METHODS OP ANALYSIS 39 



(b) Another portion of the liquid is mixed with a solution of 

 potassium or sodium bicarbonate containing no normal carbonate 

 or free C0 2 . The mixture is titrated with AgN0 3 as before, the 

 reaction as regards HCN being: 



2KHC0 3 +AgN0 3 +2HCN=KAg(CN) 2 +KN0 3 +2C0 2 +2H,0. 



The difference of the two titrations gives the equivalent in 

 terms of KCN of the amount of HCN present. 



Bi-carbonates do not decompose K 2 Zn(CN) 4 , but, both titra- 

 tions being subject to some indefiniteness as to the finishing point, 

 the method is not very satisfactory. 



6. Estimation of Ferro-cyanides by Means of Potassium Per 

 manganate. — The ferro-cyanide is ppt. as Prussian blue by means 

 of an acidulated solution of ferric chloride. The ppt. is collected 

 and washed thoroughly. It is then decomposed by hot caustic 

 potash, yielding ferric hydrate and potassium ferro-cyanide, fil- 

 tered, the filtrate acidulated with H 2 S0 4 , and titrated with stand- 

 ard perniariganate. The finishing point is shown by the change 

 from yellow to reddish yellow. When much ferro-cyanide is pres- 

 ent the solution must be diluted, otherwise the end-point is not 

 sharp. About 100 cc. of H,0 should be added for every 0.1 grams 

 ferro-cyanide present. The permanganate must be standardized 

 against a solution of pure potassium ferro-cyanide; 3.16 grams 

 KMnO, are equivalent to 42.2 grams of K 4 Fe(CN) 6 ,3H 2 0, the reac- 

 tion being: 



KMn0 4 +5 K 4 Fe(CN) 6 44 H 2 S0 4 =5 K 3 Fe(CN) 6 +3 K 2 S0 4 +MnS0 4 +4 H 2 0. 



A centi-normal solution (0.316 grams KMn0 4 per litre) may be 

 conveniently used. 



The chief objection to this method is the difficulty of thor- 

 oughly washing the ppt. of Prussian blue. This is absolutely nec- 

 essary, as other bodies, e. g., thio-cyanates, are almost invariably 

 present, which would likewise reduce permanganate in acid solu- 

 tion. 



7. Estimation of Thio-cyanates by Means of Iodine.* — This de- 

 pends on the fact that the thio-cyanates react with iodine in the 

 presence of alkaline carbonate, thus: 



KCNS+8I+4H 2 0=H 2 S0 4 +6HI+KI+ICN 



* Rupp & Schied. J. S. C. I.. 1902. 



