Aqua Regia. This is used in the arts, chiefly as a solvent for gold. By the mutual action of nitric and hydrochloric acids a compound of chlorine, nitrogen, and oxygen, is formed. The best proportions and strength of the acid are variously stated. Colourless nitric acid must be used. Mr. Elkington employs 21 parts of nitric acid, sp. gr. 1.45; 17 parts of hydrochloric acid, 1.15 sp. gr.; and 14 parts of water. This dissolves 5 parts of gold. For the nitro-hydrochloric acid employed by dyers as a solvent for tin, see Dyes, etc. further on.
See Pocket Formulary.
Digest by the aid of heat 1 part of treacle, or of potato starch, in 5 parts of nitric acid, sp. gr. 1.42, diluted with 10 parts of water, so long as gaseous products are evolved. By evaporation the acid is obtained in crystals, and must be recrystallized until sufficiently pure. Mr. Lewis Thompson directs 28 oz. of sugar, 184 oz. of nitric acid of 1.245 sp. gr., to be digested at 125° F. It yields 30 or 31 oz. of acid. M, Schlesinger directs 4 parts of dry sugar, and 33 of nitric acid of 1.38 sp. gr. to be boiled to one sixth of the original volume, and allowed to crystallize. This is the best method of operating on a small scale, when the amount of product is not the principal object. Considerable quantities of oxalic acid are made, on the large scale, by heating sawdust with a mixture of caustic potash, and soda, and by subsequently decomposing the oxalate of soda formed.
Phosphoric Anhydride. See Pocket Formulary. Dry phosphoric acid is thus obtained: On a flat plate, place a large bell glass, and under it a small porcelain cup or crucible. Introduce into the latter a piece of phosphorus, dried with blotting paper, and set it on fire by a heated wire. Let the bell glass be raised on one side to admit sufficient air to maintain combustion; and as the phosphorus is consumed, introduce successive pieces, taking care that the glass does not become too hot. When the quantity of acid is considerable, knock it from the plate with an iron spoon, and put it into stoppered bottles. Several glasses may be used at once. Dry phosphoric acid is used as a desiccating body, having the strongest attraction for water of any known substance. It is used also in making a stopping for teeth - see Teeth Cements.
Heat powdered nutgalls in a dish covered with thin filtering paper pasted to its edges, and surmounted with a bell-formed receiver, kept cool. A solution of the condensed acid, decolorized by animal charcoal, and mixed with spirit, is used to stain the hair (and skin) brown.
This is only made on a large scale; but the commercial acid requires purification for many chemical as well as pharmaceutical purposes. See Acidum Sulphuricum purum, Pocket Formulary.
Into a retort, placed in a freezing mixture, and having a receiver attached, put some dry phosphoric acid (see above), and add 3/4ths of its weight of strong sulphuric acid. Remove the retort from the freezing mixture, and place the receiver attached to it there; a gentle heat being now applied to the retort, the anhydrous acid is obtained in silky crystals.