ELECTROLYTIC DETERMINATIONS AND SEPARATIONS.

[Footnote: NOTE.--Each of these determinations was accompanied by a series of results in which the practical determinations obtained from the method described were compared with the theoretical contents of the solutions of the various elements. These, however, would take up too much room for insertion in these columns.]

By ALEX. CLASSEN and M.A. VON REIS; translated by M. BENJAMIN, Ph.B., F.C.S.

Ever since the electrolytic method for the estimation of copper came into general use, numerous chemists have endeavored to adapt this peculiarly simple and elegant method to the determination of other metals. According to the experiments which have been made up to the present time, it has been found that the separation of copper is best effected in a nitric acid solution, while that of nickel and cobalt takes place most readily in an ammoniacal solution, and for the precipitation of zinc and cadmium a potassium cyanide solution is the best. The accuracy of the results depend chiefly upon the following of certain fixed rules, such as, for instance, that the precipitation of copper only takes place when there is a definite amount of nitric acid in the solution; that of cobalt and nickel when a certain quantity of ammonium hydrate and ammonium sulphate is present. The electrolytic decomposition of the chlorides has not yet been successfully accomplished, so that prior to the operation it is necessary to convert them into sulphates. The experiments which have been made for the purpose of investigating the application of the electric current in quantitative analyses are very few, about the only exception being the separation of copper from the metals which are not precipitated from a nitric acid solution, or which are deposited as peroxides at the other electrode. We shall endeavor to show in that which follows, that copper, zinc, nickel, and cobalt, and even iron, manganese, cadmium, bismuth, and tin, whether they be present as sulphates, chlorides, or nitrates, may be precipitated and separated from each other by electrolytic methods much more rapidly than by any previously known process.

DETERMINATION OF COBALT.

Neutral potassium oxalate is added in excess to the solution of a cobalt salt, and the clear solution of cobalt potassium oxalate submitted to electrolysis. The intense red color of this solution is soon changed into a dark green; the latter diminishing in intensity as the metal is deposited at the negative electrode. The electric current decomposes the potassium oxalate into the carbonate, so that a precipitate of cobalt carbonate is simultaneously formed with the separation of the metallic cobalt. This precipitate may be dissolved by adding oxalic acid or dilute sulphuric acid; the further action of the current will change the solution to an alkaline reaction, upon which the treatment with acid is repeated until all the cobalt has been separated out in its metallic condition. The electrolytic separation of cobalt is much more easily and rapidly effected when the potassium oxalate is substituted by the corresponding ammonium salt, as the latter forms a soluble double salt with the cobalt compounds. If the ammonium oxalate added is just sufficient to form the double salt, a red cobalt oxalate (which is only slowly reduced by the current) will separate out in addition to the cobalt. In order to obviate this difficulty, the solution to which the ammonium oxalate had been added in excess is heated, and then three or four grammes more of solid ammonium oxalate are added. The hot solution, when exposed to the action of the current, deposits the cobalt as a closely adhering gray film. By the aid of two Bunsen's elements, 0.2 gramme cobalt can be separated in an hour's time. When the reduction has been completed, and this is best determined by testing a small sample (removed by a pipette) with ammonium sulphide, the positive electrode[1] is removed from the solution, and the liquid poured off. The dish is immediately rinsed several times with water, and the excess of water removed at first with alcohol, and then with absolute ether. The cobalt in the dish is dried in the air bath at 100° C., and in the course of a few minutes a constant weight is obtained.

[Footnote 1: A piece of platinum foil, 4.5 cm. in diameter, is used for the positive electrode, and a deep platinum dish as the negative electrode.--Vide "Classen's Quantitative Analysis," 3d Edition, p. 46.]

DETERMINATION OF NICKEL.

This process is precisely identical with the previously described method for cobalt. The ammonium oxalate is added in excess to the solution, which is then heated, and four more grammes of the solid salt added. The separation of the nickel is as rapid as that of the cobalt. The nickel is precipitated as a gray, compact mass, tightly adhering to the electrode.

DETERMINATION OF IRON.

For this estimation, solutions of the chloride as well as those of the sulphate (ammonium, iron, alum) may be used in the manner previously described. The electrolysis is best effected in the presence of a sufficient quantity of ammonium oxalate; no separation of any iron compound takes place. The iron is deposited in the form of a bright, steel gray, firmly-adhering mass on the platinum dish. The iron may be exposed to the air for several days without any noticeable oxidation taking place.

DETERMINATION OF ZINC.

Zinc may be separated from a solution of the double salt fully as easily and rapidly as the previously mentioned metals were. The reduced zinc has a dark gray color, and adheres very firmly to the electrode. The separated metal is dissolved by using dilute acids and heating. It is only removed with difficulty, and generally leaves a dark coating on the dish, which is separated by repeated ignitions and treatment with acid.

