542 PRINCIPLES OF CHEMISTRY 



acid on the carbonate, whilst the hydroxide, caustic potash^ KHO, which 

 is in many respects analogous to caustic soda, is easily obtained by 

 means of lime in exactly the same manner as sodium hydroxide is pre- 

 pared from sodium carbonate. 1 1 Therefore, in order to complete our 

 knowledge of the alkali metals, we will only describe two salts of 

 potassium which are of practical importance, and whose analogues have 

 not been described in the preceding chapter. 



Potassium cyanide, which presents in its chemical relations a certain 

 analogy with the halogen salts of potassium, is not only formed accord- 

 ing to the equation, KHO + HCN=H 2 O + KC]Sr, but also when- 

 ever a nitrogenous carbon compound for instance, animal matter is 

 heated in the presence of metallic potassium, or of a compound of 

 potassium, and even when a mixture of potash and carbon is heated in 

 a stream of nitrogen. Potassium cyanide is obtained from yellow 

 prussiate (p. 401), which has been already mentioned and whose pre- 

 paration on a large scale will be described in speaking of iron. If 

 the yellow prussiate be ground to a powder and dried, so that it loses 

 its water of crystallisation, then it melts at a red heat, and decomposes 

 into carbide of iron, nitrogen, and potassium cyanide, FeK 4 C 6 N 6 = 

 4KCN + FeC 2 + N 2 . After the decomposition it is found that the 

 yellow salt has been converted into a white mass of potassium cyanide. 

 The carbide of iron formed collects at the bottom of the vessel. If 



and iodide before igniting it, as it facilitates the evolution of the oxygen from the iodate. 

 'The iodate may also be converted into iodide by the action of certain reducing agents, 

 such as zinc amalgam, which, when boiled with a solution containing an iodate, converts 

 it into iodide. Potassium iodide may also be prepared by mixing a solution of ferrous 

 iodide (it is best if the solution contains an excess of iodine) and potassium carbonate, in 

 which case ferrous carbonate, FeCOs, is precipitated (with an excess of iodine the pre- 

 cipitate is granular, and contains a compound of the suboxide and oxide of iron), while 

 potassium iodide remains in solution. Ferrous iodide, FeI 2 , is obtained by the direct 

 action of iodine on iron in water. Potassium iodide considerably lowers the temperature 

 (even by 24), when it dissolves in water, 100 parts of the salt dissolve in 73'5 parts of 

 water at 12'5, in 70 parts at 18, whilst the saturated solution which boils at 120 con- 

 tains 100 parts of salt per 45 parts of water. Solutions of potassium iodide dissolve a 

 considerable amount of iodine; strong solutions even dissolving as much or more iodine 

 than they contain as potassium iodide (see Chap XI. Note 64). 



11 Caustic potash is not only formed by the action of lime on dilute solutions of 

 potassium carbonate (as sodium hydroxide is prepared from sodium carbonate), but also 

 by igniting potassium nitrate with finely-divided copper (see Note 15), and also by mixing 

 solutions of potassium sulphate (or even of alum, KA1S 2 O 8 ) and barium hydroxide, 

 BaH 2 O 2 . It is sometimes purified by dissolving it in alcohol (the impurities, for example, 

 potassium sulphate and carbonate, are not dissolved) and then evaporating the alcohol. 



The specific gravity of potassium hydroxide is 2'04, but that of its solutions (sec 

 Chap. XII. Note 18) at 15 S = 9992 + 90'4p + 0'28p 2 (here p' 2 is +, and for sodium 

 hydroxide it is ). Strong solutions, when cooled, yield a crystallo-hydrate, KHO,4H 2 O, 

 which dissolves in water, producing (like 2NaHO,7H 2 O) cold, whilst potassium hydros idi: 

 .in solution develops a considerable amount of heat. 



