SURFACE COMPLEXATION OF CALCIUM MINERALS IN AQUEOUS-SOLUTION .1. SURFACE PROTONATION AT FLUORAPATITE WATER INTERFACES

The amphoteric properties of hydrous fluorapatite surfaces have been investigated. The measurements were performed as potentiometric titrations in 0.1 M NaCl medium at 25°C using a glass electrode. The experimental data could be explained by a model that assumes the surface equilibria. PO-+H+=POH logβs101(int.)=6.6±0.1(3σ). CAOH2+-H+=CAOH logβs-101(int.)=-9.7±0.1(3σ). The equilibrium constants were evaluated using the computer program FITEQL by Westall (Report 82-01, Department of Chemistry, Oregon State University, Corvallis, 1982), assuming a constant capacitance model for the electric double layer. The acidity constants result in a value of log[H+]pzc = -8.15, which shifts to log[H+]pzc = -7.13 upon exposure to atmospheric carbon dioxide. © 1991.