162 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Thiocyanic Acid. HSCN. 



Thiocyanic acid, or sulphocyanic acid, has long been known to be 

 present in the form of its salts in saliva and it has also been found 

 in other secretions of the animal body. The amount is always very 

 small. 



Thiocyanic acid is obtained by distilling its potassium salt with 

 dilute sulphuric acid, or by the action of dry hydrogen sulphide upon 

 mercuric thiocyanate. 



Thiocyanic acid is a gas and, like cyanic acid, is easily condensed 

 in a freezing mixture to a liquid which has a penetrating and acrid 

 odour and is soluble in water and alcohol. It is an unstable substance ; 

 on removal from the freezing mixture it polymerises to a yellow amor- 

 phous body. It forms soluble salts with the alkali metals and insoluble 

 salts with the heavy metals. 



Potassium Thiocyanate. 



Potassium thiocyanate is readily prepared from potassium cyanide 

 by evaporating its solution with flowers of sulphur or ammonium 

 sulphide : thus : 



10 c.c. of a I per cent, solution of potassium cyanide are boiled for 

 some minutes with flowers of sulphur and filtered. The presence of 

 potassium thiocyanate is shown by the red colour which is formed on 

 the addition of a drop of ferric chloride solution. 



Potassium thiocyanate crystallises from alcohol in long colourless 

 prisms, which deliquesce in the air. 



Sodium thiocyanate is also deliquescent. 



Ammonium thiocyanate is prepared in a similar manner to the 

 potassium salt. It is a product obtained in the manufacture of coal 

 gas from ammonium salts, hydrogen cyanide and sulphur. 



It is usually prepared by the action of carbon bisulphide upon 

 alcoholic ammonia, or ammonia under pressure. Ammonium thio- 

 carbamate is formed, and this is decomposed by steam into ammonium 

 cyanate and hydrogen sulphide : 



yNH a 



2NH 3 + CS 2 = CS< 



\S.NH 4 

 X NH 2 



CS( = H 2 S + NH 4 SCN. 



\S.NH 4 



Ammonium thiocyanate crystallises in prisms which are easily soluble 

 in water and alcohol. 

 Metallic Thiocyanates. 



(1) Ferric thiocyanate is formed on adding ferric chloride solution 

 to a soluble thiocyanate ; ferric thiocyanate is soluble and has an 

 intense red colour and is used in detecting thiocyanates. 



(2) Silver thiocyanate is thrown down as a white curdy precipitate 

 on adding silver nitrate to a solution of a thiocyanate. 



