Determination of the acid dissociation constant of bromocresol green and cresol red in water/aot/isooctane reverse micelles by multiple linear regression and extended principal component analysis

摘要

The pKa of 3',3'',5',5''tetrabromo-m-cresolsulfonephtalein (Bromocresol Green) and o-cresolsulphonephtalein (Cresol Red) was spectrophotometrically measured in a water/AOT/isooctane microemulsion in the presence of a series of buffers carrying different charges at different water/surfactant ratios. Extended Principal Component analysis was used for a precise determination of the apparent pK_a and of the spectra of the acid and base forms of the dye the apparent pK_a of dyes in water-in-oil microemulsions depends on the charge of the acid and base forms of the buffers present in the water oil. Combination with multiple linear regression increases the precision. Results are discussed taking into account the profile of the electrostatic potential in the water pool and the possible partition of the indicator between the aqueous core and the surfactant. The pK_a corrected for thee effects are independent of w_0 and are close to the value of the pK_a in bulk water. On the basis of a tentative hypothesis it is possible to calculate the true pK_a of the buffer in the pool.