58 METHODS OF QUANTITATIVE ANALYSIS. 



precipitate is formed whicli contains all of the -citric acid 

 together with some ammonium chlojide 'and a little or- 

 ganic matter. This latter may be separated by repeating 

 the precipitation. 



"The ammonium citrate is dissolved in water and con- 

 centrated to about 20 c. c. and titrated with standard 

 potassium bichromate. A solution containing 46.1 grams 

 K^Cr.Oj in a liter, is titrated against a solution of ferrous 

 ammonium sulphate, prepared by dissolving 150 gr. of 

 the salt in 700 c. c. water, adding 100 c. c. of concen- 

 trated sulphuric acid and completing the volume to one 

 liter ; 20 c. c. of this iron solution diluted with 80 c. c. 

 of water requires 'j.'j to 7.8 c. c. of bichromate solution 

 for complete oxidation. For the titration of citric acid, 

 the 20 c. c. of this citrate solution are treated with 20-30 

 c. c. of bichromate solution and 20-25 c. c. of concen- 

 trated sulphuric acid and carefully mixed by stirring. 

 After heating for about a quarter of an hour the oxida- 

 tion to carbonic acid may be considered complete (the 

 solution should not be heated to boiling). The solution 

 is then diluted with 50 c. c. of water and treated with an 

 excess of ferrous ammonium sulphate until the reddish 

 brown color is completely changed to green. With the 

 standard bichromate the solution is titrated back until no 

 more ferrous salts can be detected with potassium ferri- 

 cyanide. Theoretically 4.61 grams of potassium bichro- 

 mate are equivalent to i gram of citric acid. Numerous 

 titrations have shown, however, that in practice 4.61 

 grams KjCr^O, represent 1.02 grams of citric acid. The 

 lactic acid contained in the milk does not affect the re- 

 sults because this acid is not precipitated by alcoholic am- 



*See also "Zeitschift fur Analytische Chemie," 1899, page 

 718; also "Analyst," 23, page 161; and also "Revue Chim. Analyt, 

 Appl." VI (7) no. — Translators. 



