Fused with potassium bisulphate in an ignition tube the violet vapors of iodine are evolved, and thus iodides may be distinguished from chlorides and bromides.
29. Nitrates.—If a nitrate be heated upon charcoal before the Bp., violent deflagration occurs. If the substance containing the nitric acid be mixed with a very small quantity of finely powdered potassium cyanide, the deflagration is accompanied with ignition and detonation.
If the substance be mixed in a dry condition with dry potassium bisulphate, and is then heated in an ignition tube, red-brown nitrous fumes are evolved. This reaction takes place if there is but a small quantity of nitrate present.
30. Phosphates.—Phosphates impart to the flame a bluish green color. The color is made more intense by moistening the substance with sulphuric acid, and then taking the paste so formed on the platinum wire and placing it in the Bunsen flame.
31. Silicates.—Silicates, when treated with microcosmic salt on a platinum wire, suffer decomposition; the bases unite with the phosphoric acid to form a transparent glass in which the silica may be seen floating as a cloudy mass.
The bead must only be examined for silica while hot, since on cooling it becomes opaque.
32. Sulphides.—Many sulphides, when heated in an ignition tube, volatilize and give a sublimate of sulphur in combination with the metallic portion of the substance.
A very delicate test for sulphur in whatever combination it may be found in a substance, and which may be performed with great ease, is to mix the finely powdered assay with four parts, Na2CO3, and fuse in an ignition tube. When thoroughly fused the tube is broken, and the fused mass is placed on a bright silver coin, and a drop of water is added. If the substance contains sulphur, a black spot will be observed on the coin where the fused mass was placed.
Before going on to the next chapter, the student should assure himself of his familiarity with the reactions just given, and he should practise with various substances, the nature of which is unknown to him.