The thermometric titration of phosphoric acid is based on the exothermic neutralization reaction with sodium hydroxide. A standard solution of c(NaOH) = 2 mol/L is used as titrant, which is added at a constant rate to the sample. Once all phosphoric acid has been titrated, the temperature of the sample decreases, indicating the endpoint.
Any calcium present in the fertilizer interferes with the titration of phosphoric acid by precipitating as calcium hydroxide. This can be eliminated by adding a saturated oxalate solution prior to the titration, resulting in the precipitation of calcium oxalate.
