According to my textbooks, the dihydrate can be rapidly oxidized to the sulfate if not tightly capped. It therefore appears to be less soluable as Potassium Sulfate is less soluable.

Might be a thought to consider.

Also, 50% by weight is an incomplete spec in this case. Due to the high solubility of some items the full spec of wt/vol or wt/wt must be used for accuracy.