The effect of carboxylic acids on the dissolution of calcite in aqueous solution. Part 2.—d-, l- and meso-Tartaric acids

Abstract

The effects of the dianions (A^2–) of d-, l- and meso-tartaric acids on the dissolution reaction which takes place between calcite and the corresponding monoanions (HA^–) has been investigated: (a) by using channel-electrode shielding measurements and (b) through the characterization of the resulting dissolution etch-pit morphologies. For the d- and l-tartaric acid system, under conditions where A^2– is present at a concentration of 20 × 10^–3 mol dm^–3, dissolution is found to be described by the following rate law: j_Ca²⁺= 0.01 cm s^–1[HA^–]₀ where [HA^–]₀ is the concentration (mol dm^–3) of HA^– at the calcite/water interface and j_Ca²⁺ is in mol s^–1 cm^–2. This demonstrates partial inhibition of the dissolution reaction as compared to the rate of dissolution in mineral acids. This is attributed to the adsorption of A^2–. The shape of the etch pits resulting from treatment of calcite with A^2–/HA^– was used to infer the likely adsorption sites for the dianion. In the meso-tartaric acid system, much stronger inhibition is found and the rate of dissolution is strongly dependent on the concentration of A^2– present, the rate law being given by: j[graphic omitted] = 0.04 cm s^–1[HA^–]₀//_{1 +K[A^2–]} where K= 1.4 × 10³ dm³ mol^–1. Etch-pit morphologies indicated inhibition of both the lateral dissolution and the etch-pit nucleation process at dislocations on the calcite surface.