Samuel Y. Shen and Alfred J. Oilman Long Island University Brooklyn, New York
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Potentiometrir Titration of Aspirin in Ethanol
Methods for determining the acid content of aspirin have been developed in both aqueous (1-5) and nonaqueous (6-9) solutions using a variety of methods. This experiment utilizes this interesting analysis to illustrate the potentiometric end point in uonaqueous acid-base titrations. Twenty tablets of commercial aspirin are ground into a fine powder, accurately weighed, dissolved in 95% ethanol, and made up to volume in a 500-ml volumetric flask. A 50-ml aliquot is titrated with a standard solution of alcoholic KOH. The titration is followed with a pH meter equipped with a glass electrode. Readings are recorded a t regular increments and the curve plotted to obtain the end point. Preliminary work with pure acetyl salicylic acid determined the feasibility of the titration. A typical curve for a monobasic acid was obtained. Recovery of the pure acid averaged 99.81'% for six determinations, a standard deviation of 0.1%. Error due to hydrolysis was negligible Thymol blue was used to check the end point. The sharp color change from yellow to blue coincided with the potentiometric end point. Filler from the commercial aspirin tablets formed a suspension in solution, but did not interfere with the titration. The table shows the varied results obtained with six differentbrands of aspirin.
540
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Journal o f Chemical Education
Literature Cited (1) JENKINS, GLENNL., CHRISTIAN, J . E., AND HAGER,G. P., "Quantitative Pharmaceutical Chemistry," 4th ed., McGram-Hill Book Co., New York, 1953, p. 139. (2) TINKER,R. B., AND MCBAY,A. J., J. Amer. Phar. Assoe. Sei. Ed., 43, 315 (1954). Z. B., AND SHCEENNIKOYA, (3) K o R s ~ u ~ o A., v , KNUZNETBOVA, M. K., Zhur. Anal. Khirn., 6 , 96 (1951) (Through Chem. Abs. 45, 6968 (1951)). JAMES S., AND ROBERTS,JOENE., J. CHEM.EDUC. (4) PROCTOR, 38, 471 (1961). (5) BJORLING,C. D., Fam. R a y , 48, 281 (1949) (Through Chem. Abs. 43, 5703 (1949)). (6) Moss, M. L., E L L I ~J.~H., , iuvo HALL, R. T., Anal. Chem., 20, 784 (1948). (7) FRITZ,J. S., AND LISICKI,N. M., Anal. Chem., 23,589 (1951). (8) MARKUNAS, P. C., AND RIDDICK,J . A., Anal. Chem., 23, 337 (1951). (9) WOLLISE,E. G., ET AL.,Anal. Chem.. 26,1753 (1954).
Determinatian of Acetyl Salicylic Acid in Aspirin Brand of aspmn
Ava. wt. of one tablet m s )
Wt. acetvl salicylic aEid found (gms)
Acetvl salicvlic aEid pertablet (%)
