Ammonium carbonate

Ammonium carbonate is the commercial salt, formerly known as sal volatile or salt of hartshorn. Ammonium carbonate is used when crushed as a smelling salt. It can be crushed when needed in order to revive someone that has fainted. It is also known as "baker's ammonia" and was a forerunner to the more modern leavening agents baking soda and baking powder.

Production
Ammonium carbonate was historically obtained by the dry distillation of nitrogenous organic matter such as hair, horn, decomposed urine, etc.

Currently, it is produced by heating a mixture of ammonium chloride, or ammonium sulfate and chalk, to redness in iron retorts, the vapours being condensed in leaden receivers. The crude product is refined by sublimation, when it is obtained as a white fibrous mass, which consists of a mixture of ammonium bicarbonate, NH4HCO3, and ammonium carbamate, NH2COONH4, in molecular proportions; on account of its possessing this constitution it is sometimes called ammonium sesquicarbonate. It possesses a strong ammoniacal smell, and on digestion with alcohol the carbamate is dissolved and a residue of ammonium bicarbonate is left; a similar decomposition taking place when the sesquicarbonate is exposed to air.

Ammonia gas passed into a strong aqueous solution of the sesquicarbonate converts it into normal ammonium carbonate, (NH4)2CO3, which can be obtained in the crystalline condition from a solution prepared at about 30 °C. This compound on exposure to air gives off ammonia and passes back to ammonium bicarbonate.

Uses
As well as in smelling salts, as mentioned, ammonium carbonate is still used for leavening in particular recipes, particular northern european and scandinavian. It can sometimes be susbstituted with baking powder, but the finished product will never be as airy and light as the original recipe. Icelandic loftkökur (air biscuits) for instance simply cannot be made with anything other than ammonium carbonate.