Solubility Product of Calcium Sulfate

Gypsum scale is calcium carbonate's nastier sibling. With K_sp = 4.93 × 10⁻⁵ for the anhydrous form — the dihydrate, gypsum proper, sits lower at 3.14 × 10⁻⁵ — calcium sulfate is roughly ten thousand times more soluble than calcite, so it only appears when a system is concentrated hard. That makes it the limiting scale in high-cycle cooling towers and in reverse-osmosis reject streams, where sulfate and calcium are both concentrated several-fold.

The reason operators fear it is that the usual remedy makes it worse. Dosing sulfuric acid to control carbonate scale adds sulfate, pushing the calcium sulfate ion product up, and unlike carbonate scale gypsum cannot be dissolved with acid at all — once laid down it has to be removed mechanically or with a chelant. Its solubility is also unusual in passing through a maximum near 40 °C and then falling, which is why gypsum deposits show up on hot surfaces in exactly the places the carbonate model says are safe.