METHODS OF ANALYSIS 43 



known as ''protective alkali." This means, in practice, the alka- 

 linity which the solution shows to phenol-phthalein, after sufficient 

 AgN0 3 has been added, to convert all the cyanogen of the free 

 cyanides into the double silver salt. (1) Titration of Total Alka- 

 lies. — A measured quantity of the liquid is titrated with N/10 acid 

 (any mineral acid may be used, but HN0 3 is preferable), a few drops 

 of a .1$ solution of methyl orange being used as indicator. If the 

 addition of acid should cause a ppt. (as when Zn, Cu or Ag salts 

 are present) it is better to add a moderate excess of acid, make 

 up to a definite volume, and titrate an aliquot portion with N/10 

 caustic alkali. The substances determined are : Cyanides, hydrates, 

 carbonates, bi-carbonates, sulphides, zincates, etc., of the alkali 

 and alkaline earth metals and of ammonium. 



The double cyanides of Zn, Ag, Cu, and perhaps some other 

 metals give ppts. which represent a consumption of acid propor- 

 tional to the amount of such bodies as may be present, e. g. 



K 2 Zn(CN) 4 +2HN0 3 =Zn(CN) 2 +2KN0 3 +2HCN 



HCN and carbonic acid do not affect methyl orange. Ferro- 

 cyanides ferri-cyanides, and thio-cyanates of potassium, sodium, 

 etc., are neutral to all indicators. (2) Titration of Protective Alka- 

 lies. — This has already been described. (See page No. 36.) All 

 results should be calculated as the equivalent of KOH. 



The following method for estimating cyanogen in commercial 

 cyanide is said to be more accurate than Liebig's, and is recom- 

 mended by Adair.* 



The method to be described was originally devised for the 

 estimation of ferro-cyanide in pot-metal (a very impure product). It 

 can be readily adapted for the estimation of cyanogen in commer- 

 cial cyanide, and is preferable to the silver methods for this pur- 

 pose, because, although not quite so quick, only the useful cyanide 

 is estimated. Cyanates, sulpho-cyanides, sulphides and chlorides, 

 even in large percentages, do not sensibly affect the result. 



The outline of the method is to convert the cyanide as such into 

 ferro-cyanide — next to oxidize with KMn0 4 , in the presence of 

 H 2 S0 4 . The ferro-cyanide is oxidized to ferri-cyanide only, whereas 

 cyanates, sulpho-cyanides and other impurities are either dis- 

 tinctively oxidized or converted into substances which do not inter- 



* Journal of the Chemical Society of South Africa, January, 1903. 



