Action of hydrochloric acid on aluminum hydroxide-magnesium hydroxide gels and magaldrate: 27Al NMR and pH-stat studies.

Neutralization of mixtures of aluminum hydroxide-magnesium hydroxide gels and of magaldrate by hydrochloric acid were studied by 27Al NMR under conditions of both equilibrium and kinetic control. Under conditions where equilibrium has been attained, an aluminum NMR signal is detectable for suspensions of the mixed gels and magaldrate only after enough acid has been added to exhaust the acid-neutralizing capacity of the magnesium hydroxide. Mixed gels seem to form several soluble aluminum-containing species as neutralization proceeds. Under the conditions of the modified Beekman neutralization procedure, in which the species concentrations reflect neutralization kinetics, mixed gels show a sharp burst of the hexaaquoaluminum cation as acid is added followed by a slow loss of that cation from solution and an accompanying slow rise in pH. Magaldrate shows a steady increase in the hexaaquoaluminum cation with added acid. Differences between magaldrate and mixed gels are also apparent in pH-stat titrations in which magaldrate displays a biphasic response, contrasting to the two burst phases with an intervening lag phase observed for mixed gels. The results of the 27Al NMR and pH-stat titrations are consistent with the hypotheses that magaldrate is a homogeneous substance with a hydrotalcite-like structure and that mixed gels consist of a magnesium hydroxide core surrounded by aluminum hydroxide.