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 neutralization (α). 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 release of Zn, Mg and P decreased with increasing α except for oxalic 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 dissolved Zn or Mg existed as hydrated ions. While in acids with high complexing ability such as citric acid and oxalic acid, both of the phosphate matrix and the unreacted ZnO particles simultaneously dissolved at lower degree of neutralization. Zn or Mg dissolved to form stable complexes with those acid anions. At high degree of neutralization, a driving force for the dissolution was complexing ability of acids. The extent of the cement matrix erosion increased, with increasing tendency of the stable complex formation. Hopeite, Zn3(PO4)2&middot4H2O crystallized on the surface of the cement immersed in acetic acid with α from 0 to 0.9 and zinc oxalate monohydrate, Zn(COO)2&middotH2O did in oxalic acid with α from 0 to 0.6.