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I found the following reaction for formation of magnesium pyrophosphate:

$$\ce{H2PO4 + Mg^2+ + NH4 -> Mg(NH4)PO4.6H2O ->[\Delta] Mg2P2O7},$$

but its stoichiometry looks incorrect, as well as some products are missing, which I assume are $\ce{NH4+}$ and $\ce{H2O},$ which the texts do not mention.

Other formation reactions are completely different than this one. Can someone please explain whether this reaction is completely incorrect, or there are only few inaccuracies?

Edits:- I am more confused about the reaction
$\ce{H₂PO₄+ Mg^2+ + NH₄^+ -> Mg(NH)₄PO₄.6H₂O}$

Is this reaction correct or not and why? In some texts and online sources,(not so authentic) I also got the reaction to be
$\ce{NaH₂PO₄+ Mg^2+ + NH₄^+ -> Mg(NH)₄PO₄.6H₂O}$

And in some,

$\ce{MgH₂PO₄+ Mg^2+ + NH₄^+ -> Mg(NH)₄PO₄.6H₂O}$
Please let me know.

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    $\begingroup$ Could you please add at least one source for the aforementioned reaction? Also, are you interested in the most efficient or simple synthesis, or solely in troubleshooting the provided scheme? Feel free to reflect that by editing the question. $\endgroup$
    – andselisk ♦
    Commented Jul 29 at 13:34
  • $\begingroup$ .What is H2PO4? $\endgroup$
    – Nilay Ghosh
    Commented Jul 29 at 15:00
  • $\begingroup$ And what is NH4? $\endgroup$
    – Nilay Ghosh
    Commented Jul 29 at 15:55

2 Answers 2

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The reaction you are referring to is a common salt analysis test for magnesium ion called $\ce{Na2HPO4}$ test. From Vogel Qualitative Analysis, 5th ed.:

$\ce{Na2HPO4}$ test: white crystalline precipitate of magnesium ammonium phosphate $\ce{Mg(NH4)PO4.6H2O}$ in the presence of ammonium chloride (to prevent precipitation of magnesium hydroxide and ammonia solution)

The simplified way of writing the reaction (not involving spectator ions and writing $\ce{NH4Cl}$ as $\ce{NH3 (aq.)}$) is:

$$\ce{Mg^2+ + NH3 + HPO4^{2-} -> Mg(NH4)PO4 \downarrow }$$

If this is decomposed further, you get magnesium pyrophosphate which is well described in Oscar's answer.

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Often in decomposition reactions volatile by-products that come off are ignored, as the material of interest is the calcined product left behind.

Here the plausible volatile products are $\ce{P4O_{10},NH3,H2O}$. But we cannot expect to lose any $\ce{P4O_{10}}$ when the phosphorus/magnesium ratio is unchanged by the thermal decomposition process. (It is actually difficult to displace $\ce{P4O_{10}}$ from phosphates thermally, so this property is an advantage for forming the pyrophosphate.)

That leaves $\ce{NH3}$ and $\ce{H2O}$, and so we render

$\ce{Mg(NH4)PO4\cdot 6H2O -> Mg2P2O7 + NH3 + H2O}$

We can easily balance this equation through atom balances. Placing $2$ before the magesium ammonium phosphate balances both magnesium and phosphorus; then the coefficients on ammonia and water are rendered by balancing nitrogen and oxygen. With all elements in fixed oxidation states the balance on hydrogen is guaranteed.

We end up with

$\ce{2Mg(NH4)PO4\cdot 6H2O -> Mg2P2O7 + 2NH3 + 13H2O}$

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  • $\begingroup$ Thanks for the clarified response. You're absolutely correct about the formation of Magnesium pyrophosphate from Magnesium Ammonium Phosphate. However, I am more confused about the reaction H₂PO₄+ Mg²⁺ + NH₄⁺ ----> Mg(NH)₄PO₄.6H₂O Is this reaction correct or not and why? Please let me know. $\endgroup$
    – Omkar Deshmukh
    Commented Jul 29 at 16:02
  • $\begingroup$ @OmkarDeshmukh First of all, H2PO4 doesn't exist. The reaction you are referring is $\ce{Mg^2+ + NH3 + HPO4^{2-} -> Mg(NH4)PO4}$ $\endgroup$
    – Nilay Ghosh
    Commented Jul 29 at 16:14
  • $\begingroup$ @NilayGhosh Yes now I get it. Thanks. $\endgroup$
    – Omkar Deshmukh
    Commented Jul 29 at 16:46
  • $\begingroup$ @OmkarDeshmukh if this helped you, you can accept either of our answers. You have to click the checkmark beside the answer body. $\endgroup$
    – Nilay Ghosh
    Commented Jul 29 at 17:01
  • $\begingroup$ Producing magnesium pyrophosphate occurs in two steps, with two equations. The fist is the equation given by Nilay Ghosh $$\ce{Mg^{2+} + NH3 + HPO4^{2−} ⟶ Mg(NH4)PO4}$$ The second reaction is produced when heating $\ce{Mg^(NH4)PO4}$ at high temperature. This salt is then transformed into magnesium pyrophosphate according to $$\ce{2 Mg(NH4)PO4 -> Mg2P2O7 + 2 NH3 + H2O}$$ $\endgroup$
    – Maurice
    Commented Jul 29 at 17:01

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