The rapid dry-plate processes in photography, which are at present exciting considerable attention among the more advanced classes of those engaged in the art, have created a demand for neutral potassium oxalate that cannot be supplied through the ordinary trade channels. The preparation is simple, involving no special apparatus. There are three oxalates of potassium known to chemists - the neutral salt here referred to, and which contains two atoms of potassium to one molecule of acid; the binoxalate, the ordinary salt of sorrel of the drug stores, and that which is found in many plants, containing one atom of potassium to one of acid; and the quadroxalate, a salt not frequently prepared or used, in which the proportions of potassium and acid are one to two. The neutral salt is the only one used in photography. It crystallizes in rhombic prisms, is stable in the air, contains 2 molecules of water of crystallization 'which may be driven off by heat, and is soluble in about 3 times its weight of cold water.
It is evident that the easiest mode of preparing this salt is by neutralizing a solution of carbonate of potassium by oxalic acid. Some have recommended that the ordinary salt of sorrel (sal acetosella) be rendered neutral by the addition of the carbonate, but this is certainly a roundabout and expensive plan, not only as involving the use of more costly material, but unnecessary evaporation. The most expeditious method will be found to be as follows: - Dissolve a quantity - say 1 lb. - of carbonate of potassium in an equal weight of cold water, decanting the clear solution from any undissolved sediment, if such should remain. This residue consists of potassium sulphate or silicate, and is commonly present in the ordinary salts of tartar of commerce. Put the clear solution in an enamelled iron, porcelain, or wedgwood dish, add a quantity of water equal to that first employed, and heat to the boiling-point.
Add carefully, and by small portions, avoiding mishap by effervescence, sufficient powdered oxalic acid to neutralize the carbonate, testing carefully towards the close with test-paper. If necessary, filter the solution while hot, and set aside to crystallize. A fresh crop of crystals may of course be obtained by evaporating the mother-liquor.
The quantity of oxalic acid required cannot be definitely stated, as both acid and carbonate are generally impure; but theoretically, 174 parts of carbonate should require 90 of acid, and produce 202 of neutral oxalate. The product will practically be always considerably less than this, seldom equalling more than the weight of the carbonate employed.
As has been stated, the neutral oxalate is soluble in about 3 times its weight of water, and as photographers use a saturated solution, there is no reason, if time be an object, why a liquor should not be prepared extemporaneously, or at least that the operation of crystallization might not be omitted. The specific gravity of such a solution is, at ordinary temperatures, 1*220, and 10 oz. of the salt, when dissolved, measure 26 fl. oz. Such a solution, except made with distilled water, will of course require filtering, as the lime present in ordinary water is precipitated as oxalate. (E. B. Shuttleworth.)