Purification of Supporting Electrolytes for Polarographic Trace Analysis by Contralled Potential Electrolysis at Mercury Cathode LOUIS MEITES Department o f Chemistry, Yale University, New Haven, Conn.
inum anode in the cell used (3) or in any other nondiaphragm cell, but the flow of a rapid stream of nitrogen through the solution keeps the concentration of dissolved oxygen always very small.] The recording polarograph used has been described (4).
A simple and generally applicable method for the removal of heavy metal impurities from supporting electrolytes to be used in polarographic trace analysis consists of electrolyzing the solution with a mercury cathode whose potential is kept constant at a suitable value, and in each of the cases studied reduces the concentration of the impurity to a polarographically undetectable value within 45 minutes of unattended electrolysis.
APPLICATIONS O F THE METHOD
Removal of Zinc from Sodium Hydroxide. During the development of a polarographic method for the determination of zinc in lead and its compounds, it was found that the percentage of zinc in the best available sodium hydroxide was of the order of 0.001%. Thus the zinc wave in the blank was approximately equal in height to the wave n hich xould have been secured from a sample containing 0.01% of zinc, and would have seriously affected the accuracy and precision of the determination of small percentages of zinc. A typical polarogram of a 2 N sodium hydroxide solution, secured a t one tenth of the highest sensitivity of the polarograph, is shown as curve a in Figure 1. The diffusion current of zinc measured from this curve, 0.083 Ga., corresponded to 0.012mM zinc. A portion of this solution was subjected to electrolysis as described above, with the potential of the mercury cathode kept constant a t -1.8 volts us. S.C.E After 30 minutes, a portion of the solution was withdrawn from the cell: Its polarogram is shown as curve b. Since no trace of a zinc wave could be detected even at the full sensitivity of the polarograph, it is estimated that the concentration of zinc remaining in solution must have been less than 0.0002mM. The fact that the current on curve b is appreciably lower than that on curve a even a t potentials preceding the zinc wave shows that the original sodium hydroxide also contained traces of other reducible impurities which were removed by the electrolysis. When a sodium hydroxide solution thus purified was allowed to stand in borosilicate glass, a small zinc wave was observed after some time, doubtless because traces of zinc were leached from the glass. Such solutions should therefore be stored in thoroughly cleaned polyethylene bottles. Removal of Nickel and Zinc from Ammoniacal Ammonium Chloride. This was of interest in connection with the develop ment of a method for the determination of nickel and zinc in copper and its salts. I n the procedure finally evolved, a stock 4M ammonia-4M ammonium chloride-lM hydrazine hydrochloride solution was prepared and diluted fourfold in the course of dissolving the sample. Curve a in Figure 2 is a polarogram of such a stock solution prepared from the beet available chemicals. Curve b is a polarogram of the same solution, recorded with slightly higher damping than curve a , after electrolysis for 30 minutes a t a cathode potential of - 1.6 volts 2's. S.C.E., and it shows no detectable wave for either element. Moreover, it is clear that the solution also contained traces of other reducible substances which were removed by the electrolysis. Removal of Iron from Citrate Medium. A recently published method ( 9 ) for the determination of iron in copper-base alloys involves the measurement of the height of the ferric iron wave in a weakly acidic 1 M citrate buffer after removal of the copper. The amount of iron present in the citric acid used for the preparation of the buffer proved a source of serious annoyance in the determination of traces of iron. A procedure for the preparation
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NE of the most important problems confronting the trace analyst is that of securing or preparing reagents which are sufficiently free from the substance being determined to give a satisfactorily low blank. Though t h k problem is by no means peculiar to polarographic analyses, it is perhaps especially severe there because of the relatively large amounts of indifferent salts used. It is not a t all uncommon to employ a 1M supporting electrolyte in a polarographic procedure in which the substance being determined may be present a t a concentration as low as 10-6 or 10-6M. For an analysis to be even feasible-let alone accurate-under these conditions, it is clearly essential that the concentration of the substance being determined in the supporting electrolyte be no greater than 10-6M. This corresponds to an impurity of about 0.001% or less in the solid salt, which is a standard not satisfied by many commercially available reagents. There is now reported the development of a method for the purification of supporting electrolytes which is more efficient, simple, and rapid than any other generally applicable method for the removal of traces of heavy metal impurities. I t consists of pre-electrolyzing a portion of the supporting electrolyte solution with a mercury cathode, using a potentiostat to maintain the cathode potential constant a t a value a t which the deleterious impurity is completely deposited. The only evident limitation of the method is the fact that it cannot be used for the removal of elements such as tungsten, vanadium, and uranium, which cannot be reduced to the metallic state by electrolysis of an aqueous solution a t a mercury cathode. However, such elements are rarely, if ever, present in significant amounts in reagents used as supporting electrolytes. EXPERIMENTAL
