBLE I N COKRECTION WITH I T S USE BY STEPHEN POPOFF, ADOLF H. K V S Z , AND R. D. S O W
Although an infinite amount of literature has been written about hydrogen ion determination by electrometric methods, very little can be found about the actual details of manipulation, methods of plating, factors affecting equilibrium, etc. That which has been published is often contradictory. After describing the plating of electrodes Clark‘ frankly states, “It must be admitted that the above description is loose. This is because the rush of experimental application has prevented a detailed examination of conditions and experience has taught details difficult to formulate in exact language.” Ellisz maintains that the use of lead in platinizing solution is objectionable. On the other hand, Beans and Hammett3 have no difficulty in preparing platinized electrodes of high activity from solutions containing lead. Lewis, Brighton and Sebastian4 find a gold or gold plated electrode superior but Beans and Hammett do not find gold plating an advantage. Ellis prescribes platinizing until the coating is thick enough to shake off; Clark until the electrode is covered with a deposit of thickness sufficient to cover the glint of polished metal and so that it adheres under a vigorous stream of water; Tolrnan and F e r g u s ~ n till , ~ the electrode assumes a velvety appearance. Similarly, authors are not agreed upon the time required for the electrodes to come to equilibrium and other details in electrometric determinations. I n view of this lack of agreement and lack of information it seemed desirable to study the conditions and thereby obviate some of the difficulties. Very often in electrometric determinations involving the use of the hydrogen electrode a poisoning of the platinum causes a decrease in potential which may even result in a reversal of polarity. This necessitates the replatinizing of electrodes, an expensive and time-consuming process. Therefore, it seemed desirable to devise some means of restoring the potential without replatinizing. I n an investigaI n this work a precision of 0.1 millivolt was sought. tion on oxidation potentials in which the temperature was kept constant to =tO.OI’, duplicate electrodes prepared by the method given showed a maximum variation of 0.01 mv.
*
Clark: “The Determination of Hydrogen Ions.” Second edition (1922). *Ellis: J. Am. Chem. SOC.,38,737 (1916). Beans and Hammett: J. Am. Chem. SOC., 47, 1216(19zj). Lewis, Brighton and Sebastian: J. Am. Chem. SOC.,39, 2245 (1917). 3 Tolman and Ferguson: J. Am. Chem. SOC.,34,232 (1913).
PREPARATIOX OF T H E HYDROGES ELECTRODE
I057
Procedure Throughout this work the usual potentiometric set-up was used with a Leeds and Korthrup Type K potentiometer. A modification of the Hildebrand form of hydrogen electrode was used with a saturated potassium chloride calomel electrode. The hydrogen was generated by the electrolysis of sodium hydroxide solution. It was purified by being passed through alkaline permanganate solution, cotton, water and the same solution of which the hydrogen ion concentration was being determined. The solution was surrounded by a bath constant t o i 0.1'. The electrodes used consisted of pieces of platinum foil 15 mm. x 5 mm. fused on to a piece of platinum wire and into a glass tube as recommended by Clark. Contact was made by filling the tube with mercury. All salts used were recrystallized and the mercury for the calomel electrode redistilled. Use of the Bovie Titration Cell When an open beaker was used for hydrogen ion determination, ten minutes were required for a thinly coated electrode to attain equilibrium. When a Bovie titration cell was substituted the time was reduced to three minutes. The latter has the advantage that the current of hydrogen stirs the solution and maintains an atmosphere of hydrogen above the solution. It is necessary to have the solution saturated with hydrogen before equilibrium can be obtained. Until then the electrode behaves more or less as an oxygen electrode. Use of Gold Plated Electrodes A preliminary plating of gold on the electrode is recommended by Lewis Brighton and Sebastian' because gold, unlike platinum, does not absorb large amounts of gases and therefore comes to equilibrium more quickly. A comparison of electrodes plated with and without gold showed no difference when placed in a solution already saturated with hydrogen, both requiring three to five minutes to attain equilibrium. This is in accord with the results of Beans and Hammett who concluded that the massive platinum need not be saturated with hydrogen. However, since a slightly more adherent coating of platinum black is obtained when the electrode is gold plated and since the deposit is more easily removed when electrolyzed in hydrochloric acid, gold plated electrodes were used throughout this work and are recommended. Use of Lead Acetate in Platinizing Solution Only a very slight deposit could be obtained on a platinum electrode when chemically pure chloroplatinic acid solution was used. This electrode gave a fluctuating reading, smaller than the proper value, when placed in a buffer solution. An unsuccessful attempt was made to obtain a deposit of platinum black from pure chloroplatinic acid solution using a I I O volt current. This only resulted in the formation of a sludge in the vessel. The minimum quantity of lead acetate required to produce a good deposit was found to be 0.5 cc. of a 0.1N solution in IOO cc. of 3 7 c chloroplatinic acid solution. Electrodes prepared in this way agreed to 0.1millivolt.
IO j8
STEPHEN POPOFF, ADOLF H. KUNZ, AND R. D. SNOW
Current Density in Plating
No noticeable difference could be obtained over a considerable range of current density but good results were obtained with a current of 2 0 milliamperes from two dry cells in series connected through a variable resistance. This current was maintained after the bubbles of gas were being evolved. As the electrodes had a surface of 1.5 sq. cm. this corresponds to a current density of about I .35 amperes per square decimeter.
Nature of Deposit Electrodes were prepared from solutions of chloroplatinic acid containing enough additional hydrochloric acid to make the total acidity 0.5 N and from solutions containing no additional acid. A comparison of the time required for equilibrium was made in a borax-boric acid buffer solution, a potassium chloride-hydrochloric acid buffer, a solution of borax and a solution of sulfuric acid. Both thinly and thickly plated electrodes were compared. The former were platinized in five minutes and the latter in forty-five.
