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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 $\mathrm{p}K_\mathrm{a}$?

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    $\begingroup$ 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 $\endgroup$ – Oscar Lanzi May 14 '17 at 19:20
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    $\begingroup$ 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. $\endgroup$ – airhuff May 15 '17 at 18:57
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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 $4$.

enter image description here

Figure 1. $\mathrm{p}K_\mathrm{a}$ values of oxy-halogen compounds $\ce{XOH}$ (•) and $\ce{XO2H}$ (◦).

References

  1. Schmitz, G. Inorganic Reactions of Iodine(III) in Acidic Solutions and Free Energy of Iodous Acid Formation. International Journal of Chemical Kinetics 2008, 40 (10), 647–652. https://doi.org/10.1002/kin.20344.
  2. Stanisavljev, D. Consideration of the Thermodynamic Stability of Iodine Species in the Bray-Liebhafsky Reaction. Berichte der Bunsengesellschaft für physikalische Chemie 1997, 101 (7), 1036–1039. https://doi.org/10.1002/bbpc.19971010708.
  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.
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