Acidity constants of some tetrazole compounds in various aqueous-organic solvent media

The acidity constants of some tetrazole compounds were determined pH-metric ally at 25 degreesC and at the constant ionic strength I = 0.1 mol dm(-3) ( KNO3) in pure water as well as in various aqueous mixtures having variable mole fractions (0.05-0.3) of organic solvents. The organic solvents used ar e methanol, ethanol, N,N-dimethylformamide, dimethyl sulfoxide, and acetone . Generally, the pK(a) values increase with an increase in the content of t he organic solvent. It was concluded that the major effects responsible for this behavior are, in addition to the electrostatic effect, the stabilizat ion of the conjugate base by hydrogen-bonding interactions with solvent mol ecules and solvent basicity. Effect of molecular structure of tetrazoles on the pK(a) values is discussed. Moreover, the thermodynamic functions (Delt aH, DeltaG degrees, and DeltaS degrees) associated with the ionization proc ess of investigated compounds in a pure aqueous medium are derived and thei r values are discussed.