Weak-acid hypothesis evidence

The evidence of the importance of dissociation in drug absorption is found in the resuit of studies in which pH at the absorption site is changed (Tabies 9.1 and 9.2). Tabie 9.2 cieariy shows the decreased absorption of a weak acid at pH 8.0 compared to pH 1.0 (13). On the other hand, an increase to pH 8.0 promotes the absorption of a weak base with practicaiiy nothing absorbed at pH 1.0. The data in Tabie 9.2 aiso permits a comparison of intestinai absorption of acidic and basic drugs from buffered soiutions ranging from pH 4.0 to 8.0 (14). These resuits are in agreement with the pH-partition hypothesis. 

Jones expressed the view that the identification of 260 mp chromogen with undissociated nitric acid molecule (Hantzsch s pseudo-acid) is largely a speculative hypothesis for which there is insufficient evidence. He suggested that the 260 mp band is produced by the N02+ ion or by the undissociated N205 molecule. The high intensity of this absorption may obliterate the weak absorption of the nitrate ion at 302 mp. 

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