TABLE I Effect of Acidity on Plating the Electrodes HzPtCle
Borax Alkaline buffer Sulfuric acid Acid buffer
Thin
Thick
3 min. 3 ”
7 min.
5
”
4
”
HzPtCle Thin
4-7 min. 4-5 ”
15-20
”
7-10
”
3
”
”
3-4
”
1-12
+ HC1Thick 20-25 1-12
min. ”
7-10
7
” ”
No great difference in time for equilibrium is caused by the acidity of the platinizing solution when the thinly plated electrode is used. The thick deposit from strongly acid solution requires more time in alkaline solutions than in acid. In all cases the thick deposit requires considerably more time for equilibrium. As the same reading is obtained with the thin deposit the latter is recommended. Another argument for the use of the thin deposit is found in the work with phthalate buffers. Merri11,l and Oakes and Salisbury* found a drifting in potential when used in a phthalate buffer. Clarks and Wood and Murdick4 could find no such drift. Draves and Tartar5 showed that the reduction is due to the use of a thickly coated electrode as employed by the former while the thin deposit used by Clark causes no reduction. The wellknown catalytic activity of finely divided platinum should be reduced to a minimum. Merrill: J. 4m. Chem. SOC.,43,2688 (1921). Oakes and Salisbury: J. Am. Chem. SOC., 44,948 (1922). BClark: J. Am. Chem. SOC.,44, 1072 (1922). ‘Wood and Murdick: J. Am. Chem. SOC.,44,2008 (1922). 6 Draves and Tartar: J. Am. Chem. SOC., 47, 1226 (1925). 1
2
PREPARATION O F THE HYDROGEN ELECTRODE
1059
Directions for Plating Electrodes The following method of plating platinum black electrodes is recommended, although other conditions may give equally good results: I. Clean electrode by electrolyzing as anode in concentrated hydrochloric acid until old deposit is removed. New electrodes should be cleaned in hot alkali. Electrolyze as cathode in a dilute potassium cyanide solution of gold. 2. The directions of Clark for preparing this solution are:-Dissolve 0.7 g. gold chloride in 50 cc. water and precipitate the gold with ammonia water, taking care to avoid excess. Filter, wash and dissolve immediately in a KCN solution consisting of 1.5 g. KCN in IOO cc. water. Boil till the solution is free from the odor of ammonia. 3. Electrolyze as cathode for five minutes in a 3%solutionof chloroplatinic acid containing 0.5 cc. of 0.1N lead acetate in roo cc. A current density of 1.35 amperes per square decimeter is employed. 4. Electrolyze as cathode in dilute sodium hydroxide solution a few seconds. 5 . Electrolyze as cathode in dilute sulfuric acid one minute. 6. Keep in distilled water till ready for use. No advantage was found in reversing the current during the deposition of platinum black. Poisoning of Electrodes The decrease in potential of the hydrogen-calomel electrode system after continued use is considered by Bovie and Hughes’ as due to a poisoning by mercuric chloride. In the calomel electrode the following decomposition takes place: H g d X +HgC& H g Evidence of this is found in the gray color the calomel assumes and in a positive test for mercuric chloride in the potassium chloride solution. The mercuric chloride diffuses into the titrating chamber where it poisons the finely divided platinum. If this is the cause, then, whether the poisoning is due to the mercuric chloride itself, a product of reduction, or a prodvct of interaction with ma-
+
TABLE I1 Removal of “Poisons” from Electrodes Poisoned by
Hg Clz soln. 11
1)
1,
7 j
,l
)I
1
Mv. 592. I 592 ’ I 593.2
587.8
Electrode soln. 11
Reading before poisoning
,,
590.5
587.6
Reading after poisoning Mv.
-2 0 0 - 44
Reading after treatment Mv.
- 238
592.2 592.3 593.1
-302
587.6
-2 0 0 - 200
590.5
Bovie and Hughes: J. Am. Chem. SOC.,45, 1904 (1923).
587 ’ 5
1060
STEPHEK POPOFF, ADOLF H. KUNZ, AND R. D. S N O W
terials in the solution, it should be removed,in most cases a t least, by nitric acid. It was observed by one of us that the potential could be restored a t least qualitatively by such treatment. Table I1 shows the results of a quantitative study of the poisoning by mercuric chloride solution and the solutions from calomel electrodes and the removal of the poison. I n all cases after treatment with nitric acid, the electrode was electrolyzed a few seconds in sodium hydroxide solution and then in sulfuric acid. This removes the nitric acid and saturates the electrode with hydrogen. The poisoning was much more readily effected when a thickly coated electrode was used, another argument for a thin deposit. Extreme difficulty was found in poisoning electrodes. Apparently, when repurified chemicals are used throughout, the poisoning danger is not as great as with ordinary chemicals. Attempts to poison the electrode with formaldehyde, phenol, and potassium cyanide were unsuccessful. An electrode poisoned with the solution from a calomel electrode was also restored by treatment with dichromate cleaning mixture.
Summary and Conclusions The hydrogen electrode attains equilibrium more quickly in a Bovie titration cell than in an open beaker. 2. A preliminary plating of gold on electrodes does not effect the time required for attainment of equilibrium but causes a more adherent coating of platinum black to be deposited which is more easily removed in cleaning. 3. A thinly coated electrode is more desirable than a heavily coated one. 4. Directions for plating electrodes are given. 5 . Poisoning due to mercuric chloride can be removed by treatment with nitric acid or dichromate cleaning mixture. I.
Analytical Laboratory,
State Universaty of Iowa.
