Prep. From the residuum of the distillation of nitric acid when made by treating nitre with sulphuric acid, which consists of bisulphate of potash (KO, HO, 2 So3), not quite pure. This is dissolved in water, and to the solution slaked lime is added until it is alkaline in reaction. The solution is then filtered, heated to the boiling point, and carbonate of potash added as long as there is any precipitate. It is then refiltered, dilute sulphuric acid added, so as to produce a neutral or slightly acid solution, evaporated till a film forms on the surface, and then set aside to crystallize.

In this process, the excess of sulphuric acid in the residue of the nitric acid process (which is a bisulphate of potash, KO, HO, 2 So3), is removed in the form of sulphate of lime, by rendering the solution slightly alkaline with slaked lime. The excess of lime is afterwards got rid of by the addition of carbonate of potash, and to the filtered fluid dilute sulphuric acid is added so as to produce a neutral or slightly acid solution (this ensures all the carbonate of potash present being converted into sulphate), and the sulphate of potash is allowed after evaporation to crystallize out.

Much of the sulphuric acid is here lost in combination with the lime, but as the large scale on which nitric acid is manufactured renders the residue of the nitric acid process very cheap, it is probably found more economical to sacrifice some of the sulphuric acid than to procure more sulphate of potash by the addition of carbonate of potash, which is expensive in comparison to the value of the salt produced.

Prop. & Comp. Hard semitransparent crystals, six-sided prisms, terminated by corresponding pyramids; of a bitter saline taste, and slightly soluble in water. Its solution, acidulated with hydrochloric acid, is precipitated yellow by bichloride of platinum, and white by chloride of barium. Crepitates and afterwards fuses at a red heat without losing weight. Yields 132 per cent. of sulphate of baryta. Composition (KO, So3). The watery solution is neutral to test paper, and is not affected by oxalate of ammonia.

Off. Prep. It is contained in Pulv. Ipecacuanhae cum Opio.

Therapeutics. Mildly purgative. Almost always given in combination; by some it is supposed to be alterative, acting on the secreting and excreting organs; latterly, evidence has been given of its acting as a poison in large doses. It is often used on account of its mechanical properties for the purpose of more intimately dividing vegetable substances.

Dose. 20 gr. to 120 gr. as a purgative; in smaller doses as an alterative.