This section is from the book "Dental Medicine. A Manual Of Dental Materia Medica And Therapeutics", by Ferdinand J. S. Gorgas. Also available from Amazon: Dental Medicine.
The Terchloride of Gold is obtained by dissolving gold in aqua regia (three parts, by measure of hydrochloric acid, and one of nitric acid), using gentle heat to hasten the solution, the acids employed being chemically pure. The solution is then evaporated to dryness, when ruby-red prismatic crystals of the terchloride of gold result.
Terchloride of gold is very deliquescent, and is soluble in water, alcohol and ether. It possesses a disagreeable, styptic taste, reddens blue litmus paper, and will impart a purple stain to the skin, which may be removed by a solution of cyanide of potassium. It is readily decomposed by many metallic and non-metallic elements, and also by saline and organic compounds, on account of its elements being held together by a feeble affinity. It is escharotic and disinfectant, and its physiological effects are similar to those of corrosive sublimate. It is not used internally.
In dental practice, the terchloride of gold, in the form of an aqueous, alcoholic, or ethereal solution, is employed for the purpose of obtunding the sensitiveness of dentine, for which it is a valuable application; and the ethereal solution possesses some advantages over the aqueous or alcoholic solutions for such a purpose. To prepare an obtunding solution: "Dissolve the crystals of the gold in pure water; fill a test-tube half full of the solution, then add an equal quantity of sulphuric ether, and agitate the mixture. Let it then rest for a few minutes, when the ethereal solution will rise to the surface, and may be poured off" into another tube or phial, and securely stoppered. It should be kept, as much as practicable, from the action of light and air. Applied to dentine, on pledgets of cotton, it acts like chloride of zinc, but more promptly and with less pain. Chlorine is more abundantly liberated during its action than from chloride of zinc; hence it is a better disinfectant than the latter. It has a fine yellow tint, and it is not absorbed by the dentine, but forms an insoluble compound with the gelatinous elements. As it is an irritant poison, care should be observed in its use. If not protected in a glass-stoppered bottle, the gold is precipitated in a metallic form.