6 



DETEKMINATION OF MALIC ACID. 



lutions were then polarized in 200 mm tubes with white light and the 

 readings compared with those of the untreated solution. After stand- 

 ing overnight the readings were repeated. 



TABLE I. Effect of uranyl acetate on the rotation of sugars. 



The results show a decided reduction in rotation which would have 

 a serious effect on the determination of small amounts of malic acid 

 in the presence of large amounts of sugar. To avoid inaccuracy from 

 this cause it is necessary in some cases to remove the malic acid as 

 completely as possible from solution (2) and then to add uranyl 

 acetate to the filtrate. For reasons which will be given later this 

 procedure is never followed when the rotation of solution (2) is 

 positive. Normal lead acetate is used to precipitate malic acid. 

 Even when the removal of the malic acid is not necessary it is ad- 

 visable to clarify the solution with lead acetate and then remove the 

 excess of lead in order to obtain an easily readable solution. In the 

 earlier work a solution of lead acetate was added to the malic-acid 

 solution before diluting to the mark. It was found, however, that 

 the volume of the precipitate in some cases seriously changed the 

 concentration of the solution, especially where sugar solutions in- 

 verted with hydrochloric acid were used, and the solid normal lead 

 acetate, Pb(C 2 H 3 O,) 2 +3H 2 O, was therefore substituted. This salt 

 contains 4.7 per cent of water of crystallization and introduces a 

 negligible dilution error in the amounts used. An attempt was made 

 to use Home's dry lead subacetate, but the results were not satis- 

 factory. 



Dry neutral potassium oxalate was at first used to remove the ex- 

 cess of lead from the solution. It was later found that the rotation 

 of sugar solutions is affected to some extent when this salt is added 

 to an invert sugar solution containing lead acetate. An approxi- 

 mately 25 per cent solution of invert sugar polarized 29 before 

 and after addition of lead acetate. After precipitating the lead with 

 potassium oxalate, however, the filterate polarized 29.6. Conse- 



