Effect of pH on the growth of Mg(OH) 2 crystals in an aqueous environment at 60°C

Abstract

Magnesium hydroxide was precipitated at 60°C at various constant pH levels in the range 8.7 to 12.5 from magnesium chloride and ammonium (or sodium) hydroxide. The particles of 0.05–0.30 μm diameter were characterized by transmission electron microscopy, electron diffraction, and surface area measurements. Increase in pH in the range 9.0 to 11.0 favored edgewise growth of the platelets. Decrease in pH tended to decrease the diameter to thickness ratio and surface area of the platelets, culminating in a change in shape to dense equiaxed particles with a “stacked-plate” morphology at pH 8.75, where excess magnesium chloride was present, and the growth rates in the c- and a-directions were nearly equal. At pH 10.0, nearly circular plates with indented outline were observed, whereas, at somewhat higher or lower pH, the plates were hexagonal in shape. The results indicated that excess OH - or Mg 2+ ions affected the growth rate in a particular direction differently, causing changes in shape, and diameter to thickness ratio. Nucleation appeared easier at higher pH.