In the calcium or phosphate stock solutions (a) or (b), both ions may again be present in similar concentrations in the range of from 0 to about 1 M, preferably from to about 0.6M, more preferably 0 to 0.5M, for example, 0.098M (Na') and 0.1 17M (CI-) in the calcium stock solution (a); and, for example, 0.108M