The rapid determination of free acid and water-soluble phosphate in calcium superphosphate

The sample of superphosphate is extracted with water, and the solution titrated with carbonate-free sodium hydroxide in two stages with appropriate indicators. The first stage gives the free acid, and the second stage an approximate result for the primary calcium phosphate, Ca(H2PO4)2. A suitable volume of the sodium hydroxide is then titrated with the superphosphate solution, giving a reading from which a more accurate value for the primary calcium phosphate can be obtained. The total soluble phosphate determined in this way agrees well with that found by gravimetric analysis.