DETERMINATION OF MALIC ACID. 



9 



It is well known that the specific rotation of aqueous solutions of 

 /-malic acid varies greatly with concentration, 1 changing from minus 

 to plus in the higher concentrations. The addition of concentrated sul- 

 phuric acid 2 will also cause a change in rotation. Some experiments 

 were made to throw light on the question as to whether such dif- 

 ferences in the specific rotation of Z-malic acid affect the rotation of 

 the uranium malic complex. A solution containing 13.95 grams of 

 Z-malic acid in 28 cc of solution polarized +1.1. This solution was 

 diluted and the malic acid determined by the uranium method, the 

 theoretical recovery being obtained. This same solution was treated 

 with concentrated sulphuric acid and after standing over night 

 polarized +2.7. After standing four days the rotation was found 

 to be the same. On dilution the theoretical recovery was again 

 obtained by the uranium method. 



DETERMINATION OF THE FACTOR. 



A large number of experiments w r ere made to determine the factor 

 for calculating the percentage of malic acid from the polarization. 

 Neutral solutions containing known amounts of malic acid were 

 treated with uranyl acetate, polarizing in a 200 mm tube with white 

 light and dividing the percentage of malic acid present by the read- 

 ing in degrees Ventzke. In some cases this determination was made 

 in the presence of invert sugar. The details of the experiment are 

 given in Table IV, page 10. It will be observed that there is a 

 gradual decrease in the factor with increase in concentration of malic 

 acid. The increase is not sufficient, however, to seriously affect re- 

 sults in the ordinary concentrations found in natural products. The 

 average of 27 determinations gave a factor 0.036, and this was accord- 

 ingly adopted for use in all calculations. A number of determinations 

 in which sodium light was used instead of white light showed a 

 slightly smaller specific rotation with the former. 



TABLE III. Specific rotation in circular degrees (10 cm tube). 



With very clear solutions containing large amounts of uranium- 

 malic complex difficulty may be experienced in obtaining the neutral 

 point on account of the decided blue color developed in one segment 

 of the field. This can be eliminated to a great extent by the use of 

 a 3 cm cell containing a 3 per cent solution of potassium bichromate. 



i Schneider, Ann. Chem. (Liebig.) 1881, 207: 262. 



2 Ibid., p. 279. 



