4 DETERMINATION OF MALIC ACID. 



the solution before and after the addition of the uranium salt and 

 multiplying the difference in the readings by a factor determined 

 experimentally. As was previously stated, the rotation of d-tartaric 

 acid is also increased by uranium salts. Consequently, this method 

 can not, without modification, be applied to a solution containing 

 this acid. We have determined that, as would be expected, inactive 

 lactic, succinic, and citric acids are unaffected by salts of uranium. 

 There are, therefore, no serious difficulties in the way of applying 

 this method to the determination of malic acid in the absence of 

 6?-tartaric acid. Work is now in progress on the estimation of tartaric 

 acid by a similar method. 



In the earlier experiments of this investigation an attempt was 

 made to use uranyl nitrate and potassium hydroxid as reagents, as 

 recommended by Walden. This author states that a maximum rota- 

 tion is obtained by the use of 4 molecules of potassium hydroxid and 

 1 to 4 molecules of uranyl nitrate [UO 2 (NO 3 ) 2 +6H 2 O] for each 

 molecule of malic acid. He prefers the nitrate to the acetate because 

 of the greater solubility of the former salt. 



Uranyl nitrate, however, has one serious disadvantage. Its solu- 

 tion in water always contains nitric acid due to hydrolysis. It is 

 impossible to obtain the maximum polarization in the presence of 

 free mineral acid and it is therefore necessary, as Walden pointed out, 

 to neutralize the solution when uranyl nitrate is used. The color of 

 the solution always makes this operation very difficult. Many at- 

 tempts were made to use uranyl nitrate, but with indifferent results. 

 After addition of the uranyl nitrate solution to that containing 

 malic acid, a solution of potassium hydroxid was added drop by 

 drop until a precipitate just began to form. This was dissolved 

 either by addition of a drop of nitric acid or of uranyl nitrate solu- 

 tion. The results, however, were very irregular. In some cases the 

 theoretical recovery of malic acid was obtained, while in others the 

 results were very much too low. Attempts to neutralize the solution, 

 using various indicators, were made with similar results. No better 

 success was attained when the amount of alkali required to neutralize 

 both the uranyl nitrate and the malic acid solution was determined 

 in advance and the calculated amount added to the mixture. 



Uranyl acetate was then substituted for the nitrate in the hope 

 that the presence of a weak organic acid like acetic acid would not 

 influence the rotation of the uranium-malic complex. As a further 

 precaution the solution to be examined was carefully neutralized 

 before the addition of the uranium salt. The concentration of H 

 ions in a solution containing acetic acid and sodium acetate is so 

 slight that it was believed its influence on the rotation would be 