DETERMINATION OF MANGANESE.

It is already known that manganese may be separated as the peroxide from its nitric acid solution. We find, however, that the precipitation is only completely effected when the quantity present is small; the amount of nitric acid must also be slight, and it is necessary to wash the dish without interrupting the current. If the manganese is converted into the soluble double salt, prepared by adding an excess of potassium, and submitted to the electric current, the whole of the manganese will be deposited at the positive electrode. When ammonium oxalate is used, the complete precipitation does not take place. As the separated peroxide does not adhere firmly to the electrode, it is necessary to filter it and convert it, by ignition, into the trimangano-tetroxide (Mn₃O₄).

DETERMINATION OF BISMUTH.

This separation presents considerable difficulty, because the metal is not precipitated as a compact mass on the platinum. The bismuth is always obtained in the same form, no matter whether it is precipitated from an acid solution, or from the double ammonium oxalate, or, finally, from a solution to which potassium tartrate has been added. As large a surface as possible must be used, and the dish piled to the rim; then, if the quantity of bismuth is small, the washing with water, alcohol, and ether may be effected without any loss of the element. If small quantities of the metal separate from the dish, they must be collected on a tared filter, and determined separately. In our experiments, an excess of ammonium oxalate was added to a nitric acid solution of bismuth. During the electrolytic decomposition, a separation of the peroxide was observed at the positive electrode, which, however, slowly disappeared. In order to prevent the reduced metal from oxidation, the last traces of water are completely removed by repeated washings with alcohol and anhydrous ether.

DETERMINATION OF LEAD.

The nitric solution of lead acts similarly to that of manganese. When the amount of peroxide separated is so large that it does not adhere firmly, and becomes mechanically precipitated on the negative electrode, it becomes impossible to complete the estimation without loss from the solution of the peroxide, and the results cannot be accepted.

If the double oxalate is submitted to electrolysis, the whole of the lead is separated out in its metallic state, but it is so rapidly oxidized by the air that it is very seldom that it can be dried without decomposition even when the operation is conducted in a current of illuminating gas. The electrolytic estimation of this element cannot be recommended.

DETERMINATION OF COPPER.

The copper may be very easily and rapidly separated from the double ammonium oxalate salt, provided a sufficient excess of ammonium oxalate is present. Weak currents cannot be employed for the determination of this element when it is present in large quantities, for under such circumstances the metal does not adhere with sufficient firmness to the electrode. We employed a current which corresponded to an evolution of 330 c.c. of gas per hour, and we were able to precipitate 0.15 gramme metallic copper in about twenty-five minutes.

DETERMINATION OF CADMIUM.

When the cadmium ammonium oxalate is submitted to the action of the electric current, the metal is thrown down in the form of a gray coating, which does not adhere very firmly to the electrode, but, however, sufficiently so as not to become separated on careful washing.

DETERMINATION OF TIN.

Tin may be easily estimated by electrolysis; it can be separated from its hydrochloric acid solution, or from its double salt with ammonium oxalate, as a beautiful silver gray coating on the platinum. When the ammonium oxalate is substituted by the potassium salt, the operation becomes more difficult, as a basic salt is formed at the opposite pole, and is not easily reduced. If the tin is separated from an acid solution, the current must not be interrupted while the washing takes place, a precaution which it is not necessary to follow when the ammonium oxalate is used. When the tin is dissolved from the platinum dish, it acts like the zinc; that is to say, a black coating is left on the electrode.

DETERMINATION OF ANTIMONY.

Antimony may be precipitated in its metallic state from a hydrochloric acid solution, but it does not adhere very firmly to the electrode. If potassium oxalate is added to a solution of the trichloride, the antimony may be readily reduced, but the metal adheres still less firmly to the electrode than it did in the first instance. An adherent coating may be obtained by adding an alkaline tartrate, but in that case the separation takes place too slowly. The precipitation of antimony may be very readily effected from solutions of its sulpho salts.

To a liquid, which may contain free hydrochloric acid, hydrogen sulphide is added, then neutralized with ammonium hydrate, and saturated with ammonium sulphide in excess. The reduction may be accelerated by the addition of some ammonium sulphate. The antimony separates out as a fine, light gray precipitate on the electrode, and which adheres very firmly, provided the precipitation has not been carried on too rapidly, i. e., if the current employed for the reduction was not too strong.

When the reduction has been completed, the supernatant liquid is poured off, and the residue washed in the ordinary manner.

DETERMINATION OF ARSENIC.

