EFFECT OF ELUENT PH ON THE IONIC AND MOLECULAR-FORMS OF THE NONSTEROIDAL ANTIINFLAMMATORY AGENTS IN REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

High-performance liquid chromatographic conditions for the best separa tion of some non-steroidal anti-inflammatory agents ionic were describ ed. The dependence of eluent pH on the (protonated) and molecular (non -protonated) forms of analysed compounds have been investigated. This paper is the study of the retention behavior of some anti-inflammatory agents depending of the eluent pH. Some derivatives of phenol (acetam inophen, aspirin, salicylamide, phenacetin and salicylic acid), pyrazo lidinedione (sulfinpyrazone, oxyphenbutazone, phenylbutazone and ketaz one), amino-2-nicotinic acid (nixylic and niflumic acid) and amino-2-b enzoic acid (mefenamic and flufenamic acid) were analysed with the iso cratic RP-HPLC separation. The different variations of mobile phase me thanol-water v/v, containing 1% acetic acid were performed in order to obtain the best conditions of the separation. For all analysed substa nces the pH, pKa and the fitted pKa values were calculated in differen t mixtures of methanol-water using the graphical method given by G.Cha rlot and B.Tremillon. The capacity factors (k' and the separation fact ors (alpha) were calculated for all of them.