METHODS OF ANALYSIS 37 



the next measurement of cyanide solution. The method of titra- 

 tion for KCN herein given is for solutions, say, over 0.10% Below 

 this strength it is not advisable to dilute with water. 



The practice varies considerably in different mills, both regard- 

 ing the strength of the silver nitrate solution, and also the amount 

 of the working cyanide solution taken for analysis. In some works 

 but 10 cc. of solution is taken, and the silver nitrate standardized, 

 so that each cc.=0.10 lbs. of cyanide per ton of solution, and in 

 other cases 1.0 lb. per ton. 



Liebig's method works admirably with pure cyanide solutions, 

 but gives uncertain and inaccurate results in ordinary working 

 solutions, particularly in the presence of zinc. As, however, it is 

 generally only necessary to obtain relative commercial results, and 

 a knowledge of the real strength of the working solution in actual 

 free KCN or its equivalents is not essential, this method is in gen- 

 eral use. 



2. Titration of Alkalinity. — KCN and other simple cyanides 

 of the same class are alkaline to ordinary indicators. The whole 

 of the alkali may be determined by titrating with standard acid, 

 using methyl orange as indicator. With phenolphthalein the end 

 point is indefinite, owing to the faint action of HCN on this indica- 

 tor. The double cyanide of zinc and potassium is likewise alkaline 

 to methyl orange. 



For practical purposes it is most important to know the alka- 

 linity exclusive of cyanide, as it is this alkali which is chiefly of 

 use in preventing the unnecessary waste of cyanide by reactions 

 due to base metal compounds, and to the carbonic acid of the air. 

 This may be done (accurately in the absence of zinc) by adding 

 silver nitrate till a slight turbidity is produced, adding phenol- 

 phthalein to this turbid solution, and titrating, without filtering, 

 with N/10 acid. The result indicates generally — 



Equivalent of hydrates in terms of N/10 acid plus equivalent of 

 half the alkali metal in normal (mono) carbonates, in terms of 

 N/10 acid. 



The reactions in a typical case are: 



KOH+HN0 3 =KN0 3 +H 2 0. 

 K 2 C0 3 +HN0 3 ==KHC0 3 +KN0 3 . 



Bi-carbonates are neutral to phenolphthalein, hence are not 

 determined. They have no protective influence in this case. 



