This is because the solubility of sodium sulfate is decreased by the presence of hydroxide ions. At high pH, there are more hydroxide ions present, which compete with sulfate ions for solvation. This reduces the amount of sodium sulfate that can dissolve.

The solubility of sodium sulfate in water at 25°C is 19.2 g/100 mL. At pH 1, the solubility of sodium sulfate is 25.6 g/100 mL. At pH 13, the solubility of sodium sulfate is 12.8 g/100 mL.