why use the pKa for H2PO3- not HPO3(2-)?

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why use the pKa for H2PO3- not HPO3(2-)?

Postby Chem_Mod » Sun Aug 21, 2011 12:18 pm

How do we differentiate to use the pKa for potassium dihydrogen phosphate and not the pKa for potassium hydrogen phosphate?

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Re: why use the pKa for H2PO3- not HPO3(2-)?

Postby Chem_Mod » Sun Aug 21, 2011 12:18 pm

Use the Henderson-Hasselbach equation, pH = pKa + log([base]/[acid]). In the case of these species, H2PO3- + H2O -> HPO3(2-) + H30+, thus the dihydrogen species is the acid, and the monohydrogen specie is the base. Given that, in the Henderson-Hasselbach equation the pKa of dihydrogen phosphate must be used.


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