Buffering and ion-release by a glass-ionomer cement under near-neutral and acidic conditions

Specimens of an experimental glass-ionomer cement were stored in water (initial pH 5.9) and aqueous lactic acid (initial pH 2.7) for storage periods of 1 week up to 6 weeks. Change in mass, solution pH and fluoride release were measured at weekly intervals, and other ions were determined at weeks 1, 2, 4 and 6. In water, cements raised the pH consistently to 6.7–6.9 from weeks 2 to 6, but this did not correspond to uniform amounts of ions released, nor even to consistent mole ratios of ions in solution. Similarly, in lactic acid, pH was raised to between 3.6 and 4.5, but without a consistent concentration of ions in solution. In near neutral conditions, calcium was found to be virtually insoluble and remained within the cement, whereas reasonable amounts of sodium, aluminium, phosphorus, silicon and fluoride were released at all time intervals, with downward trends over time. In acidic conditions, considerable amounts of calcium were released over time, and amounts of calcium, aluminium, phosphorus and silicon increased with time, reaching a maximum in week 4. This suggests that as maturation proceeds, there is an increase in the acid-soluble fraction of the cement containing these elements. Fluoride release was found to be as previously reported, i.e. greater amounts in the early stages of the experiment, and with a gradual decline, and with greater amounts in acid than in water. Determination of fluoride with and without the decomplexing reagent TISAB showed that 70–75% of the total fluoride was released in “free” form in water for most weeks, whereas in acid, it declined sharply and by week 6, an estimated total of 96% of the fluoride released was complexed.