Multiwavelength spectrophotometric determination of protolytic constants of 4-(2-pyridylazo)resorcinol (PAR) in binary dioxane plus water mixtures

Acid-base properties of 4-(2-pyridylazo)resorcinol (PAR) in 1,4-dioxane (DX)-water binary mixture were investigated using a multiwavelength spectrophotometric method. The protolytic equilibrium constants, spectral profiles, concentration diagrams and also the number of components have been calculated from the fitting of the pH-absorbance data with appropriate mass balance equations by an established factor analysis model. The binary mixture of water-DX is a good system to study changing in the donating ability and permittivity of mixed solvent components in comparison to each other, considering the fact that variation in the permittivity from pure water to pure DX is 76 units. A linear relationship is observed between pKa's of the PAR molecule and the mole fraction of DX in different solvent mixtures. A glass electrode calibration procedure based on a four-parameter equation aided Gran's method was used to obtain pH readings based on the concentration scale (p(c)H). It has been observed that there is an inverse relationship between second and third protolytic constants and mole fraction of DX. The effect of the solvent on the protolytic constants was discussed.