COMPOUNDS CONTAINING NITROGEN. 549 



By the decomposition of alkali cyanides by diluted acids : 



KCN 4- HC1 = KC1 + HCN. 

 By the action of hydrochloric acid upon silver cyanide : 



AgCN + HC1 = AgCl -f HCN. 

 By distilling potassium ferrocyanide with diluted sulphuric acid : 



2K 4 Fe(CN) 6 + 6(H 2 SO 4 ) K 2 Fe 2 (CN) 6 + 6KHSO 4 + 6HCN. 



Potassium Sulphuric Potassium ferrous Potassium acid Hydrocyanic 



ferrocyanide. acid. ferrocyanide. sulphate. acid. 



Experiment 66. Place 20 grammes of potassium ferrocyanide and 40 c.c. of 

 water into a boiling-flask of about 200 c.c. capacity ; provide the flask with a 

 funnel-tube and connect it with a suitable condenser, the exit of which should 

 dip into 60 c.c. of diluted alcohol, contained in a receiver, which latter should 

 be kept cold by ice during the operation. After having ascertained that all 

 the joints are tight, add through the funnel-tube a previously prepared mixture 

 of 15 grammes of sulphuric acid and 20 c.c. of water. Apply heat and slowly 

 distil until there is little liquid left with the salts remaining in the flask. 



Determine the strength of the alcoholic solution of hydrocyanic acid thus 

 prepared volumetrically and dilute it with water until it contains exactly two 

 per cent, of HCN. 



Pure hydrocyanic acid is, at a temperature below 26 C. (78.8 F.), 

 a colorless, mobile liquid, of a penetrating, characteristic odor resem- 

 bling that of bitter almonds ; it boils at 26.5 C. (80 F.) and crystal- 

 lizes at 15 C. (5 F.). It is readily soluble in water, and a 2 per 

 cent, solution is the diluted hydrocyanic acid, Acidum hydrocyanicum 

 dilutam. 



According to the U. S. P., this diluted acid is made by the decom- 

 position of 6 grammes of silver cyanide by 15.54 c.c. of diluted hy- 

 drochloric acid, mixed with 44.10 c.c. of water, allowing the silver 

 chloride to subside and pouring off the clear liquid. 



The diluted acid has the characteristic odor of bitter almonds, a 

 slightly acid reaction, and is completely volatilized by heating. Pure 

 absolute hydrocyanic acid may be kept unchanged, but when water 

 or ammonia is present, the acid decomposes comparatively rapidly, 

 giving ammonia, formic acid, oxalic acid, and other products. The 

 official 2 per cent, acid deteriorates appreciably within several weeks, 

 and, therefore, should not be kept in stock for a long time, but should 

 be prepared as it is needed. 



The salts of hydrocyanic acid are called cyanides, and are nearly 

 all insoluble in water. The cyanides of the alkali metals, the alka- 

 line-earth metals, and of mercury are soluble. 



Dissociation of cyanogen compounds. Hydrocyanic acid is an extremely 

 weak acid, and its aqueous solution conducts electricity very badly, that is, it 