The potentiostat used in this work was designed in collaboration with Julian M. Sturtevant. It is to be made available commercially by Analytical Instruments, Inc., of Bristol, Conn. A feature especially useful in this work was the fact that the input impedance of the amplifier is high enough to permit the use of a &inch calomel reference electrode available from the National Technical Laboratories for use with Beckman pH meters: This is not true of some of its predecessors (1). In principle, all of the purifications described could have been accomplished by the classical constant current electrolytic technique. However, the use of a potentiostat for controlling the cathode potential has a very considerable practical advantage, in that it permits the progress of the purification to be followed by simply observing the magnitude of the electrolysis current, which decreases practically to zero a t the completion of the electrolysis. To take full advantage of this indication, oxygen was removed from all solutions by a stream of nitrogen. [Sormally osygen is formed by the electrochemical reaction a t a plat416
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V O L U M E 27, NO. 3, M A R C H 1 9 5 5 of such a solutioii free from any polarographically detectable trace of iron has now been devised. The citric acid solution is merely made about 0. Llf in excess sodium hydroxide and electrolyzed at -1.75 volts v s . S.C.E. for about 30 minutes. This is well beyond the half-wave potential of the reduction wave of the ferrous complex in this medium. When the iron has been completely deposited, the solution is removed from the cell and neutralized to the desired pH with concentrated hydrochloric acid. This modification of the general procedure is necessitated by the fact that the ferrous citrate complex can be reduced to the metallic state a t a lower potential than is required to reduce hydrogen ion only in an alkaline solution. Removal of Alkali and Alkaline Earth Metals from Tetraethylammonium Hydroxide. This separation cannot be accomplished directly by recrystallization because of the high solubility of the hydroxide. Instead tetraethylammonium bromide must be recrystallized several times, then converted to the desired hydroxide by treatment with moist silver oxide. This is a tedious procedure, and the yield obtainable is low enough to make it a rather expensive one as well.
cessity in this instance, for no other type so nearly completely prevents transfer of potassium ion from the reference electrode into the solution being electrolyzed. It is convenient to reserve one electrode for such electrolysis and to store it in a tetraethylammonium chloride or hydroxide solution between electrolyses to leach most of the potassium out of the fiber.
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Figure 2. Polarograms of 4M ammonia4M ammonium chloride-lM hydrazine hydrochloride (a)Before and ( b ) after controlled potential electrolysis at - 1.80 volts us. S.C.E.
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Figure 1. Polarograms of 2.M sodium hydroxide ( a ) Before a n d ( h ; a f t r r controlled potential electrolysis a t - 1.80 vults u s . S.C.E.
In 50% ethyl alcohol containing 0.lM tetraethylammonium hydroxide the half-wave potentials of the alkali metals range from -1.99 volts for rubidium to -2.31 volts for cesium, and those of the alkaline earth metals range from -1.86 volts for barium to about -2.2 volts for magnesium, while the final current rise does not begin until about -2.4 volts. It has been found that all these metals could be removed by electrolysis a t -2.35 volts for about 45 minutes. The current does not decrease quite to zero when the purification is complete, and so the electrolysis should be discontinued when the current has remained constant at a few milliamperes for 10 or 15 minutes. The fiber-type calomel electrode mentioned is virtually a ne-
In a typical instance an electrolysis for 30 minutes served t o purify a 50% ethyl alcohol-0.1M tetraethylammonium hydroside EO thoroughly that the residual concentration of alkali and alkaline earth metals was too low to be detected polarographically. Other Applications. Though this technique has as yet been applied only to solutions to be used in polarographic work, it should be useful in various other types of procedures as well. It should, for example, prove convenient in removing such elements as lead and zinc from reagents to be used in colorimetric procedures employing dithizone, which is a ubiquitous problem of colorimetric analysis. LITERATURE CITED
(1) Lingane, J. J., “Electroanalytical Chemistry,” Interscience, New York, 1953. (2) hfeites, L., ANAL.CHEM.,24, 1374 (1952). (3) Ibid., submitted for publication. (4) Rfeites, L., J. Am. Chem. Sac., 76, 5927 ( 1 9 5 4 ) . RECEIVED for review September 29, 1954. Accepted December 4, 1954. Presented before the Division of Analytical Chemistry at the 126th Meeting of the AMERICAX CHEMICAL SOCIETY, New York, N. Y . , September 1954. Contribution 1254 from the Department of Chemistry of Yale University.
