When I tried to mix a sodium metabisulfite solution with sodium hydroxide solution, the mixture solution quickly became turbid. Before mixing both were clear solutions. Please help explain the chemistry for this observation?
Sodium disulfite can be prepared by treating a solution of sodium hydroxide with sulfur dioxide.[3] When conducted in warm water, Na2SO3 initially precipitates as a yellow solid. With more SO2, the solid dissolves to give the disulfite, which crystallises upon cooling.
https://en.wikipedia.org/wiki/Sodium_metabisulfite
Sodium metabisulfite (Na2S2O5) dissolves in water as sodium bisulfite: Na2S2O5 + H2O → 2NaHSO3. Sodium bisulfite is converted to sodium sulfite (Na2SO3) by NaOH (alkaline solution): NaOH + NaHSO3 → Na2SO3 + H2O.
The solubility of sodium sulfite (Na2SO3) in the final solution may have been exceeded due to common-ion effect, as Na+ is both contributed by the sodium salt aq. sol. and by the NaOH aq. sol.
We may address this effect quantitatively, as follows.
If not found in the literature, the solubility in the final solution can be predicted based on the Na2SO3 solubility product (Ksp = [SO32-]·[Na+]2), which can be calculated after its solubility in water (S; given in M units, i.e. mol/dm3 of solution). Unlike solubility (in water), Ksp can be applied both to the pure salt solution in water and to the final mixed solution (same temperature).
Precipitation can be predicted for [SO32-]f·[Na+]f2 > Ksp = S3, where the subscript (f) denotes the final mixed solution.
Sodium balance is: CNaOH + 2CNa2SO3 = [Na+]f. Sulfur balance is: CNa2SO3 = [SO32-]f. C denotes nominal (i.e. formal) concentrations at the final mixed solution (before precipitation, if occurs).
Hence, we can predict precipitation for CNa2SO3·(CNaOH + 2CNa2SO3)2 > S3.
After precipitation, if occurs, CNa2SO3 should decrease, so that last inequation transforms to the corresponding equation.
- Badges
- Science topic
- Similar topics
- Chemistry