Medium effect on the acid dissociation constants of some heterocyclic azo compounds

The effect of different organic solvent + water mixtures on the acid dissoc iation constants of protonated 5-amino-4-(arylazo)-3-methyl-1-phenylpyrazol e derivatives were studied. The organic solvents used are methanol, ethanol , acetone, and dimethylformamide. The results obtained were discussed in te rms of the solvent characteristics. The pK(BH)+ values of the dyes in quest ion were determined and found to depend on both the amount and nature of th e organic cosolvent. The decrease of the pK(BH)+ values as the amount of or ganic solvent in the medium is increased can be accounted for in terms of t he high stabilization of the protonated form of the compounds by dispersion forces rather than by hydrogen bonding. Also the effect of solvent polariz ability and hydrogen-bonding interaction on the pK(BH)+ values and thus on the spectra of the charge-transfer band observed have been discussed. The a bsolute pK(BH)+ value for each compound despite the presence of more than o ne basicity center has also been discussed.