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.




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](http://nepis.epa.gov/Exe/ZyNET.exe/910124WK.txt?ZyActionD=ZyDocument&Client=EPA&Index=Prior%20to%201976&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5CZYFILES%5CINDEX%20DATA%5C70THRU75%5CTXT%5C00000018%5C910124WK.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=33)


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](http://www.pnas.org/content/75/9/4262.short) Proc. Natl. Acad. Sci. USA vol. 75 pages 4262–4264.


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I found this article (TW. Shin, K. Kim, IJ. Lee, *J. Solution Chem.* **1997,** *26*, 379. doi: [10.1007/BF02767677](https://doi.org/10.1007/BF02767677)) with precise analysis of the p*K*a 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 p*K*a 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.


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