DETERMINATION OF MALIC ACID. 



TABLE II. Determinations on a normal solution of sucrose containing 0.24 



cent of malic acid. 



i Difference between (1) and (2). 2 Difference between (1) and (3). 



It will be seen that when the difference between readings (1) and 

 (2) is used in the calculations a somewhat high recovery is obtained. 

 This is to be expected since the decrease in rotation due to the effect 

 of the uranium salt on sugar is added to that due to the rotation of 

 the uranium malic complex. Consequently, Reading (1) is lower, 

 and the difference between (1) and (2) is higher, than it should be. 

 Reading (3) is decreased by the combined effect of the action of the 

 uranium salt on sugar and also on the traces of malic acid present. 

 Hence, the difference between (1) and (3) is too small. The per- 

 centage recovery obtained by using the difference between readings 

 (1) and (2) is not excessively high and this is the largest error that 

 would be obtained due to the action of uranium salts on sucrose, 

 assuming that no more concentrated solution than one of normal 

 sugar is examined. Therefore, while not theoretically correct, it is 

 practically permissible to dispense with Reading (3) entirely when 

 working with solutions in which Reading (2) is positive. It would, 

 of course, be possible to avoid error from this source by inverting 

 the solution before beginning the determination, then neutralizing, 

 and diluting to a definite volume. This has not been found to be 

 necessary in practice, however. When it is known that the solution 

 under examination contains either less than 10 per cent of invert 

 sugar or more than 0.25 per cent of malic acid it is unnecessary to 

 make Reading (3). 



Solutions are frequently encountered which are too dark to polarize 

 without clarification. It was found that alumina cream can be used 

 for this purpose without affecting the results. 1 



The question arises as to whether uranium salts produce the same 

 effect on levo, dextro, and inactive malic acid. For ordinary work on 

 natural products this question is of little importance, since, as far as 

 the literature goes, malic acid appears to occur in nature only in 

 the levo form. W. Gintl 2 reports the presence of the inactive form 

 in the leaves of Fraxinus excelsior L., but this discovery does not 

 appear to have been confirmed since that time. 



1 With very dark solutions such as fruit juices, C. W. Clark, of this laboratory, has 

 found that a few drops of bromin may often be used to decolorize without affecting the 

 results. 



2 Jahresber. Chem., 1868, p. 800. 



