§ 64. hydeocya:n^ic acid. 97 



carbonate. Of course, neither of these reagents should 

 contam any chlorine. Greater accuracy is secured, more- 

 over, by using the same volume of a solution containing 

 about the same amount of chlorine as in determining the 

 standard of the argentic solution in the beginning — that 

 is, about 25 c.c. containing about 0.15 grm. of chlorine. 



It is well, also, to have on hand an accurately titrated 

 solution of sodic chloride, containing exactly 5.85 grms. 

 of sodic chloride in the litre, and which, therefore, is ex- 

 actly equivalent, cubic centimetre for cubic centimetre, to 

 the argentic solution. Then, if it is feared in any case 

 that too much argentic nitrate has been added, a cubic 

 centimetre of this solution can be put in, when the red 

 coloration will disappear, and argentic nitrate can be add- 

 ed again more cautiously ; finally, when the desired result 

 is obtained, subtract one from the number of cubic centi- 

 metres of argentic solution used. 



HYDKOCTANIC ACID HCy. 



64. Cyanides of manganese, zinc, and copper, are in- 

 soluble in water. 



Reactions. — Cyanides give a white precipitate, AgCy, 

 with argentic nitrate, insoluble in dilute nitric acid, and 

 somewhat difiicultly soluble in ammonia ; when heated, 

 it is decomposed, metallic silver being left behind. 



If a cyanide is treated with dilute sulphuric acid in a 

 watch-glass, and another watch-glass, with a drop of 

 lammonic sulphide charged with excess of sulphur in its 

 'centre, is quickly inverted over the first glass, the hydro- 

 cyanic acid evolved from the cyanide is absorbed by the 

 ammonic sulphide, and on evaporating the drop in the 

 upper glass to dryness at a very gentle heat, ammomc 

 sulphocyanate is left, which, if moistened with a drop of 

 ferric chloride, gives a deep red color. 

 5 



