Hydrate forms causing complications with gravimetric analysis of ferric phosphate

Question

I'm planning to determine phosphate concentrations in a solution via adding Iron(III) and weighing the precipitated ferric phosphate.

However, I read that ferric phosphate has a dihydrous form, which is slightly soluble. Furthermore, temperatures of over 200 degrees C are required to dehydrate this.

This is inconvenient for me, and I'm wondering if there is a way to correct for the presence of the dihydrate which means not all the phosphate will be precipitated, as well as the fact that the precipitate will be a mixture of the anhydrous and dihydrous forms. Another method to dehydrate it would work, or maybe a table of what proportions the anhydrous and dihydrous forms exist in under different conditions, so I can just factor it in.

Answer 1

As Poutnik suggested, you shouldn't use unestablished methods for gravimetric analysis for analytic determination. The Ref.1 have showed one of the original published method for phosphate analysis by gravimetric determination. Accordingly to Ref.2, which developed this method for undergraduate experiment:

This method consists of first converting all the phosphorus containing species in the sample to soluble orthophosphate $(\ce{PO4^{3‒}})$ ion by oxidation and hydrolysis in acid solution. Then an acidic quimociac reagent is added to the prepared orthophosphate sample and a bright yellow precipitate forms. The resulting precipitate is filtered, dried and weighed. Precipitation follows the reaction: $$\ce{H3PO4 + 12H2MoO4 + 3C9H7N -> (C9H7N).3H3PO4.12MoO3.H2O + 11H2O}$$ When the precipitate is dried, the water of hydration is removed, leaving a stable, anhydrous, yellow product with a molar mass of $\pu{2213 g mol-1}$ (Ref.3).

Since resultant product is a fairly large molecule, the accuracy of the method is well established.

References:

1. H. N. Wilson, "The accurate determination of “phosphoric anhydride” by means of quinoline phosphomolybdate," Analyst 1951, 76(899), 65-76 (ODI: DOI https://doi.org/10.1039/AN9517600065).
2. Lee Alan Shaver, "Determination of Phosphates by the Gravimetric Quimociac Technique," J. Chem. Educ. 2008, 85(8), 1097-1098 (ODI: https://doi.org/10.1021/ed085p1097).
3. Wesley W. Wendlandt and William M. Hoffman, "Thermal Properties of Quinolinium Phosphomolybdate," Anal. Chem. 1960, 32(8), 1011–1012 (DOI: https://doi.org/10.1021/ac60164a035).