*3.1.2 Ester hydrolysis*

Hydrolysis of drugs with an ester functional group (e.g. procaine, atropine, etc.) forms one of the most common types of drug instability. It is usually a bimolecular reaction involving acyl-oxygen cleavage. Ester hydrolysis is (H<sup>+</sup> ) or (OH<sup>−</sup>) ion catalyzed and is dependent on the specific compound and the pH of the solution. Atropine hydrolysis is totally pH dependent and this was characterized by the slopes of −1 and +1. In some cases, the hydrolysis of the drug can show a pH-profile with three regions: a hydrogen ion (proton) catalyzed region, (slope = −1), an uncatalyzed region (solvent dependent, slope = 0) and a hydroxyl ion-catalyzed region (slope = +1) (**Figure 1**) [5].
