CHAPTEE VII. 



POTASSIUM HYDROXIDE (CAUSTIC POTASH)— POTASSIUM SULPHIDES 

 (LIVER OF SULPHUR) — POTASSIUM CHLORIDE (MURIATE OF 

 POTASH)— POTASSIUM NITRATE— POTASSIUM SULPHOCARBONATE 

 — POTASSIUM XANTHOGEN ATE— POTASSIUM CYANIDE (PRUSSIC 

 ACID)— POTASSIUM SULPHOCYANIDE. 



30. Caustic Potash, KHO. — Preparation. — By decomposing 

 carbonate of potash in solution in water by lime a precipitate of carbonate 

 of lime is formed, and potassium hydroxide enters into solution : — 



KoCOg + Ca(0H)2 = 2K0H + CaCOa 

 Potassium Calcium Potassium Calcium 

 carbonate, hydroxide, hydroxide carbonate. 



Properties. — Caustic potash is a white deliquescent solid ; exposed 

 to moist air it absorbs water and carbonic acid. Potash, even in dilute 

 solution, is a strong caustic which softens and gradually dissolves the 

 skin ; it traverses and perforates the mucous membranes ; it is a 

 strong poison, which should be handled carefully. It is used in 

 surgery as a cautery. [Antidote, vinegar.] 



Use. — Potash solutions have been used in America against different 

 plant lice, which, owing to the chitinous protections covering them, 

 gi'eatly resist insecticides. The caustic nature of potash gives access 

 to the insect by removing these obstacles. 



Psylla Pyricola, Forst. (psylla of the pear). — Slingerland tried to 

 destroy the eggs of this insect, but, contrary to anticipation, caustic 

 potash had not the desired effect. Bolley tried this substance to de- 

 stroy potato scab, but found that a 0*5 per cent solution injured the 

 formation of the eyes and germs, and that in 1-5 per cent solution it 

 almost completely destroyed them. 



31. Potassium Sulphides, K^S to K^S^,. — Preparation.— Potas- 

 sium monosulphide is prepared from caustic potash ; by saturating a 

 30 per cent solution with hydrogen sulphide, potassium hydrogen sul- 

 phide and water are formed : — 



KOH + H,S = KHS + H,0. 



The same amount of 30 per cent caustic potash is afterwards added, 

 which reduces the hydrosulphide to the monosulphide : — 



KHS + KOH = K.,S + H2O. 



To obtain the polysulphides it suffices to heat the monosulphide with 

 1, 2, 3, 4 atoms of sulphur. 



(112) 



