Thermodynamic Model for SnO2(cr) and SnO2(am) Solubility in the Aqueous Na+–H+–OH−–Cl−–H2O System

摘要

The solubility of SnO2(cassiterite) was studied at 23±2 °C as a function of time (7 to 49 days) and pH (0 to 14.5). Steady state concentrations were reached in 7 days. The data were interpreted using the SIT model. The data show that SnO2(cassiterite) is the stable phase at pH values of ∼11.7. These extensive data provided a log 10 K 0 value of −64.39±0.30 for the reaction (SnO2(cassiterite) +2H2O⇄Sn4++4OH−) and values of 1.86±0.30, ≤−0.62, −9.20±0.34, and −20.28±0.34 for the reaction ( $mathrm{Sn}^{4+} + nmathrm{H}_{2}mathrm{O} rightleftarrows mathrm{Sn}(mathrm{OH})_{n}^{4 - n} + nmathrm{H}^{+}$ ) with values of “n” equal to 1, 4, 5, and 6 respectively. These thermodynamic hydrolysis constants were used to reinterpret the extensive literature data for SnO2(am) solubility, which provided a log 10 K 0 value of −61.80±0.29 for the reaction (SnO2(am)+2H2O⇄Sn4++4OH−). SnO2(cassiterite) is unstable under highly alkaline conditions (NaOH concentrations 0.003 mol⋅dm−3) and transforms to a double salt of SnO2 and NaOH. Although additional well-focused studies will be required for confirmation, the experimental data in the highly alkaline region (0.003 to 3.5 mol⋅dm−3 NaOH) can be well described with log 10 K 0 of −5.29±0.35 for the reaction Na2Sn(OH)6(s)⇄Na2Sn(OH)6(aq).