[Vol. 10 



210 AN'NALS OF THE MISSOURI BOTANICAL GARDEN 



cc. of cone. ILSO4 and diluting with 100 cc. of distilled water. 

 To about 5 cc. of the solution to be tested, containing a small 

 amount of citric acid, is added about 1 cc. of the mercury solution, 

 the solution heated almost to boiling, and 2 per cent KMnO« 

 added drop by drop with shaking. After a few drops have been 

 added a white cloudy precipitate is formed if citric acid is present. 

 If the KM n0 4 continues to be used up but no precipitate is formed 

 it is likely that sugar, oxalic acid, or some other compound oxid- 

 ized by permanganate solution more easily than citric acid, is 

 present. If these compounds are present in small quantity, 

 citric acid may be detected by continuing to add KMn0 4 solution 

 slowly until they have been completely oxidized, when the KM11O4 

 will react with the citric acid. If the interfering substances are 

 present in considerable amount it may be necessary to precipitate 

 the citric acid with barium acetate and 50 per cent alcohol, and 

 after washing the precipitate, dissolve it in sulphuric acid, filter 

 off the BaSO«, and apply the test. 



According to Yoder ('11), succinic, tartaric, and malic acids 

 do not give this test, but aconitic acid does. Amberg and Mc- 

 Clure ('17) stated that pyruvic, ita- and citraconic acids gave 

 positive tests, but a large number of others tried, including tri- 

 carballylic, succinic, etc., did not give positive tests. Oxalic 

 acid gave a white precipitate on addition of the reagent, and this 

 must be filtered off before proceeding with the test. 



QUANTITATIVE DETERMINATION 



There is as yet no satisfactory gravimetric method for the 

 determination of citric acid, though a number of such methods 

 have been offered. The lead salt which is commonly used for the 

 primary separation is very soluble in water, and Yoder ('11) 

 points out that at least 3.6 volumes of alcohol are necessary for 

 a quantitative precipitation of citric and malic acids, whereas 1 

 volume has usually been used. 



For gross work citric acid may be precipitated from a neutral 

 solution with calcium acetate or chloride, filtered hot, washed 

 sparingly with hot water, and the precipitate weighed as calcium 

 citrate or converted to CaSO< and weighed. This general method 

 is valuable only for concentrated solutions, but is commonly 



