kept. It forms a comparatively unimportant class of salts (citrates). It
is used in the determination of phosphoric acid, chiefly for the purpose
of preventing the precipitation of phosphates of iron and alumina by
ammonia, and in a few similar cases. The commercial crystals are used;
they should be free from sulphuric acid and leave no ash on ignition.

~Hydrochloric Acid~, HCl in water, (sp. gr. 1.16. It contains 32 per
cent. of hydrogen chloride).--It is sometimes called "muriatic acid,"
and when impure, "spirit of salt." The acid solution should be
colourless and free from arsenic, iron, and sulphuric acid. It forms an
important family of salts, the chlorides. It is the best acid for
dissolving metallic oxides and carbonates, and is always used by the
assayer when oxidising agents are to be avoided. The acid is used
without dilution when no directions are expressly given to dilute it. It
has no action on the following metals: gold, platinum, arsenic, and
mercury; it very slightly attacks antimony, bismuth, lead, silver, and
copper. Tin is more soluble in it, but with difficulty; whilst iron,
zinc, nickel, cobalt, cadmium, and aluminium easily dissolve with
evolution of hydrogen and the formation of the lower chloride if the
metal forms more than one class of salts. All the metallic oxides,
except a few of the native and rarer oxides, are dissolved by it with
the formation of chlorides of the metal and water.

~Dilute Hydrochloric Acid~ is made by diluting the strong acid with an
equal volume of water. This is used for dissolving precipitates obtained
in the general course of analysis and the more easily soluble metals.

~Hydrofluoric Acid, HF.~--A solution in water may be purchased in
gutta-percha or lead bottles. It is of variable strength and doubtful
purity. It must always be examined quantitatively for the residue left