A potentiometric and spectrophotometric study on acid?base equilibria in ethanol-aqueous solution of acetazolamide and related compounds

E.CHUFAN; F.SUVIRE; ENRIZ R.D.; J.PEDEREGOSA

TALANTA

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1999 vol. 49 p. 859 - 868

0039-9140

Acid?base equilibria in ethanol-aqueous solution of 5-acetamido-1,3,4-thiadiazole-2-sulfonamide (acetazolamide, H2acm), 5-tertbutyloxycarbonylamido-1,3,4-thiadiazole-2-sulfonamide (B-H2ats), 5-amino-1,3,4-thiadiazole-2-sulfonamide (Hats) and 5-amino-1,3,4-thiadiazole-2-thiol (Hatm) at 25°C, 0.15 mol dm3 ionic strength (NaNO3), have been investigated by potentiometry and UV spectrophotometry. The ionization constants were calculated with SUPERQUAD program from potentiometric measurements and by a method according to Edsall et al. using the mole fractions determined by complementary tri-stimulus colorimetry (CTS). The constants obtained by potentiometry were: B-H2ats, pka1 7.33(3) and pka2 9.27(1); Hats, pka1 2.51(3) and pka2 8.49(1); Hatm, pka1 1.92(1) and pka2 6.81(1); whereas the constants determined by spectrophotometry were: H2acm, pka1 7.78(1) and pka2 9.57(2); B-H2ats, pka1 7.71(2) and pka2 9.61(2); Hats, pka1 2.19(3) and pka2 8.61(2); Hatm, pka2 6.90(2). Theoretical calculations using MO semiempirical and ab-initio RHF:6-31G* computations for the compounds were also performed. It was possible to clarify the preferred deprotonation mechanism of acetazolamide and B-H2ats in which the first deprotonation takes place at the carbonamido group.