# What is the acid dissociation constant of cacodylic acid?

What is the $\mathrm pK_\mathrm a$ of cacodylic acid? I cann't seem to find one answer on the internet. I have read 1.67, 6.37, and 4.8 in my $\mathrm pK_\mathrm a$ table. What is the reason for this? Also, the problem I just did had it as 0.34.

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Cacodylic acid is amphoteric, so it has a $K_\mathrm a$ and $K_\mathrm b$.

See table 1 of this EPA report on cacodylic acid

The $\mathrm pK_\mathrm a$ is 6.2.

The $\mathrm pK_\mathrm b$ is 12.4, which corresponds to the $\mathrm pK_\mathrm a$ of the conjugate acid being 1.6.

Also, according to "Weed Control" by the US National Research Council at page 167:

Cacodylic acid is normally monobasic, but in strong sodium hydroxide it forms the disodium salt, $\ce{(CH3)2AsO3HNa2}$, of a tribasic acid. Cacodylic acid is somewhat amphoteric, forming a hydrochloride $\ce{(CH3)2AsO2H.HCl}$ by direct action with hydrogen chloride gas.

Also, the value for the $\mathrm pK_\mathrm a$ of the conjugate acid being 1.6 was confirmed by electrophorisis is Arsoniumphospholipid in algae Proc. Natl. Acad. Sci. USA vol. 75 pages 4262–4264.

I found this article (TW. Shin, K. Kim, IJ. Lee, J. Solution Chem. 1997, 26, 379. doi: 10.1007/BF02767677) with precise analysis of the pKa of cacodylic acid - it says that in 25 °C it's estimated to be 6.27. Values lower than 6 are wrong, but found often. Sometimes it's said to be a diprotic (which it isn't) with first pKa 1.57 and second 6.27 - pubchem says 1.57 citing Lide, D.R. (ed.). CRC Handbook of Chemistry and Physics. 76th ed. Boca Raton, FL: CRC Press Inc., 1995-1996., p. 8-45 - it might be one of the sources of problem.