On the effect of impurities on the metastable zone width of phosphoric acid

Abstract

The experimental data published in the literature on the metastable zone width, as determined by the maximum supercooling Δ T max using the conventional polythermal method, of phosphoric acid aqueous solutions containing impurities were analyzed to understand an increase in Δ T max/ T 0 with an increase in saturation temperature T 0 of solute–solvent system and the effect of impurities on the metastable zone width. For the analysis the following relations were used: ln(Δ T max/ T 0)= Φ+ βln R (K. Sangwal, Cryst. Res. Technol. 44, 2009, 231−247) and ( T 0/Δ T max) 2= F(1− Zln R) (K. Sangwal, Cryst. Growth Des. 9, 2009, 942−950; J. Cryst. Growth 311, 2009, 4050−4061), where Φ, β, F and Z are constants. Analysis of the experimental data revealed that: (1) the parameters Φ and F strongly depend on saturation temperature T 0 and concentration c i of impurities, but the constants β and Z are independent of T 0 and depend on c i, (2) the dependence of the parameters Φ and F on T 0 follows an Arrhenius-type equation with activation energy E sat, (3) the activation energy E sat for diffusion of ions/molecules of phosphoric acid containing impurity ions is equal to the differential heat of adsorption Q diff for these impurities, (4) the effectiveness of an impurity is directly connected with the values of their differential heat of adsorption Q diff; the lower the values of Q diff for an impurity, the lower is its effectiveness in promoting nucleation, (5) the activation energy E sat is not related with its heat of dissolution Δ H s and (6) the increase in Δ T max/ T 0 with an increase in T 0 for phosphoric acid is associated with the activation energy E sat for diffusion of solute molecules in the solution such that E sat<0.