Selective inhibition of aragonite growth by citrate and isocitrate at moderate supersaturations, as measured by an optical-microscope flow-cell assay

An optical flow-cell assay was developed which allowed for simultaneous measurement of calcite and aragonite growth under conditions of constant concentrations of constituent ions and growth-modifying additives. Using this assay, it was determined that l-citrate and d,l-isocitrate selectively inhibited the growth of aragonite crystal seeds, while having little or no effect on calcite growth rates at supersaturations in the range of σ=2.5±0.4. Tricarballylate, which differs from citrate and isocitrate by the absence of a hydroxide functionality, had very little effect on calcite or aragonite growth. For citrate on aragonite, an effective surface binding constant of K=2.5±0.6×105 M−1 was derived from the kinetic data; for isocitrate, K=2.9±0.8×105 M−1. The similarity of these effective binding constants argues against precise stereochemical recognition of the aragonite surface.