Effect of a Citrate Ion Impurity on the Size and Shape of Calcium Phosphate Crystals

Abstract

IT is commonly observed that a change in the size and shape of a crystal occurs when an impurity is added to the solution from which the crystal is grown. One such change, or habit modification, is that of crystals of gypsum (calcium sulphate), which in the presence of sodium citrate change from long needles to short stubby rods¹. The mechanism of such habit modification of crystals is obscure, but the impurity is believed to be adsorbed on the surfaces of the growing crystal, and thus to cause an alteration in their rates of growth which results in the observed change in size and shape. Since the calcium and citrate ions associate in solution to form a complex, it was possible that other calcium compounds than gypsum adsorb the citrate ion, leading to habit modification of their crystals. In fact, the surface of the calcium phosphate crystals in bone has been supposed to be the site of the bone citrate². It is therefore of interest to investigate the effect of citrate on the growth of calcium phosphate crystals.