DETERMINATION OF CITRIC ACID. 57 



salt oxidized at the same time to carbon dioxide and 

 w ater. 



A solution of ferrous sulphate is used to indicate the 

 end reaction. At the moment when all the citric acid is 

 decomposed, the excess of chromic acid oxidizes the fer- 

 rous salt to the ferric condition, the change being recog- 

 nized by the action of potassium ferricyanide (ferric salts 

 give no precipitate with potassium ferricyanide, while 

 ferrous compounds give a blue precipitate). The pro- 

 cess is carried out in the following manner : 



"400 c. c. of milk* are mixed with 4 c. c. of sulphuric 

 acid (2j4 times normal strength) and heated to boiling; 

 10 grams of kaolin are mixed with enough water to form 

 a thin paste and added to the above. The mixture is 

 again boiled and then allowed to cool. When cold it is 

 diluted to 500 c. c. and filtered. If the filtrate is not clear 

 it is again treated with kaolin. To 100 c. c. of the fil- 

 trate add enough barium hydroxide solution to com- 

 pletely precipitate the sulphuric acid present, and evap- 

 orate to a syrup. This residue, which should be stirred 

 to render as homogeneous as possible, is treated with 

 2.2 c. c. of sulphuric acid (23 ' normal) and 20 c. c. of 

 absolute alcohol, and after allowing to stand for a short 

 time is mixed with 50 c. c. of ether. The mixture is then 

 filtered through cotton. In this way the lactose which 

 settles out as a cry.stalline precipitate, is completely sepa- 

 rated. The filtrate is now treated with alcoholic am- 

 monia, the reagent being added as long as any turbidness 

 is produced in the solution. The ether is distilled off, 

 leaving about 20 c. c. of residue. This is treated with 

 60 c. c. of absolute alcohol and 10 c. c. of alcoholic am- 

 monia. By this means the citric acid separates out as 

 the triammonium salt. On standing a well crystallized 



*From Hoppe & Sevier's "Handbuch der Physiologisch— und 

 Pathologisch— Chemischen .\nalyse," \'l. page 4/1 



