3M water and less than 39% water. At water concentrations greater than 0.2M, the nickel ion in solution is found to be predominantly hexaquonickel(ll), instability constant = 4.6 x 10At a concentration of water of 39 wt. %, a transition in the solid phase in equilibrium with the solution occurs between an anhydrous form and a dihydrate. A maximum solubility of 2.56M is obtained in 31% HF solutions in water which corresponds to the phase change between this dihydrate and the tetrahydrate previously reported ( 70). The tetrahydrate persists to 100% water. The Ksp 0.006M water was found to be 2 x 10 '.