Four hundred and ninety-third meeting.

March 12, 1861.- MONTHLY MEETING.

The PRESIDENT in the chair.

Professor Horsford exhibited excellent drawings, by Mr. Hand, of New York, of magnified dissections and preparations of the grain of wheat, especially of the coats and superficial portion or bran, which has been long since ascertained to contain the principal part of the gluten, this indicating the importance of retaining as much of the bran in the bread as possible.

Professor C. W. Eliot read the following paper :

On the Chromate of Chromium, and analogous Chromates, with a New View of the Constitution of the Black Oxide of Manganese. By FRANK H. STORER and CHARLES W. ELIOT.

I. CHROMATE OF CHROMIUM (Cr2Oз CrO3).

1. When a solution of monochromate of potash is mixed with a solution of any ter-salt of chromic oxide, the mixture immediately becomes brownish red, a bright brown precipitate subsides, and when this precipitate has been completely deposited, the liquid separated by filtration will present the clear yellowish-red color of bichromate of potash. The chrome salt may be chrome alum, or sulphate of chromic oxide, or hydrated sesquichloride of chromium, and, if a sufficient excess of chromate of potash be added to the solution, the precipitate and the filtrate will present the appearances described. This filtrate may be evaporated and crystallized; the resulting crystals will be a mixture of bichromate of potash and of sulphate of potash or chloride of potassium, as the case may be often crystals of monochromate of potash will also present themselves. We prepared the precipitate for analysis by mixing a solution of chrome alum with an excess of monochromate of potash. The brown precipitate was washed with cold water: the color of the wash-water, at first bright yellow, became gradually paler, but never colorless. Twice in the course of the seven days during which the washing was continued, the precipitate was transferred from the filter to a mortar, stirred up with water, and thrown upon a fresh filter. This pro

cess deepened the yellow color of the wash-water in each case. It being quite evident that the color of the wash-water was due to chromic acid abstracted from the precipitate, the washing was stopped on the eighth day, the precipitate was dried at 50° to 55°, and analyzed as follows. A portion was dissolved in very dilute nitric acid by the aid of a gentle heat; the color of the solution was a red brown. Ammonia, added in very slight excess to this solution, kept at the boiling point for half an hour, produced a brownish-green precipitate of chromic oxide, which looked the browner because it floated in a bright yellow liquid. This precipitate was filtered off and thoroughly washed with hot water; it then presented the common appearance of chromic oxide, and was ignited and weighed in the usual manner. The filtrate from this precipitate and the wash-waters were concentrated by evaporation, and acidified with acetic acid. Acetate of lead then precipitated the whole of the chromic acid contained in the yellow liquid, and after the subsidence of the precipitate the supernatant liquid was perfectly colorless. The chromate of lead was washed and weighed on a tared filter as usual. The result of the analysis was

This examination showed, first, that the washed precipitate was a compound of chromic acid and chromic oxide; secondly, that it was more basic than any supposable definite compound of these two bodies; thirdly, that the prolonged washing removed chromic acid from the substance originally precipitated. To study the effect of the washing upon the precipitate, we prepared a considerable quantity of the brown precipitate by mixing a concentrated solution of pure chrome alum with a large excess of a concentrated solution of monochromate of potash. The precipitate as first collected upon the filter of course retained a considerable quantity of the bichromate of potash, chromate of potash, and sulphate of potash which existed in the filtrate. The precipitate was quickly rinsed with cold water until these mechanically adhering salts seemed to have been removed. During this short washing the substance underwent no change in color or in any other external property. A portion was now removed from the filter, air-dried on a brick, and submitted to analysis (analysis a). The rest of the precipitate was washed for six hours with cold water; another portion 25

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was then taken from the filter, air-dried, and analyzed (analysis b). Through the remainder (perhaps two teaspoonfuls) of the precipitate five litres of water were passed; another portion was then dried at 100° and analyzed (analysis c). When dried at 100° the precipitate is much less readily soluble in dilute nitric acid, than when not so heated, but the difficulty is easily overcome. The method of analysis was that described above.

A comparison of these three determinations teaches that the abstraction of chromic acid by washing with cold water stops at no definite point, and probably has no limit except the complete change of the original precipitate into chromic oxide. The composition of the substance a must be a close approximation to the composition of the original precipitate, for the slight rinsing which was intended merely to remove the chromates belonging to the filtrate could hardly have abstracted much of its combined chromic acid. In the chromate of chromium whose formula would be Cr2O, CrO3, the ratio of the chromic oxide to the chromic acid would be that of 3: 2; and we find that the ratio of the chromic oxide to the chromic acid in analysis a is but little larger than that of 3:2. The substance a is probably therefore a chromate of chromium of the formula Cr2O3 CrO3, from which a little of the chromic acid has been removed by washing.

