Assay of ethambutol in pharmaceutical preparations

Ethambutol tablets of 200 and 400 mg denominations were assayed by the standard non-aqueous titration method and a simpler calorimetric method. With the titrimetric method, assay values, appreciably higher than the stated content (117% or more), were obtained with the products of 4 companies, while all the values were within 6% of the stated content by the calorimetric method. Rifampicin and pyrazinamide interfered with the estimation of ethambutol by both methods: isoniazid, however, caused an overestimation with the titrimetric method only