ANALYTICAL INVESTIGATIONS ON ISONICOTINIC-ACID HYDRAZIDE (ISONIAZID) .4. COLORIMETRIC DETERMINATION OF MICROQUANTITIES OF ISONIAZID USING 2,3,5-TRIPHENYLTETRAZOLIUM CHLORIDE

A new high-sensitive colorimetric method for the determination of microquantities of isoniazid (INH) is described. The method is based on the reduction of 2,3,5-triphenyltetrazolium chloride (TPTC) by INH in an alkaline ethanolic medium. The formazan formed from this reduction exhibits an absorption maximum at lambda = 485 nm, with an apparent molar absorptivity of epsilon = 2.10 x 10(4) L mol-1 cm-1 and the corresponding Sandell's sensitivity of 6.52 ng of INH per cm2. Beer's law is obeyed over the range of 0.2-12.0-mu-g ml-1 and the optimum concentration range is 0.5-10.0-mu-g ml-1. The regression line equation was calculated as: A = 1.56 x 10(-1) C + 1.46 x 10(-3) with a correlation coefficient of 0.9999 (n = 5). The accuracy and the precision are satisfactory, since the mean standard analytical error (SD/square-root n) and the mean rsd (%) do not exceed the values of 0.0272-mu-g ml-1 and 1.27%, respectively. Finally, the new method proposed, was applied for the determination of INH in pure form and in pharmaceutical formulations. The results from this analysis were compared statistically with those of the USP XXI official method applied simultaneously on samples of the same batches. The data of Student's t-test and of the variance ratio F-test demonstrate that there were no significant differences between the sets of these analytical results.