Ampicillin adsorption by some antacids

In certain situations of peptic ulcers ampicillin has been co-administered with bismuth carbonate with an implication for adsorption of the ampicillin. To quantify this effect the kinetics and extent of adsorption of ampicillin by some commonly used antacids were measured; these are bismuth carbonate, magnesium trisilicate and aluminium hydroxide. The adsorption of ampicillin by bismuth carbonate followed the Langmuir adsorption isotherm, which suggests chemisorptions. It was characterized by a strong adsorption at a low adsorbate (ampicillin) concentration but the % adsorption decreased with increase in adsorbate concentration, which is a feature of a saturated monolayer adsorption. On the other hand, the adsorption by magnesium trisilicate and aluminium hydroxide followed the Freundlich adsorption isotherm characterized by a low adsorption at a low adsobate concentration but this increased slightly with increase in adsorbate concentration, suggesting a weak physical adsorption. The adsorption capacities (mg/g) of the adsorbate were 1.64 (bismuth carbonate) 0.04 (magnesium carbonate) and 0.03 (aluminium hydroxide). Bismuth carbonate thus gave by far the highest degree of adsorption. The conclusion is that the co-administration of ampicillin and bismuth carbonate in the treatment of certain peptic ulcers is erroneous.