THE APPLICATION OF ELECTROLYSIS TO QUALITATIVE ANALYSIS.

By CHARLES A. KOHN, B.Sc., Ph.D., Assistant Lecturer in Chemistry, University College, Liverpool.

The first application of electrolysis to chemical analysis was made by Gaultier de Claubry, in 1850, who employed the electric current for the detection of metals when in solution. Other early workers followed in this direction, and in 1861 Bloxam published two papers (J. Chem. Soc., 13, 12 and 338) on "The application of electrolysis to the detection of poisonous metals in mixtures containing organic matters." In these papers a description is given of means for detecting small quantities of arsenic and of antimony by subjecting their acidulated solutions to electrolysis. The arsenic was evolved as hydride and recognized by the usual reactions, while the antimony was mainly deposited as metal upon the cathode. The electrolytic method for the detection of arsenic, in which all fear of contamination from impure zinc is overcome, has since been elaborated by Wolff, who has succeeded in detecting as little as 0.00001 grm. arsenious oxide by this means (this Journal, 1887, 147).

In a somewhat different manner the voltaic current is made use of in ordinary qualitative analysis for the detection of tin, antimony, silver, lead, arsenic, etc., by employing a more electro-positive metal to precipitate a less electro-positive one from its solution.

The quantitative electrolytic methods of analysis, some of which I had the honor of bringing before the notice of the Society some time back (this Journal, 1889, 256), have placed a number of methods of determination and separation of metals in the hands of chemists, which can be employed with advantage in qualitative analysis, especially in case of medical and medico-legal inquiry. These methods are not supposed to supersede in any way the ordinary methods of qualitative analysis, but to serve as a final and crucial means of identification, and thus to render it possible to detect very small quantities of the substances in question with very great certainty. As such they fulfill the required conditions admirably, being readily carried out, comparatively free from contamination with impure reagents, and capable of being rendered quantitative whenever desired.

In conjunction with Mr. E.V. Ellis, B.Sc., I have examined the applicability of the electrolytic methods for the detection of the chief mineral poisons (with the exception of arsenic, an electrolytic process for the detection of which has already been devised, as described), viz., antimony, mercury, lead, and copper.

Antimony.—The method employed in the case of antimony is that adopted in its quantitative estimation by means of electrolysis, a method which insures a complete separation from those metals with which it is precipitated in the ordinary course of analysis—arsenic and tin. This fact is of considerable importance in reference to the special objects for which these methods have been worked out.

The precipitated sulphide is dissolved in potassium sulphide, and the resultant solution, after warming with a little hydrogen peroxide to discolorize any poly-sulphides that may be present, electrolyzed with a current of 1.5-2 c.c. of electrolytic gas per minute (10.436 c.c. at 0° and 760 mm. = 1 ampere), when the antimony is deposited as metal upon the negative electrode. One part of antimony (as metal) in 1,500,000 parts of solution may be thus detected, a reaction thirty times more delicate than the deposition by means of zinc and potassium. The stain on the cathode, which latter is best used in the form of a piece of platinum foil about 1 sq. cm. in diameter, is distinct even with a solution containing 1/28 mgrm. of antimony; and by carefully evaporating a little ammonium sulphide on the foil, or by dissolving the stain in hot hydrochloric acid and then passing a few bubbles of sulphureted hydrogen gas into the solution, the orange colored sulphide is obtained as a satisfactory confirmatory test. The detection of 0.0001 grm. of metal can be fully relied on under all conditions, and one hour is sufficient to completely precipitate such small quantities.

Mercury.—Mercury is best separated from its nitric acid solution on a small closely wound spiral of platinum wire. The solution to be tested is acidified with nitric acid and electrolyzed with a current of 4-5 c.c. (c.c. refer to c.c. of electrolytic gas per minute). The deposition is effected in half an hour. The deposited metal is removed from the spiral by heating the latter gently in a test tube, when the mercury forms in characteristic globules on the upper portion of the tube. As a confirmatory and very characteristic test, a crystal of iodine is dropped into the tube, and the whole allowed to stand for a short time, when the presence of mercury is indicated by the formation of the red iodide. 0.0001 grm. of mercury in 150 c.c. of solution can be clearly detected.

Wolff has applied this test under similar conditions, using a special form of apparatus and a silver-coated iron anode (this Journal, 1888, 454).

Lead.—Lead is precipitated either as PbO₂ at the anode from a nitric acid solution or as metal at the cathode from an ammonium oxalate solution. In both cases a current of 2-3 c.c. suffices to effect the deposition in one hour.

Here, again, 0.0001 grm. of metal in 150 c.c. of solution can be easily detected. With both solutions this amount gives a distinct discoloration to the platinum spiral, on which the deposition is best effected. As a confirmatory test the deposited metal is dissolved in nitric acid and tested with sulphureted hydrogen, or the spiral may be placed in a test tube and warmed with a crystal of iodine, when the yellow iodide is formed. This latter reaction is very distinct, especially in the case of the peroxide.

Of the above two methods, that in which an ammonium oxalate solution is used is the more delicate, although it cannot be employed quantitatively, owing to the oxidation of the metal that takes place.

