Kansas Acadcnuj of Science. 175 



A METHOD FOR THE QUANTITATIVE DETERMINATION 



OF SALICYLIC ACID IN THE PRESENCE OF 



ACETYL SALICYLIC ACID. 



G. N. Watson. 



DURING the past few months both state and federal labora- 

 tories have found many adulterated samples of acetyl 

 salicylic acid (asperin), one of the most common adulterants 

 of this product being salicylic acid. 



Among the qualitative tests for the differentiation of acetyl 

 salicylic acid and salicylic acid is the test with bromine water. 

 Bromine water gives a precipitate with salicylic acid, but does 

 not give a precipitate with acetyl salicylic acid.^ This fact 

 and the method of Fr. Freyer- for determination of salicylic 

 acid by means of a N/10 bromine solution were utilized for the 

 quantitative determination of salicylic acid in mixtures of the 

 two drugs. The method of Freyer is practically that of the 

 U. S. P. for the determination of phenol. The end product, 

 tri-brom-phenol, is the same with both drugs. 



Two solutions were prepared, one containing 100 mg. of 

 salicylic acid in 100 cc, and the other 100 mg salicylic acid 

 and 100 mg. acetyl salicylic acid in 100 cc. Ten cc. of the solu- 

 tion, equivalent to 10 mg. of U. S. P. salicylic acid, were used 

 for a determination. Twenty cc. of N/10 bromine, U. S. P., 

 were added, followed by 75 cc. water and 5 cc. hydrochloric 

 acid. The mixture was allowed to stand, with occasional 

 shaking, in a glass-stoppered flask for ten minutes, when 5 cc. 

 of potassium iodide, T. S., were added, and the liberated 

 iodines titrated with NIO sodium thiosulphate, V. S. The 

 two solutions required the same number of cc. of N/10 thio- 

 sulphate and showed the presence of 9.9881 mg. of salicylic 

 acid in 10 cc. samples of each solution. 



A 50 mg. sample of the same salicylic acid showed, by direct 

 titration with N/10 KOH, a purity of 99.9 percent, which 

 result checks very closely with the preceding. 



It must be remembered, however, that a cold aqueous solu- 

 tion of acetyl salicylic acid decomposes slowly, forming sali- 

 cylic acid, and if heated the reaction is comparatively rapid. 



1. Fuller, Qualitative Analysis of Medicinnl Preparations, p. 26. 



2. Chem. Zeitun?, vol. 20, p. 820. 



