# Comparing Acidic Strength of H3PO4 and H3AsO4

The $$\mathrm{p}K_\mathrm{a}$$ values for $$\ce{H3AsO4}$$ and $$\ce{H3PO4}$$ are $$2.30$$ and $$2.12$$ which indicate that $$\ce{H3PO4}$$ is a stronger acid compared to $$\ce{H3AsO4}$$. I looked up their electronegativity values to find that they are almost the same for both arsenic and phosphorus. I feel $$\ce{H3AsO4}$$ should be the stronger acid because arsenic atom is larger in size and the charge is spread out in much more volume which should make the conjugate base more stable. What am I missing out?

• Charge is mostly on oxygens, BTW such a difference as this is negligible . Aug 3, 2018 at 22:09
• When comparing acidic strength for oxoacids with the same number of oxygens, typically the acid with the central atom of higher electronegativity - in this case P, will be more acidic. This is because the electrons are slightly withdrawn from the OH bond as the electronegativity of the central atom increases. As bonding electrons are pulled closer to the OH bond, the molecule becomes more polar, and so the molecule becomes a stronger acid. Aug 3, 2018 at 23:00
• Wikipedia lists the electronegativity of phosphorous as 2.253 and arsenic as 2.211 on the Allen scale. Wikipedia's values for the pKa1, pKa2 and Ka3 of phosphoric acid are 2.148, 7.198 and 12.319. For arsenic acid the values are 2.19, 6.94, and 11.5. Consider the pka2 and pKa3 values I find the electronegativity argument dubious.
– MaxW
Aug 4, 2018 at 0:35

Now consider $$\ce{H3PO4}$$. The structure has one $$\ce{P=O}$$ bond and $$\ce{3 P-OH}$$ bonds.
The conjugate base would be one Hydrogen short from an $$\ce{OH}$$ group. Similarly with $$\ce{H3AsO4}$$. However, the conjugation (resonance) will be better in the case of $$\ce{P=O}$$ bond, since it is an orbital overlap of oxygen's $$\mathrm{2p}$$ and phosphorus' $$\mathrm{3p}$$ as against arsenic's $$\mathrm{4p}$$. $$\mathrm{2p}$$ and $$\mathrm{3p}$$ are of comparable size and account for a better overlap and better resonance, distribution of electronic charge and hence a more stable conjugate base.
Consequently, $$\ce{H3PO4}$$ is better than $$\ce{H3AsO4}$$ as an acid.
• The OP has in mind $\ce{H3AsO4}$, not $\ce{H3AsO3}$. Apr 6, 2019 at 11:55