— 153 — 



But in doing so he leaves out of account that there is a difference between 

 dissolving an oil in alcohol of high strength and adding water afterwards, 

 and endeavouring to dissolve the oil in an alcohol which has previously 

 been diluted with that volume of water. In the last-named case the 

 absorbing-power of the solvent towards essential oil is much less. 



A few years ago, L. W. Andrews 1 ) described a volumetric determination 

 of hydrocyanic acid which was based upon the reaction of hydrocyanic acid 

 and mercuric chloride according to the equation 2 HCN + HgCl 2 = Hg(CN) 2 

 + 2 H CI. The free hydrochloric acid generated in the reaction is titrated 

 with decinormal potash liquor, p-nitrophenol being the indicator. The 

 complexity of this method lies in Andrews' directions that after being diluted 

 with corrosive sublimate solution, the liquid has to stand for an hour. 

 Another drawback is the want of clearness in the colour-change of the 

 nitrophenol. According to Rosenthaler 2 ) the colour-change may be more 

 easily observed if iodo-eosin is employed as indicator. Rosenthaler also 

 found that the result was equally satisfactory if the action was limited 

 to quite a short time. 



The following are required for the titration: 



1. Decinormal potash liquor and decinormal sulphuric acid. 



2. Iodo-eosin (in 0,2 °/ alcoholic solution) and ether. 



3. Sublimate solution. 



The determination proceeds as follows: The liquid under examination 

 which should preferably contain about l°/ hydrocyanic acid, should be 

 diluted with iodo-eosin solution, neutralised with alkali or acid until the 

 solution just shows a pink tinge, after which add sublimate solution and 

 titrate back immediately with alkali, until the colour reappears. Whether 

 enough sublimate solution has been added is shown by the titrated liquid 

 no longer discolouring after the addition of a few drops of sublimate 

 solution. If decoloration should take place it is necessary to add more 

 sublimate and to titrate again with potash liquor. If there has been over- 

 titration, add an excess of acid and titrate back again. 1 cc. decinormal 

 potash liquor = 2,7018 mg. hydrocyanic acid. 



The determination of the total hydrocyanic acid in liquids which 

 contain free hydrocyanic acid and benzaldehyde cyanohydrin offers no 

 difficulties. Neutralise again with iodo-eosin, dilute with excess of deci- 

 normal potash solution and again shake for one minute. Then add acid 

 until decoloration has been reached and titrate with alkali until the pro- 

 cess is completed. In all cases where solutions of unknown strength are 

 concerned it is advisable, after finishing the titration, once more to add 

 liquor and sublimate solution and, after allowing the mixture to stand for 



*) Americ. chem. Journ. 30 (1903), 187. 

 2 ) Arch, der Pharm. 248 (1910), 529. 



