Equilibrium Solubility, Optical, and Thermodynamic Properties of (Na, K) Pyrophosphate Solutions at Various Temperatures

Abstract

-The equilibrium solubility and physicochemical properties of potassium and sodium pyrophosphates in aqueous solutions were investigated at various temperatures. Water activity measurements for the binary systems were carried out from dilute to saturated solutions at temperatures 298.15 and 353.15 K. The osmotic and activity coefficients were evaluated using thermodynamic models, which reproduce accurately the experimental data and describe the non-ideal behavior over the studied molality and temperature ranges. Based on water activity, the Pitzer, extended ion interaction, and Clegg–Pitzer–Brimblecombe models were developed to evaluate the thermodynamic properties of K₄P₂O₇(aq) and Na₄P₂O₇(aq). Good agreement was obtained with the osmotic coefficient data over the molality and temperature ranges. The solubilities of Na₄P₂O₇ and K₄P₂O₇ in aqueous solutions were measured at each temperature between 298.15 and 353.15 K; they vary from 0.232 to 1.498 mol·kg^–1 for Na₄P₂O₇ and from 2.456 to 5.211 mol·kg^–1, for K₄P₂O₇ over the temperature range. The solid precipitates were characterized by X-ray diffraction. The thermodynamic solubility product ln K°_sp(calc), as well as the standard Gibbs molar energies of dissolution ΔG°_diss and formation ΔG°_fwere evaluated at different temperatures for these binary systems. Optical absorption spectra and energy band gaps E_g of solid phases were also investigated across the temperature range.