Given a solution consisting of disodium hydrogen phosphate + sodium hydroxide (approaching a pH of 12), will the solid formed after dehydration of the solution be hygroscopic at STP? Which salts are likely to precipitate? In the event that the resulting solid is hygroscopic, how might the addition of KCl or NaCl salts to the solution, before dehydration, affect the extent to which the dehydrated solid absorbs moisture if exposed to a humid atmosphere?

  • 1
    $\begingroup$ pKa3 of phosporic asic is 12.67. so there would be significant initial concentration of PO4^3-. While dehydrated pH would increase and more HPO4^2- would get neutralized. $\endgroup$
    – Poutnik
    Commented Aug 23, 2022 at 9:27

1 Answer 1


The solid is made of trisodium phosphate $\ce{Na3PO4}$. Trisodium phosphate has a pH equal to $11.9$ in a $1$% aqueous solution, according to the Merck Index, in article $8736$. If evaporation is made near room temperature, the solid is a dodecahydrate $\ce{Na3PO4·12H2O}$. It is a little hygroscopic. Caking appears after some weeks. It looses $4$ molecules water by heating.

  • $\begingroup$ Thanks, Maurice. If you estimate that caking would occur after some weeks in an open atmosphere. Would you caking could be eliminated if stored in a plastic bottle with an equivalent mass of dessicant? How can we go beyond qualitative estimates of hygroscopicity? $\endgroup$
    – John Chris
    Commented Aug 23, 2022 at 12:58
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    $\begingroup$ Yes. Caking can be eliminated if stored in presence of a desiccant, like blue silica gel. $\endgroup$
    – Maurice
    Commented Aug 23, 2022 at 13:12
  • $\begingroup$ How does the degree of hydration (e.g. the 12 water molecules) of the resulting solid vary as a function of the temperature at which evaporation occurs? What might be a good recommended dehydration temperature? $\endgroup$
    – John Chris
    Commented Sep 4, 2022 at 15:01
  • $\begingroup$ I see that it "Crystallizes with 8 and 12 mols of H2O" $\endgroup$
    – John Chris
    Commented Sep 4, 2022 at 15:04

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