# What is the pKa of iodous acid?

Searching the web readily gives the $$\mathrm{p}K_\mathrm{a}$$ of hydroiodic acid, hypoiodous acid, and iodic acid, however, extensive searching did not result in any value of the $$\mathrm{p}K_\mathrm{a}$$ of iodous acid. So I am wondering if this value has been documented before, and if it hasn't, then what progress has been made to determine its $$\mathrm{p}K_\mathrm{a}$$?

• That's because iodous acid is less stable than the others you mention. Sorry I could not provide a more direct answer, but iodous acid is barely even known. What is known, now, is what you are up against. See en.m.wikipedia.org/wiki/Iodous_acid – Oscar Lanzi May 14 '17 at 19:20
• There is a question posted on the Q&A site CheggStudy that reads: "Determine the pH of a 50 mL solution of a 0.15 M iodous acid (HIO2). The Ka of iodous acid is 2.9×10−5. ". Although the answer to the question is behind a paywall, if we trust the question itself then the pKa of iodous acid is 4.5. This is a somewhat dubious source though. – airhuff May 15 '17 at 18:57

According to Schmitz [1], $$\mathrm{p}K_\mathrm{a}(\ce{HIO2})\approx 6$$ at $$\pu{25 °C}$$ (numerical references for the bibliographical entries have been updated):
Stanisavljev [2] has proposed a correlation between thermodynamic values of oxo-halogen ions and has obtained $$ΔG_\mathrm{f}^\circ (\ce{IO2-}) = \pu{−51.8 kJ mol−1}$$. He has also estimated the dissociation constant of $$\ce{IO2H}$$, $$\mathrm{p}K_\mathrm{a}(\ce{IO2H}) = 4$$, and obtained $$ΔG_\mathrm{f}^\circ (\ce{IO2-}) = \pu{ −75 kJ mol−1}$$. However, this value introduced in the above equation leads to $$K_\mathrm{C}^\circ = \pu{2.8e12}$$ and an impossibly large value of $$k_\mathrm{+C}^\circ$$. His correlation supports the idea that $$k_\mathrm{+C}^\circ$$ is very large but overestimates $$ΔG_\mathrm{f}^\circ (\ce{IO2-})$$ and probably also the dissociation constant of $$\ce{IO2H}$$. Figure 1 shows known $$\mathrm{p}K_\mathrm{a}$$ values of oxy-halogen acids, including the value of $$\mathrm{p}K_\mathrm{a}(\ce{BrO2H})$$ [3] not considered by Stanisavljev, and suggests that $$\mathrm{p}K_\mathrm{a}(\ce{IO2H})$$ is closer to $$6$$ than to $$4$$.
Figure 1. $$\mathrm{p}K_\mathrm{a}$$ values of oxy-halogen compounds $$\ce{XOH}$$ (•) and $$\ce{XO2H}$$ (◦).
3. Faria, R. B.; Epstein, I. R.; Kustin, K. Kinetics of Disproportionation and $$\mathrm{p}K_\mathrm{a}$$ of Bromous Acid. J. Phys. Chem. 1994, 98 (4), 1363–1367. https://doi.org/10.1021/j100055a051.