1923] 



CAMP — CITRIC ACID AS A SOURCE OF CARBON 225 



If it is not desirable to use the centrifuge, the alcoholic solution 

 may be warmed over the water bath, the flask or beaker being 

 kept covered. After a few minutes of warming the precipitate 

 will begin to flocculate out. When the solution has almost reached 

 the boiling point of the alcohol mixture it should be set aside 

 over night and filtered the next day. In filtering, the precipitate 

 should be washed as much as possible by decantation, using 26 

 per cent alcohol, the washing completed on the filter (2 or 3 wash- 

 ings are sufficient), and the filter drained to remove most of the 

 alcohol. The filter with precipitate should be transferred to a 

 beaker, warmed with 50 cc. of water and 3 cc. of 5 N ILSCh, 

 and washed with more warm water. Continue as previously 

 described with the preliminary distillation or boiling. 



Method 2. Precipitation as calcium citrate. — The method to 

 be followed is essentially that of L. and J. Gadais ( '09), and where 

 a large water bath or, better still, a sand bath is available the 

 method is very convenient. A small beaker is used instead of the 

 crucible, and the color after neutralization with NaOH is des- 

 troyed with acetic acid and the precipitation carried out with 

 calcium acetate instead of the chloride, giving a more easily 

 filtered precipitate. The calcium precipitate is dissolved in 50 cc. 

 warm water and 5 cc. of 5 A 7 " HuSO, and transferred to the dis- 



flask and the filter-paper washed with 50-75 cc. of 



H.,0 



The calcium precipitation is not quite as complete as the barium 

 precipitation, but in some ways is much more satisfactory, the 

 precipitate of calcium citrate being crystalline and readily filtered 

 and washed. The disadvantage lies in the slowness of concen- 

 trating the solution, after washing, to the requisite small volume. 



Where only citric acid, and no other carbonaceous substances 

 precipitated by calcium, is present, or where the only other sub- 

 stances so precipitated are inorganic anions, it is both more con- 

 venient and more efficient to determine the total carbon in the 

 precipitate by the use of the Friedeman carbon apparatus as 

 described below. This method is also very satisfactory, in many 

 cases, with the barium precipitation, but it must be borne in 

 mind that other organic acids are more likely to have relatively 

 insoluble Ba salts than Ca salts, and the precipitate must be 



