Crystal habit modification studies with ammonium and potassium dihydrogen phosphate

Face rates of growth of ammonium and potassium dihydrogen phosphate crystals have been measured for crystallisation from pure solutions and from solutions that contained known amounts of foreign ions. The level of supersaturation and the solution pH both effect the crystal habit. Low supersaturation at normal pH (3.8) tends to give tapered crystals, and at a slightly higher pH (?5), the linear growth rates of both principal axes are considerably increased. The presence of foreign ions such as Cr³⁺, Fe³⁺ and Al³⁺ in low concentrations (? 1 × 10^?3 g ion/1 or ? 50 ppm) also causes a tapering habit. A mechanism of habit modification based on the presence of aquo ions at the crystal face is suggested.