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I know that metal carbonates decompose to a metal oxide and carbon dioxide upon heating and I was curious if phosphates (eg K3PO4) displayed similar behaviour? Or are they like sulphates where some decompose, such as Iron (II) sulphate, while others such as sodium sulphate do not (this behaviour also confuses me). I realise that unlike carbon and sulphur oxides, phosphorus oxide is not a gas, so will this have an impact? Or will it just not decompose for some reason?

So far my online research hadn't resulted in anything substantial, with searching of 'decomposition of sodium phosphate' resulting in academic papers that are very hard to understand (and don't appear to directly answer my question) and a single result that it will decompose to Na + PO4 (AI generated nonsense, clearly wrong).

Thanks

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2 Answers 2

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Metal phosphates containing the $\ce{PO4^{3-}}$ ion are in general thermally stable at temperatures commonly achieved in laboratory and industrial processes. In principle the phosphorus oxide component, $\ce{P4O10}$, is volatile and can be evolved as a gas; but the entropy of this gas evolution is not great enough to balance the large enthalpy release of the acid-base reaction between metal oxides and $\ce{P4O10}$.

Hydrogen-phosphates, where the phosphate ion has more complex stoichiometry, can be decomposed by forming denser polyphosphate salts. Banach and Makara [1] describe the decomposition of sodium hydrogen-phosphate mixtures, $\ce{Na5P3O10}$, involving several stages and ultimately leaving the metaphosphate salt $\ce{NaPO3}$ (in which the phosphate functions are fully polymerized). Decomposition all the way to the metaphosphate requires a temperature of only $320°\text{C}$.

Reference

  1. Marcin Banach and Agnieszka Makara (211). "Thermal Decomposition of Sodium Phosphates". J. Chem. Eng. Data, 56, 7, 3095–3099. https://doi.org/10.1021/je200381z
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In order for a salt to be decomposed by heating it to high temperature, it must produce a gaseous substance by heating. This is the case for carbonates who deliver CO2, and for nitrates who deliver nitrogen oxides at high temperature. But it is not the case for phosphates, as phosphoric oxide is not easily gasified.

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    $\begingroup$ Volatility of $\ce{P4O10}$ itself is not an issue, sublimating at $\pu{360 ^{\circ}C}$. Rather releasing of the oxide from the salt ionic matrix. $\endgroup$
    – Poutnik
    Commented Apr 5, 2023 at 12:51

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