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How will the reaction of $\ce{H2SO4}$ with ($\ce{NH4)2HPO4}$ [diammonium phosphate, commonly used DAP fertiliser in agriculture] proceed? Will phosphoric acid be produced, or a complex phosphate-sulphate salt would be the result?

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Depending on the molar ratio,

$$\ce{HPO4^2- ->[H+]H2PO4- ->[H+]H3PO4}.$$

Also remember there are no $\ce{H2SO4}$ nor $\ce{(NH4)2HPO4}$ in water solution, but $\ce{,}$ $\ce{HSO4-(aq)}$, $\ce{SO4^2-(aq)}$ , $\ce{NH4+(aq)}$ , $\ce{HPO4^2-(aq)}$, $\ce{H2PO4^-(aq)}$ and $\ce{H3PO4(aq)}$, depending on the ratio and $\mathrm{pH}$.

For direct reaction without water, then

$$\ce{2 H2SO4 + (NH4)2HPO4 -> 2 NH4HSO4 + H3PO4},$$

with some $\ce{(NH4)2SO4}$ too.

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  • $\begingroup$ Thank you, that’s very helpful :). From your explanation, I conclude that if the objective is to get P04 3- (soluble form of phosphate ion in acidic PH for the crop) from DAP, then a higher concentration of H2S04 should be used than an equal moles of both, and the reaction should be carried out in water. $\endgroup$ Nov 18, 2022 at 22:26
  • $\begingroup$ PO4^3- is present only at very alkalic solutions. (pK_a3= 12.37) $\endgroup$
    – Poutnik
    Nov 19, 2022 at 0:43

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