Confocal fluorescence spectrometry for observation of crystal dissolution kinetics

Abstract

We have developed a confocal fluorescence microscope for observation of the dissolution kinetics at various crystal faces under varying conditions. Natural calcite (CaCO 3) was chosen for the experimental samples and the dissolution kinetics was estimated from the increase in the pH value of the solution using a fluorescent pH indicator, seminaphthorhodafluors-1. Using this instrument, we investigated not only the difference in dissolution rate among various crystal faces but also the effects of the surface roughness, subsurface structure (such as lattice defects) and inhibitor on the dissolution rate. Among the (1 0 1 ̄ 4) , (1 0 1 ̄ 0) and (2 1 3 ̄ 4) faces of calcite samples, the (1 0 1 ̄ 4) face dissolved most slowly and the (1 0 1 ̄ 0) face fastest. Increasing the surface area of (1 0 1 ̄ 4) face in contact with water by scratching increased the dissolution rate. Introduction of lattice defects on the (1 0 1 ̄ 4) face by He + irradiation significantly increased the dissolution rate. Inhibition of the calcite dissolution by La 3+ occurred in the order of (1 0 1 ̄ 4) > (1 0 1 ̄ 0) > (2 1 3 ̄ 4) .