DISSOLUTION ANISOTROPY IN NICKEL SULFATE ALPHA-HEXAHYDRATE CRYSTALS

The dissolution anisotropy of well-formed crystals of NiSO4 α oH2O grown under controlled conditions was studied using a single crystal dissolution method. Dissolution rates, proportional to an observed apparent rate constant, Kobs, were determined by measuring the movement of the (111) and (112) crystal faces with time in a flowing solvent, using a travelling microscope. Kobs for the (112) face was greater than for the (111) face at all flow rates studied but anisotropy was less pronounced at the lower flow rates. The apparent rate constants for the transport and surface controlled reactions. Kt and Kr, were of the same order of magnitude suggesting that the overall dissolution reaction was under mixed control at the lower flow rates. Activation energies were slightly higher than the normal range for transport processes. Kr(112) > Kr(111) indicating that anisotropy was due mainly to differences in the rate of the surface reaction. At high flow rates there was a change to a predominantly surface controlled reaction and activation energies were within the accepted limits for these reactions. It is likely that dissolution anisotropy is due to the differences in activation energy for the two faces. © 1979.