Active Dissociated Species of Citric Acid for Growth Suppression of a Sodium Chloride Crystal

The equilibrium growth-starting supercooling ΔTGeq for the (100) and (111) faces of a sodium chloride crystal was measured at different pHs in the presence of citric acid. The equilibrium growth-starting supercooling ΔTGeq is defined for a given crystallographic face as the degree of supercooling at which the crystal face starts to grow when the solution is cooled continuously after the equilibrium impurity adsorption has been reached. It is a measure of the effectiveness of an impurity for adsorption and growth suppression. Different values of ΔTGeq were obtained at different pHs. Through the analysis of the data, the first, second, and third dissociated species of citric acid were determined to be simultaneously active for both adsorption and growth suppression. Magnitude of the activity of each dissociate was on the order of the third, second, and first dissociates.