DISSOLUTION OF DENTAL ZINC PHOSPHATE CEMENT IN ORGANIC-ACID SOLUTIONS

Dissolution of dental zinc phosphate cement was investigated in acetic , lactic, oxalic and citric acid solutions with different degrees of n eutralization (alpha). The dissolution process was discussed based on the ion equilibrium in acid solution. Dissolution was controlled by H ion concentration and/or complexing ability of the acid anions. The r elease of Zn, Mg and P decreased with increasing alpha except for oxal ic acid. In acid solutions with low complexing ability such as acetic acid, the unreacted ZnO and/or MgO particles dissolved preferentially by the attack of H+ ion and most of the dissolved Zn or Mg existed as hydrated ions. While in acids with high complexing ability such as cit ric acid and oxalic acid, both of the phosphate matrix and the unreact ed ZnO particles simultaneously dissolved at lower degrees of neutrali zation. Zn and Mg dissolved to form stable complexes with those acid a nions. At high degrees of neutralization, a driving force for the diss olution was complexing ability of acids. The extent of the cement matr ix erosion increased, with increasing tendency of the stable complex f ormation. Hopeite, Zn3 (PO4)2 . 4H2O Crystallized on the surface of th e cement immersed in acetic acid with alpha from 0 to 0.9 and zinc oxa late monohydrate, Zn(COO)2 . H2O did in oxalic acid with alpha from 0 to 0.6.