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

The experimental data published in the literature on the metastable zone width, as determined by the maximum supercooling ΔTmax using the conventional polythermal method, of phosphoric acid aqueous solutions containing impurities were analyzed to understand an increase in ΔTmax/T0 with an increase in saturation temperature T0 of solute–solvent system and the effect of impurities on the metastable zone width. For the analysis the following relations were used: ln(ΔTmax/T0)=Φ+βln R (K. Sangwal, Cryst. Res. Technol. 44, 2009, 231−247) and (T0/ΔTmax)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 T0 and concentration ci of impurities, but the constants β and Z are independent of T0 and depend on ci, (2) the dependence of the parameters Φ and F on T0 follows an Arrhenius-type equation with activation energy Esat, (3) the activation energy Esat for diffusion of ions/molecules of phosphoric acid containing impurity ions is equal to the differential heat of adsorption Qdiff for these impurities, (4) the effectiveness of an impurity is directly connected with the values of their differential heat of adsorption Qdiff; the lower the values of Qdiff for an impurity, the lower is its effectiveness in promoting nucleation, (5) the activation energy Esat is not related with its heat of dissolution ΔHs and (6) the increase in ΔTmax/T0 with an increase in T0 for phosphoric acid is associated with the activation energy Esat for diffusion of solute molecules in the solution such that Esat<0.