Arsenic cannot be completely separated from either its aqueous hydrochloric acid, or from a solution to which ammonium oxalate has been added in excess. From its aqueous as well as from its oxalate solution, a portion of the metal may be separated, but if the current is passed through its hydrochloric acid solution for a sufficient length of time, all the arsenic will be volatilized as arsenious hydride (AsH₃).

SEPARATION OF IRON FROM MANGANESE.

If a solution of ferric oxide and manganese ammonium oxalate is submitted to electrolysis, without the previous addition of ammonium oxalate, the characteristic color of permanganic acid immediately makes its appearance, and the peroxide gradually precipitates itself on the positive, while the iron is deposited on the negative electrode. When the examination is made in the above manner, it is impossible to separate the two metals, for the peroxide will bring down with it a considerable quantity of ferric hydrate. The separation of the two metals is only possible when the precipitation of the manganese peroxide is prevented, until the greater portion of the iron has been deposited. This result may be attained by adding sodium phosphate, or, better still, by the addition of ammonium oxalate in great excess. In both cases the characteristic coloration from permanganic acid is developed by the action of the current at the positive pole; this, however, disappears in the direction of the negative electrode. After the greater portion of the ammonium oxalate has been converted into carbonate, the coloration and necessarily the formation of manganese peroxide begins.

Ammonium oxalate is added to the solution, and heat applied; then three or four grammes more of ammonium oxalate are dissolved in the liquid, which is then immediately submitted to electrolysis. When the amount of manganese is small, the separation of the two elements takes place very rapidly, and the results are accurate. If the amount of manganese is more than double that of iron, the separation of the latter will take a much longer time. Then, in order to effect a complete separation of the two elements, it is necessary to redissolve the deposited manganese in oxalic acid (the acid is added, without interrupting the current, until the liquid becomes red), and the current is allowed to continue its action.

It was found desirable, in effecting this separation, not to employ too strong a current (two Bunsen elements will suffice), and only to increase the strength of the current when it is necessary, in consequence of a large amount of manganese being present, to redissolve the peroxide.

When the process is completed, it is not advisable to allow the current to act any longer, for otherwise some of the peroxide may adhere firmly to the iron, and the latter (after previously having poured off the liquid) must be redissolved in oxalic acid, that is to say, the electrolysis must be repeated. As has been already mentioned in the determination of manganese as peroxide, its precipitation from ammonium oxalate is not complete. The solution which contains the greater portion of manganese, suspended as peroxide, must first, therefore, be boiled to decompose the ammonium carbonate; the remainder of the ammonium oxalate is neutralized with nitric acid, and the manganese converted into the sulphide by ammonium sulphide. The manganese sulphide is then ignited in a current of hydrogen, and weighed as such.

SEPARATION OF IRON AND ALUMINUM.

The quantitative separation of iron from aluminum, which presented many difficulties according to the older methods, may be easily performed by electrolysis. If a solution of iron ammonium oxalate and aluminum oxalate, to which an excess of ammonium oxalate has been added, be submitted to the action of the electric current, the iron will be deposited as a firmly adhering coat on the negative electrode, while the aluminum oxide remains in solution, just so long as the quantity of ammonium oxalate is in excess of the quantity of ammonium carbonate produced. When, finally, a precipitation of aluminum oxide takes place the liquid is almost free from iron. From time to time, the solution, in which the aluminum oxide is suspended, is tested for iron by ammonium sulphide, and the current is interrupted when no further reaction is observed. The best method of procedure is to add ammonium oxalate in excess to a neutral, a slightly acid solution, or to one which has been neutralized by the addition of ammonium hydrate (a hydrochloric acid solution is not well adapted for this purpose); then as much more solid ammonium oxalate is added until for every 0.1 gramme there is 2 to 3 grammes of the oxalate present. The hot solution is then directly submitted to the action of the electric current. After the iron has been precipitated, it is best to stop the action of the current before all the aluminum oxide is thrown down, for otherwise a portion of the latter may adhere firmly to the iron, and be difficult to remove.

In such a case, as was mentioned previously in the separation of iron from manganese, it is necessary to redissolve the iron (after previously having poured off the liquid) in oxalic acid, and then the electrolysis is continued.

In order to effect the complete precipitation of the aluminum oxide from the solution which was poured from the iron, ammonium hydrate is added, and the solution boiled for some time, and then the aluminum oxide is determined in the usual manner. When the quantity of aluminum is less than that of iron, this method may be relied upon to give exact results. With the reverse (i. e., an excess of iron) the precipitate of aluminum oxide must be dissolved in oxalic acid (without the interruption of the current), and the electrolysis continued.--Berichte der Deutschen Chemischen Gesellschaft, 14, 1662.