In the case of copper, the colour reactions with potassium ferrocyanide or ammonia are usually employed; traces of ammonia are estimated with Nessler's reagent; sulphur in iron and steel is determined by the tint assumed by a silver-copper plate suspended in the gases liberated when the metal is dissolved in sulphuric acid (Eggertz's test) (see W.
The most modern and the most generally accepted method is volumetric, and is based on the reaction between zinc chloride and potassium ferrocyanide, by which insoluble zinc ferrocyanide and soluble potassium chloride are formed; the presence of the slightest excess of potassium ferrocyanide is shown by a brownish tint being imparted by the solution to a drop of uranium nitrate.
Phys., 18 79 (5), 18, p. 380); by passing induction sparks through a mixture of acetylene and nitrogen; by the dry distillation of ammonium formate; by the decomposition of the simple cyanides with mineral acids; and by distilling potassium ferrocyanide with dilute sulphuric acid (F.
Potassium selenocyanide, KSeCN, is obtained by the action of selenium on a concentrated aqueous solution of potassium cyanide, or by heating selenium with anhydrous potassium ferrocyanide (W.
It is prepared by oxidizing potassium ferrocyanide with a diluted nitric acid.
Ammonia gives a characteristic blue coloration when added to a solution of a copper salt; potassium ferrocyanide gives a brown precipitate, and, if the solution be very dilute, a brown colour is produced.