[Determination of neutralization capacity of antacids in gastric juice]

Observations made during intragastric titration studies performed with antacids that have an identical neutralizing capacity but differ in their chemical compositions suggested that aluminum hydroxide loses a large portion of its neutralizing capacity when applied postprandially. To further elucidate the mechanism involved in-vitro studies were performed whereby the reactivity of magnesium hydroxide, aluminum hydroxide, and calcium carbonate was estimated either in aliquots of 10 ml of water or 10 ml of 5% oxo solution. The effectively available neutralizing capacity of the respective antacids was measured for the pH range of 1 to 5. The results revealed that magnesium hydroxide as well as calcium carbonate can fully react with acid whereby their theoretically available neutralizing capacity is fully available for the pH of 1 to 4.5, this in water as well as in 5% oxo solution. In contrast, aluminum hydroxide loses a large proportion of the theoretically available neutralizing capacity already in water but especially in oxo solution in a pH dependent manner. The loss far exceeds that calculated when one takes into account the pKa dependent hydrolysis of aluminum hydroxide. This loss of reactivity increased from 20% at pH 1 in a near linear fashion up to above 80% at pH 3.5. A food induced decrease of aluminum hydroxide solubility as well as stabilisation of the macromolecular aluminum hydroxide complexes through anions and organic acids are likely to be responsible for this loss of reactivity.