Assuming for the present that the precipitate produced by the reaction of chrome alum on monochromate of potash is in truth this chromate of chromium, let us inquire into the nature of the reaction between these two salts by which this precipitate could be formed, and bichromate of potash left in the filtrate. Concentrated solutions of pure chrome alum and of pure monochromate of potash were prepared of known strengths, in order to determine the amount of either solution required to produce a precipitate in the other. A single drop of chrome alum solution produces an immediate and permanent precipitate in the chromate solution; but if the process be reversed and the solution of monochromate of potash be dropped into the alum solution, no permanent precipitate is formed till an apparently large excess of chromate

of potash has been added. By accurate quantitative experiments we have proved that five equivalents of chromate of potash must be added to one equivalent of chrome alum to effect precipitation. If a less quantity be added, any precipitate which may form will at once redissolve when the mixture is made complete by agitation.* The reaction between the two salts may therefore be expressed by the formula,

5 (KO CrO3)+KO SO3, Cr2O, 3 SO3 = 4 (KO SO3)+2 (KO 2 CrO3) +Cr2O3 Cro3,

and if to one equivalent of chrome alum more than five equivalents of chromate of potash be added, the excess above five remains inactive.

x (KO CrO3)+KO SO3, Cr2O, 3 SO3=4 (KO SO3)+2 (KO 2 CrO3) +Cr2O3 Cro3+(x-5) (KO CrO3).

If any other normal salt of chromic oxide be used, a similar formula will express the reaction; thus:

Cr2Cl, +5 (KO CrO3) = 3 (K Cl) +2 (KO 2 CrO3)+ Cr2O, CrO3. Cr2O2 3 SO3 + 5 (KO CrO3) = 3 (KO SO3) + 2 (KO 2 CrO3) + Cr2O3 CrO3.

With the exact proportions of chrome alum and chromate of potash which are by the formula necessary for the precipitation of chromate of chromium we prepared a quantity of the precipitate for a second series of analyses in corroboration of analyses a, b, and c. The precipitate thrown upon a filter was very slightly rinsed with cold water, a portion of it was taken off, pressed between folds of filter-paper under heavy weights, and air-dried for analysis (analysis d); the rest of the precipitate was washed for six hours till no trace of the filtrate could possibly be supposed to be retained by the precipitate, when another portion was taken out for analysis, pressed, and air-dried (analysis e); lastly five litres of water were passed through the remaining precipitate, which was then dried at 100° and submitted to analysis (analy

* In this connection we would call attention to the inaccuracy of the statement made by Berzelius in his Traité de Chimie, (Paris, 1846, Vol. II. 307), to the effect that a compound whose formula is Cr2O3 3 CrO3 is precipitated, when a few drops of the solution of monochromate of potash are added to the solution of a neutral chrome salt. No permanent precipitate whatever is produced under these circumstances, as any one may satisfy himself by repeating the simple experiment described above.

sis f). The process of analysis was the same described above, and the results were as follows:

This set of analyses corroborates the first series in every respect, and the ratio of the chromic oxide to the chromic acid of the substance d is almost precisely the ratio 3: 2 of the chromic oxide to the chromic acid in the chromate of chromium Cr2O, Cro3. We again see that there is no definite limit to the removal of chromic acid by prolonged washing, and that the substance may be readily made more basic than any imaginable definite compound of chromic oxide and chromic acid would be.

In these analyses the precipitates had been somewhat washed before any analysis was made; it remained to analyze the substance precipitated with so much of the adhering filtrate as could not be removed by pressure between folds of filter-paper without washing. One precipitate (analysis g) was prepared by mixing a solution of one equivalent of chrome alum with eight equivalents of chromate of potash in solution, and a second by mixing a solution of one equivalent of chrome alum with sixteen equivalents of chromate of potash in solution (analysis h). In both cases the solutions used were concentrated and the precipitates air-dried. The method of analysis which we had heretofore employed was open to one objection, a trace of chromic oxide was liable to be dissolved in the excess of ammonia, to be again separated when the filtrate and wash-waters from the chromic oxide precipitate were concentrated by evaporation, and the process would obviously be altogether inadmissible in any case where the sulphates of the mother-liquor had not been removed by washing before submitting the precipitate to analysis, since sulphate of lead would be formed and vitiate the determination of the chromic acid. We therefore resorted to Rose's method of separating chromic oxide from chromic acid, by means of the nitrate of the suboxide of mercury.* The solution of the substance to be analyzed in dilute nitric acid was nearly neutralized with carbonate of potash, and when the carbonic acid had been allowed sufficient time to escape, nitrate of the suboxide of mercury was added,

* Handbuch der Analytischen Chemie, (Braunschweig, 1851,) II. 379.