DETERMINATION OF MALIC ACID. 3 



quently. Filter through a folded filter until clear and polarize if 

 possible in a 200 mm tube. If the solution is too dark to read in a 

 200 mm tube, a 100 or 50 mm tube may be used. It is desirable, how- 

 ever, to use the longest tube possible in order to obtain the maximum 

 rotation. This solution and reading will hereafter be designated 

 as (1). 



(2) Treat the remainder of the original filtrate with powdered 

 normal lead acetate until no further precipitation results. Cool in 

 an ice bath and filter through a folded filter until clear. Warm the 

 filtrate to room temperature and add a small crystal of lead acetate 

 to determine whether the precipitation is complete. If no further 

 precipitate results, remove the excess of lead completely with 

 anhydrous sodium sulphate, filter until clear, and polarize. Desig- 

 nate this filtrate as Solution (2) and its polarization as Reading 

 (2). Care should be taken to add no more lead acetate to the solu- 

 tion than is necessary for complete precipitation, as lead malate is 

 soluble in an excess of lead acetate. Solutions which are sufficiently 

 clear and contain less than 10 per cent of sugar may be polarized 

 directly without treatment with lead acetate. 



(3) If Reading (2) is negative treat a portion of Solution (2) 

 with uranyl acetate in the manner described under (1) and polarize. 

 Designate this as (3). If Reading (2) is positive, Reading (3) need 

 not be made. 



Polarize at room temperature with white light, taking care that 

 all solutions are polarized at the same temperature. Make at least 

 six readings in each case and take an average of these. In this work 

 a standard. Lippich type, triple field saccharimeter was used, the light 

 being furnished by an electric bulb placed behind a ground-glass 

 plate. Calculate all readings to the basis of a 200 mm tube. If 

 Reading (3) is numerically less than Reading (2), the latter should 

 be discarded; otherwise use Reading (2) in the subsequent calcula- 

 tion. Multiply the algebraic difference between this reading and 

 Reading (1) by 0.030. The product will equal the percentage of 



rCH 2 COOH i 

 malic acid { .1 [in the solution as polarized. 



[CHOHCOOHj 



EFFECT OF URANIUM SALTS. 



The method just described was developed in the course of some 

 work on the composition of the apple. If uranium salts produce 

 an increase in the rotation of malic acid only, it would theoretically 

 be possible to estimate the amount of malic acid in a solution con- 

 taining sugars and other organic compounds by simply polarizing 



