METHODS OF ANALYSIS 13 



nitric acid) and evaporate slowly to dryness. Now add 10 grams 

 of ammonium chloride crystals and 20 cc. of ammonia, boil, filter, 

 wash with ammonia and then with water. To the filtrate add a 

 small piece of litmus paper; make acid with HC1, and then add 5 cc. 

 HC1 in excess. Now add 2 to 3 grams of test lead and boil. Titrate 

 with potassium ferro-cyanide, using uranium acetate indicator. 

 Add about 3 cc. of the sodium sulphite solution before titrating. 

 (See standardizing the solution.) 



Some chemists evaporate to dryness, first, with nitric acid, then 

 take up and boil with nitric acid, adding a small pinch of potas- 

 sium chlorate, this method giving steady results. 



*Sources of error in the ferro-cyanide titration — Losses may 

 result from : 



(1). Volatilization of Zn as the chloride. 

 (2). Recombination of Zn with SiO,. 

 (3). Imperfect decomposition by acids. 

 (4). Occlusion by ferric-hydroxide, etc. 

 (5). The use of H 2 S for separating Cu, Cd, etc. 

 (6). Failure to make the final titration under the same condi- 

 tions as in standardization. 

 (7). Insufficient dilution of the solution. 



(8). Too great haste in titrating, especially with cold solu- 

 tions. 

 On the other hand, results may be too high, owing to: 

 (9). The presence, in the solution, of Cd, Cu, Sb, Mn, Al, or 

 some organic acid as tartaric, oxalic, etc. 

 (10). The decomposition of the ferro-cyanide solution by CI, 



Br, nitrous oxides, H 2 2 , etc. 

 (11). The addition of an inordinate excess of acid to the 



solution. 

 (12). The use of an incorrectly standardized solution of ferro- 

 cyanide. 

 Magnesium. — To J gram of slag add 15 cc. water, then 10 cc. 

 HC1 and 5 cc. nitric acid, boil, precipitate the iron with ammonia 

 and add 5 cc. bromine water (or hydrogen peroxide) and filter off 

 the iron. The iron is then redissolved with 5 cc. of HC1, diluted, 

 and again precipitated with ammonia and bromine, as before. The 

 filtrates from the two precipitates are then combined and boiled. 

 Ammonium oxalate is now added to precipitate the lime, which is 



* By Geo. Waring, Jour. Am. Chem. Soc, Vol. XXVI. 1 Jan., 1904. 