An addition of 1 grm. of ammonium oxalate to the suspected solution is sufficient.

Copper.—0.00005 grm. of copper can be very readily detected by electrolyzing an acid solution in the usual way. A spiral of platinum wire is employed as the cathode, and the presence of the metal confirmed for by dissolving it in a little nitric acid, diluting with water and adding potassium ferrocyanide.

To detect these metals in cases of poisoning, the organic matter with which they are associated must first be destroyed in the usual way by means of hydrochloric acid and potassium chlorate, and the precipitates obtained in the ordinary course of analysis, then subjected, at suitable stages, to electrolysis. As the solutions thus obtained will be still contaminated by some organic matter, it is necessary to pass the current for a longer time than indicated above. On the other hand, urine can be tested directly for these poisons.

The presence of mercury or of copper may be detected by acidifying the urine with 2-3 c.c. of nitric acid (conc.), and electrolyzing as described. 0.0001 grm. of metal in 30 c.c. of urine can be detected thus, or 1 part in 300,000 of urine.

Lead does not separate well as peroxide from urine, but if ammonium oxalate be added, and the lead deposited as metal, the reaction is quite as delicate as in aqueous solution, and 0.0001 grm. of lead can be thus detected.

With antimony it is advisable to precipitate it first as sulphide, but it can be detected directly, though not so satisfactorily, by acidifying the urine with 2-3 c.c. of sulphuric acid (dil.), and electrolyzing with a current of 1-5 to 2 c.c. In this case also it is precipitated as metal upon the cathode (cp. Chittenden, Proceedings Connecticut Acad. Science, Vol. 8).

In the presence of urine it is advisable to continue the passage of the current for about twice the time required in the case of aqueous solutions.

That an approximately quantitative result can be obtained under the above conditions was shown in several cases in which deposition of 0.001 grm. of metal was confirmed with considerable accuracy, the spiral or foil being weighed before and after the experiment.

A comparison of the delicacy of these tests with the ordinary qualitative tests for antimony, mercury, lead, and copper by means of sulphureted hydrogen, showed that the two were equally delicate in the case of antimony and of copper, but that in that of mercury and of lead the electrolytic test was at least eight times the more delicate. These comparisons were made in aqueous solutions. In testing urine the value of the electrolytic method is still more evident, for here the color of the liquid interferes materially with the reliability of the ordinary qualitative tests when only very small quantities of the metals referred to are present.

Beyond the detection of mineral poisons, qualitative electrolysis can only offer attraction to analysts in special cases, and the data on the subject are to be found in the many electrolytic methods already published. Beyond testing for gold and silver in this manner, I have not therefore examined the applicability of these methods further.

The detection of small quantities of gold and silver is of considerable importance, and advantage can be taken of the ease with which they are separated from potassium cyanide solution by the electric current for this purpose.

Silver.—Silver is obtained as chloride in the course of analysis. To confirm for the metal electrolytically, this precipitate is dissolved in potassium cyanide and the resulting solution electrolyzed with a current of 1-1.5 c.c. A spiral of platinum wire is employed as the anode, from which the silver may be dissolved by means of nitric acid, and tested for by hydrochloric acid or by sulphureted hydrogen. 0.0001 grm. of silver in 150 c.c. of solution can be detected thus, and one hour is sufficient for the deposition.

Gold.—Gold is deposited under similar conditions to silver from cyanide solutions. The deposit, which is rather dark colored, can be dissolved in aqua regia and confirmed for by the Cassius' purple test. Here again 0.0001 grm. of metal in 150 c.c. of solution can be detected without any difficulty.

As gold and silver are both extracted from quartziferous ores by treatment with potassium cyanide solution according to the MacArthur-Forrest process of gold extraction (this Journal, 1890, 267), this electrolytic method should prove very useful. By electrolyzing the resulting solution a mixture of gold and silver will be deposited upon the cathode, which can then be parted by nitric acid and tested for as described.

DISCUSSION.

The chairman said that there was little doubt but that further investigation into electrolytic methods of chemical analysis would give even more valuable results than those already obtained. Systematic investigations of the subject, such as have been given by Dr. Kohn, would go far to prove the adaptability of this method as a substitute for or aid in ordinary qualitative examinations. The remarks of Dr. Kohn respecting quantitative examinations were very interesting, and well worth following up by other practical work.

Professor Campbell Brown said that Dr. Kohn had shown that electricity brought the same kind of elegance, neatness, and simplicity into analysis that it did into lighting and silver plating.

In its applications to the detection of poisons, he understood Dr. Kohn to say that the poisons must first be extracted by chemical means. That would not be sufficient, and he had no doubt that if the subject was pursued farther they would have a paper from him (Dr. Kohn) some day, indicating that he had obtained arsenic and such poisons without the previous separation of the metal from organic matter. It was a very great desideratum to have a method for detecting arsenic and separating it from the contents of the stomach and food directly without previous destruction of the organic matter, and he hoped Dr. Kohn would pursue his work in that direction.

Dr. Hurter said he was about to construct a new laboratory, and he would assure them that one of its arrangements would be the installation of electricity, by which to carry out researches similar to those described. He was very glad to learn that the presence of arsenic, etc., could be readily proved by means of electrolysis.