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

The solubility of SnO2(cassiterite) was studied at 23 +/- 2 A degrees 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 < similar to 11.7. These extensive data provided a log (10) K (0) value of -64.39 +/- 0.30 for the reaction (SnO2(cassiterite) +2H(2)Oa double daggerSn4++4OH(-)) and values of 1.86 +/- 0.30, a parts per thousand currency signa'0.62, -9.20 +/- 0.34, and -20.28 +/- 0.34 for the reaction (Sn4++nH(2)O reversible arrow Sn(OH)(n)(4-n)+nH(+)) 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)+2H(2)Oa double daggerSn4++4OH(-)). SnO2(cassiterite) is unstable under highly alkaline conditions (NaOH concentrations > 0.003 mola <...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 mola <...dm(-3) NaOH) can be well described with log (10) K (0) of -5.29 +/- 0.35 for the reaction Na2Sn(OH)(6)(s)a double daggerNa2Sn(OH)(6)(aq).