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I have searched the web pretty much all day because I want to work out what the pH will be of various salts when they are in water solution. I have searched for references related to pKa and pH, but I find nothing. Is it really true that there doesn't exist a database anywhere in the world were these numbers have been collected? The salts I'm looking for in my case are polyphosphates; dipotassium dihydrogenpyrophosphate and tetrapotassium pyrophosphate for example.

If you can provide a reference for literature/database/table that is sufficiently large that it contain common salts at the same time as these compounds, I will accept it as the answer. If you can provide any other reference that in some way may be used to calculate the pH for the same compounds, that is helpful too.

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Pyrophosphoric acid is unstable in water, hydrolyzing at the $\ce{P-O-P}$ bond, but only slowly without catalysis or acidity.

$$\ce{K2H2P2O7 + H2O -> 2H2KPO4}$$ $$\ce{K4P2O7 + H2O -> 2 HK2PO4}$$

pH would be based on the concentration of the hydrolyzed forms, buffering at their respective pKa's.

A 1% solution of tetrasodium pyrophosphate has a pH of 10.3 as compared to pH 11.9 of a $\ce{Na3PO4}$ 1% solution.

Apparently, the $\ce{P-O-P}$ linkage reduces the basicity of the $\ce{O- Na+}$ in the pyrophosphate compared with the phosphate.

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  • $\begingroup$ It is better to write $\ce{KH2PO4}$ since it is ionic $\ce{K+H2PO4-}$. I left for you to edit that. $\endgroup$
    – Nilay Ghosh
    Oct 20, 2022 at 11:30

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