# Phosphate group acidic or not?

I would like to ask whether or not the "Phosphate Functional Group" itself is acidic. I understand that the compound Phosphoric Acid is very acidic, with several hydrogens. However the Phopshate itself lacks hydrogens to donate. Please clear up any misudnerstandings I am having right now. Thank you so much.

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No, the phosphate functional group, $\ce{P(O)(OR)_3}$, itself is not acidic. Only protic phosphates are acidic. Phosphates that aren't phosphoric acids (i.e. $\ce{P(O)(OH)_{x}(OR)_{3-x}}$) are actually Lewis bases. The carbonyl oxygen is able to coordinate metals and other Lewis acids.
• Right, but doesn't quite go far enough. In aqueous solution the phosphate ion itself is actually basic, not acidic. Thus $$\ce{PO4^{3−} + H2O -> HPO4^{2−} + OH^{-}}$$ – MaxW Jan 29 '16 at 0:30
• Curious but pK2 of phosphate is 7.198 according to Wikipedia which would make the $\ce{HPO3^{2-}}$ slightly basic too. but all the second ionization of all(?) $\ce{R-PO(OH)2}$ compounds seems to be acidic. (I'd wonder if there isn't some weird molecule where the second ionization pKa would be more than 7 in aqueous solution.) – MaxW Jan 29 '16 at 1:19
• I believe you meant $\ce{HPO^{2-}_4}$. – SendersReagent Jan 29 '16 at 1:35
• absolutely. thanks for that correction and it should be $\ce{R-O-PO(OH)2}$. – MaxW Jan 29 '16 at 1:43