While it has long been known that calcium hydroxide is more insoluble than either strontium or barium hydroxide, especially at temperatures near the boiling point of water, this fact has not found practical application as the basis of a qualitative precipitation reaction for calcium, owing to the usual carbonate content of alkali hydroxide solutions applicable as reagents. However, if hydroxyl ions are generated by the interaction of mercuric oxide and potassium iodide within a solution to be tested for calcium, under conditions that exclude the presence of any carbonate ion, the slight solubility of calcium hydroxide may be safely utilized for the detection of calcium in the presence of the other two alkaline earth ions.