Determination of The Ionization Constant of Local Anaesthetics in Organic Solvent-Water Media by The Spectrofluorimetric Method

The ionization constant (pKa) in general is a chief parameter for evaluating and measuring the reaction estimates, biological effects and retention, biological conveyance and ambient consequences. Therefore, the fluorimetric parameters of some local anaesthetics such as dibucaine, procaine and tetracaine in certain percentages of acetonitrile-water and methanol-water mixtures were optimized and then the pH and solvent percentage effects on fluorimetric parameters were investigated. In the optimal excitation and emission wavelengths and solvent percentage, pKa values of these compounds have been determined by plotting the pH-fluorescence intensity graphs in each solvent media. Almost all ionic species of tetracaine and the acidic species of dibucaine and procaine were fluorescent, but the basic species of dibucaine and procaine were non-fluorescent. The pKa values of the local anaesthetics in methanol-water media were very similar to those in acetonitrile-water media. Moreover, the pKa values determined by the spectrofluorimetric method were very close to the values predicted by the SPARC on-line pKa calculator.