Aurichloric Acid, HAuCl₄

A solution of Aurichloric Acid, HAuCl₄, is produced by the action of hydrochloric acid on auric chloride, or by addition of excess of this acid to a solution of gold in aqua regia to decompose the nitric acid present. It crystallizes in yellow needles, which deliquesce in moist air to a yellow solution. The acid exists in the form of two distinct hydrates, one with three and the other with four molecules of water, the individuality of the two being indicated by the heats of solution. In the solution of this acid the gold is present in the anion AuCl₄'. Its action on potassium ferrocyanide has been investigated.

A great number of aurichlorides is known.

Potassium aurichloride, KAuCl₄, is prepared in the anhydrous form by evaporating a solution of auric chloride in concentrated hydrochloric acid in presence of potassium chloride. It melts at the temperature of boiling mercury. It is also known as semihydrate and dihydrate. It dissolves readily in water. Heating converts it into potassium aurochloride, KAuCl₂.

Sodium aurichloride, NaAuCl₄,2H₂O, forms rhombic columnar crystals or laminae. Its water of crystallization cannot be expelled without decomposing the salt, a distinction from the potassium derivative. Its solubility in ether is another characteristic point of difference. It can be employed as a test for iodides in presence of bromides, the liberated iodine imparting a violet colour to chloroform.

Two ammonium aurichlorides are known:

4NH₄AuCl₄,5H₂O and 2NH₄AuCl₄,5H₂O.

The first is prepared in yellow, monoclinic plates from a solution of auric chloride in concentrat ed hydrochloric acid in presence of ammonium chloride; the second in yellow laminae from a neutral or slightly acidic solution of auric chloride. At 100° C. they give up their water of crystallization, and at higher temperatures decompose.

Salts of lithium, rubidium, caesium (which melts at a somewhat higher temperature than the potassium salt), calcium, strontium, barium, magnesium, zinc, manganese, and complex salts of the last three metals and nickel and cobalt have been prepared. Other complex derivatives are also known. An example is the caesium compound of the formula Cs₅Au₃Cl₁₄, minute, deep-red crystals formed by the action of excess of a concentrated solution of caesium chloride on auric chloride in presence of strong hydrochloric acid. The triple chloride formulated by Pollard as (NH₄)₈Ag₃Au₄Cl₂₃ is represented by Wells as (NH₄)₆Ag₂Au₃Cl₁₇. An amorphous series of triple salts of the general formula Cs₄MAu₂'''Cl₁₂ has been prepared by Wells, M representing Ag₂, Zn, Hg'', Cu'', or Au₂'. It exemplifies the isomorphous replacement of two univalent atoms by one bivalent atom. Triple chlorides of gold, silver, and rubidium (or caesium) are also known.

Auric chloride also forms a number of addition-products. A substance of the formula AuCl₃,PCl₅ is obtained in lemon-yellow needles by chlorination of the phosphorus-trichloride addition-product of aurous chloride, and also by heating a solution of aurous chloride in phosphorus trichloride with the pentachloride under pressure. Sulphur tetrachloride yields a compound of the formula AuCl₃,SCl₄, unstable, yellow needles decomposed by moist air. Prolonged heating of nitrosyl chloride at 100° C. under pressure with finely divided gold gives a derivative AuCl₃,NOCl. Addition products with selenium chloride, antimony pentachloride, stannic chloride, silicon tetrachloride, and titanic chloride have also been prepared.