DETERMINATION OF MALIC ACID. 7 



quently anhydrous sodium sulphate was substituted for potassium 

 oxalate with satisfactory results. 



Lead malate is somewhat soluble in water, lead acetate, and sugar 

 solutions. This fact has caused difficulty at two points during the 

 development of the method. It was early discovered that when a 

 solution of sodium malate is treated with lead acetate the filtrate has 

 a positive polarization amounting to as much as one degree. Previous 

 to this discovery it was not thought necessary to remove the lead from 

 solution (2) before polarization. A more serious difficulty was en- 

 countered when it was found necessary to treat solution (2) with 

 uranyl acetate. The presence of even a slight amount of malic acid 

 in this solution after removal of the lead will of course cause a change 

 in rotation when the uranium salt is added. To obtain the best pos- 

 sible separation of lead malate, therefore, the solution is cooled in an 

 ice bath before filtering. This procedure is sufficient to remove most 

 of the malic acid when quantities less than 0.2 per cent are present, 

 but if large amounts of the acid are present a very considerable error 

 may be introduced by the change in rotation caused by the formation 

 of the uranium-malic complex. This is illustrated by the following 

 experiment : 



Four solutions containing known amounts of malic acid, as sodium 

 malate, were treated with lead acetate at room temperature, the lead 

 removed with potassium oxalate, and the filtrate treated with uranyl 

 acetate for one-half hour, with frequent shaking. The filtrates were 

 polarized with the following results: 



Per cent of malic acid: V. 



0.099 0.1 



.199 1.1 



.99 3.2 



1. 99 15. 5 



When the solution, which has been treated with lead acetate, is 

 cooled in ice water before filtering, the rotation with uranyl acetate 

 is much decreased, so that if small amounts of malic acid occur in 

 the presence of large amounts of invert sugar, Reading (3) is less than 

 Reading (2). When comparatively large amounts of malic acid are 

 present. however, Reading (3) is likely to be greater than (2), and for 

 this reason both readings are made when the approximate amount of 

 invert sugar or malic acid are unknown, and the smaller is used in 

 calculating the percentage of malic acid present. 



In the presence of large amounts of sucrose the error due to the 

 effect of the uranium salt on sugar is in the same direction as that 

 due to the presence of traces of malic acid. This will be clearly seen 

 from the following determinations on an approximately normal solu- 

 tion of sucrose containing 0. 24 per cent of malic acid . 



