the phosphonate molar fraction in solution, for an initial (H2PP/Zr) molar ratio of 4.16, would be never lower than 0.5, since the maximum amount of phosphate species that the solid can release to the solution is half the initial phosphonate amount. Under these conditions, according to the data of Table 2, the precipitation would lead to the formation of solids with x > 1.5, much richer in phosphonate than the solids that were actually obtained.