A study of ion-pair formation between erythromycin and bromothymol blue, methylthymol blue, and thymol blue and their use for assaying erythromycin in dosage forms

The ion-pair formation between erythromycin (E) and bromothymol blue (BTB), methylthymol blue (MTB), and thymol blue (TB) has been studied spectrophotometrically. The three dyes have been found to form ion-pair associates with E in acid and neutral solutions. The optimal pH values being 2.8, ca. 4, and ca. 4 for the associates with BTB, MTB, and TB, respectively. The associates were extractable with chloroform and exhibited absorption maxima at 415, 430, and 550 nm, respectively. The composition of the associates was established by Job's method and by spectrophotometric titration. The ion-pair associates E-BTB and E-MTB were subsequently utilized for assaying erythromycin in two dosage forms.