SPONTANEOUS CRYSTALLISATION FROM SUPERSATURATED META-STABLE SOLUTIONS

The supersolubility range Ts - Tc for inorganic salts follows closely the nature of the solubility curves. For substances whose temperature coefficient of solubility is constant throughout the range from 0º to 100º, the value of T_s -T_c is constant at different temperatures. A change in the temperature coefficient of solubility denotes also a change in the value of time limit of supersaturation. Substances like KCIO₄, NH₄CI and PbAc₂, which have meta-stable regions in their normal solubility range, display tendency to stabilisation against spontaneous crystallisation, if heated beyond the meta-stable region and cooled. Saturated solutions of hydrates, which have transition temperatures and dissolve in their water of crystallisation on heating, maintain high supersolubility and stabilise against spontaneous crystallisation on heating and cooling possibly due to high solubility of the less hydrated species at low temperatures.