CAUSTIC POTASH AND SODA. 
§§ I08-II0.] 
127 
num (NH 4 Cl) 2 PtCl 4 . The distillate is exactly neutralised by HC1, 
evaporated to near dryness, and an alcoholic solution of platinic chloride 
added in sufficient quantity to be always in slight excess, as shown by 
the yellow colour of the supernatant fluid. The precipitate is collected, 
washed with a little alcohol, dried, and weighed on a tared filter ; 100 
parts of the salt are equal to 7-6 of NH 3 . 
VI.—Caustic Potash and Soda. 
§ 108. There is so little difference in the local effects produced by 
potash and soda respectively, that it will be convenient to treat them 
together. 
Potash (potassa caustica). —Hydrate of potassium (KHO), com¬ 
bining weight 56, specific gravity 2*1. 
Properties. —Pure hydrate of potassium is a compact, white solid, 
usually met with in the form of sticks. When heated to a temperature 
a little under redness, it melts to a nearly colourless liquid ; in this state 
it is intensely corrosive. It rapidly absorbs moisture from the air, and 
moist potash also absorbs with great avidity carbon dioxide ; it is power¬ 
fully alkaline, changing red litmus to blue. It is soluble in half its 
weight of cold water, great heat being evolved during solution ; it forms 
two definite hydrates—one, KH0-j-H 2 0 ; the other, KH0+2H 2 0. It 
is sparingly soluble in ether, but is dissolved by alcohol, wood-spirit, 
fusel oil, and glycerin. 
§ 109. Pharmaceutical Preparations. —Potassium hydrate, as well 
as the solution of potash, is official in all pharmacopoeias. The liquor 
jjotassce, or solution of potash, of the British Pharmacopoeia is a strongly 
alkaline, caustic liquid, of 1*045 specific gravity, and containing 5 grms. 
per 100 c.c. of KHO. It should, theoretically, not effervesce when 
treated with an acid, but its affinity for C0 2 is so great that all solutions 
of potash, which have been in any way exposed to air, contain a little 
carbonate. Caustic sticks of potash and lime used to be official in 
the British Pharmacopoeia. Filho’s caustic is still in commerce, and 
is made by melting together two parts of potassium hydrate and one 
part of lime in an iron ladle or vessel ; the melted mass is now moulded 
by pouring it into leaden moulds. Vienna paste is composed of equal 
weights of potash and lime made into a paste with rectified spirit or 
glycerin. 
§ 110. Carbonate of Potash (K 2 C0 3 +1JH 2 0), when pure, is in the 
form of small white crystalline grains, alkaline in taste and reaction, 
and rapidly deliquescing when exposed to moist air ; it gives all the 
chemical reactions of potassium oxide and carbon dioxide. Carbonate 
of potash, under the name of salt of tartar , or potashes, is sold at oil- 
