I may here introduce a few words with regard to the preservation of hydrate of potash for use in analysis. Its preservation in the solid form is evidently no difficult matter; but when we attempt to keep the solution in glass bottles for the sake of convenience in using it as a re-agent, we generally find that it very soon takes up silica from the glass. I have found, however, that flint glass bottles will preserve such a solution much {35} longer than any other, lead glass not being easily acted upon, probably because it contains very much less silica. It might not be useless to make a trial of bottles made of thin soft iron, or sheet iron, for this purpose; but it is probable that pure silver is the true material for bottles, in which solution of potash is to be preserved. A very thin shell of silver might first be made, and afterwards strengthened by coating it thickly with galvanic copper.
The ease with which sulphate of potash can be obtained, in a state of purity, has long ago suggested its use as a material from which to prepare pure potash. Schubert[6] proposed to treat pulverized pure sulphate of potash with a concentrated solution of pure baryta, the latter somewhat in excess, and during the evaporation of the solution of hydrate of potash, thus obtained, the excess of the baryta is precipitated by the carbonic acid of the air. This, however, appears to be very expensive process. I have devised another method of treating pure sulphate of potash so as to obtain pure hydrate of potash therefrom; it consists merely in operating upon the sulphate of potash in a manner similar to that in which sulphate of baryta is operated upon to procure hydrate of baryta, that is in converting the sulphate into sulphide by the conjoined application of a reducing agent and a red heat, and to decompose the aqueous solution of the sulphide by the action of an oxide of a metal whose sulphide is insoluble, such as oxide of iron (?), oxide of copper or deutoxide of manganese. I use as a reducing agent, instead of charcoal, oil, rosin, etc., coal gas. This application of coal gas was proposed by Dr. Wolcott Gibbs. If it is found that the decomposition is not perfect, and that the solution of sulphide of potassium contains some sulphate of potash, or if a little sulphate is formed in the solution by oxidation, it is removed by the introduction of a little solution of baryta, according to the method of Schubert. I am not yet prepared, however, to give the details of this method.
[6] Erd. und Mar. Jour. 26, 117.
{36}
ON THE PREPARATION OF CHEMICALLY PURE HYDRATE AND CARBONATE OF SODA. BY HENRY WURTZ, A. M.
The remarks made in the last article with reference to the presence of silica in alcoholic potash, apply also, though not so generally, to commercial alcoholic soda. Few specimens of this product are met with which are so free from silica that it cannot be detected by saturation with chlorohydric acid, evaporation to dryness in a platinum vessel, and redissolution in water. Whatever may be the origin of the silica in this case, it is very often present.
I have selected, as the most convenient substance from which to prepare pure hydrate and carbonate of soda, a product which occurs very abundantly in commerce under the name of “carbonate of soda.” It is in the form of a very fine white powder, and on examination turns out to be the ordinary monohydrated bicarbonate of soda.
NaO, 2 C.O2 + H.O.
For, upon ignition, 7.756 grns. lost, 2.7595 grns. of carbonic acid and water, which is equal to 35.60 per cent.; bicarbonate of soda should lose 36.88 per cent. It may be mentioned, in this connection, that a preparation sent over here by Button, a London Pharmaceutist, under the appellation of “Chemically Pure Carbonate of Soda,” upon examination turned out to be also bicarbonate. 2.324 grns. lost by ignition; 0.845 grn. = 36.45 per cent.