How can I determine the concentration of ammonium dihydrogenphosphate solution?

In this compound, the ammonium cation and the dihydrogen anion are both weak acids.

I have a weak solution of this salt, $C_A\approx 0.05\mathbf{M}$, that I need to quantitatively determine.

My first thought was acid-base titration because it's cheap and easy but the presence of two weak acids complicates things.

Would it be possible to determine the concentration of ammonium and dihydrogen in one equivalence point, as $2\times C_A$?


1 Answer 1


The pH can be measured vs. the amount of $0.1$ M NaOH added to your solution with a pHmeter. Then two titrations would be visible : first the titration of dihydrogenphosphate ($p\ce{K_a = 7.21}$) and then the titration of ammonium ($p\ce{K_a = 9.22}$). If you don't have a pHmeter, you would probably use a colored indicator. But the end of the first titration will not be determined with precision, as the two $p\ce{K_a}$'s are not different enough. You will only be able to determine the end of the titration of the two acids together. To do this, a rather basic indicator is needed, like Alizarine yellow whose $p\ce{K_a = 10.5}$.

  • $\begingroup$ Thank you for your answer which is much along my own thinking. I do have a pH meter and have used it for AB titrations a lot (but rather a long time ago). So that settles it and I'll accept your answer. $\endgroup$
    – Gert
    Commented Jan 12, 2021 at 17:15

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