256 METALS AND THEIR COMBINATIONS. 



this has entered into the soil, whence it is taken up by plants as one 

 of the necessary constituents of their food. 



In the plant potassium enters largely into the combination of 

 organic compounds, and when the plant is burned ashes are left 

 containing the potassium, now in the form of carbonate. By ex- 

 tractincr such ashes with water, the potassium carbonate, along with 

 small quantities of chlorides and sulphates of potassium and sodium, 

 is obtained in solution, by the evaporation of which to dryness an 

 impure article is obtained, known as crude potash. Formerly this 

 was the chief source of potassium compounds, but about the year 

 1850 the inexhaustible salt mines of Stassfurt, Germany, were discov- 

 ered. The salt there mined contains, besides the chlorides and sul- 

 phates of sodium, magnesium, calcium, and other salts, considerable 

 quantities of potassium chloride, and the Stassfurt mines at present 

 are practically the source of all potassium compounds. 



Potassium hydroxide, Potassii hydroxidum, KOH == 55.74 

 (Caustic potash), may be obtained by the action of the metal on water : 



K -h H,O = H + KOH 



The usual process for making potassium hydroxide is to boil together 

 a dilute solution of potassium carbonate or bicarbonate and calcium 



hydroxide : 



K 2 C0 3 + Ca(OH) 2 = CaC0 3 + 2KOH. 



Large quantities of high-grade potassium hydroxide are now 

 manufactured directly from the chloride by electrolysis. 



Experiment 20. Add gradually 5 grammes of calcium hydroxide (slaked 

 lime) to a boiling solution of about 5 grammes of potassium carbonate in 50 

 c.c. of water, and continue to boil until the conversion of potassium carbonate 

 into hydroxide is complete. This can be shown by filtering off a few drops of 

 the liquid, and supersaturating with dilute hydrochloric acid, which should not 

 cause effervescence. Set aside to cool, and when all solids have subsided, pour 

 off the clear solution of potassium hydroxide, which may be used for Experi- 

 ment 21 . What quantities of K 2 CO 3 and Ca(OH) 2 are required to make one liter 

 of a 5 per cent, solution of potassium hydroxide ? 



Potassium hydroxide is a white, hard, highly deliquescent sub- 

 stance, soluble in 0.5 part of water and 2 parts of alcohol ; it fuses 

 at a low red heat, forming an oily liquid, which may be poured into 

 suitable moulds to form pencils; at a strong red heat it is slowly 

 volatilized without decomposition; it is strongly alkaline and a 

 powerful base, readily combining with all acids ; it rapidly destroys 

 organic tissues, and when taken internally acts as a powerful corrosive, 

 and most likely otherwise as a poison- 



