The formula of ammonium phosphomolybdate

Question

It is written in my qualitative chemistry textbook that phosphate could be tested by introducing phosphate, nitric acid and ammonium molybdate which gives (NH₄)₃H₄[P(Mo₂O₇)₆].

I found out in the qualitative chemistry book of professor Nordman Joseph that the yellow precipitate would be widely accepted as (NH₄)₃[P(Mo₃O₁₀)₄].

*Question:

1. Is there any strong evidence for the existence of the precipitate whose formula is given in my textbook?

2. The yellow precipitate could be further reduced to blue complex when added reducing agent like SnCl₂ or FeCl₂. I cannot seem to find any chemical equation which could depict this statement.

Answer 1

The simplest way of writing the formula of ammonia phosphomolybdate is the Brutto formula, $\ce{(NH4)3[PMo12O40]}$. The phosphomolybdate ion is a polyoxometalate but we tend to stick with $\ce{[PMo12O40]^3-}$ for simplicity. Sometimes hydrate/water of crystallization is appended to the formula or the individual components are written separately like below:

• $\ce{(NH4)3PO4.12MoO3.6H2O}$
• $\ce{(NH4)3[PO4(MoO3)12]}$

Older versions had wackier formula probably because they didn't realize the "complex" factor:

• $\ce{12MoO3.(NH4)3PO4.2NHO3 +H2O}$
• $\ce{(NH4)3PO4.10MoO3.3/2H2O}$
• $\ce{(OH)2PO-O-[MoO2-O]11-MoO2(OH)}$

Regarding the reduction of ammonia phosphomolybdate with tin(II) chloride, unfortunately I couldn't find any single step reaction, possible because the product molybdenum blue is very loosely defined and thus there is no concrete formula. It can be presented with a formula $\ce{[(MoO3)154(H2O)70]^{y-3}}$.

References

1. Pharmaceutical Chemical Analysis: Methods for Identification and Limit by Ole Pedersen, CRC Press, 2006
2. Ionic Equilibria in Analytical Chemistry by Jean-Louis Burgot Springer Science & Business Media, 2012
3. A Dictionary of Chemical Solubilities: Inorganic by Arthur Messinger Comey, Macmillan and Company, 1896
4. Analytical Chemistry by Nikolaĭ Menshutkin, Macmillan and Company, 1895