# Chemistry - What does HClO2 aqueous solution decompose to in an open environment?

Per Wikipedia on Chlorous acid to quote:

The pure substance is unstable, disproportionating to hypochlorous acid (Cl oxidation state +1) and chloric acid (Cl oxidation state +5):

$$\ce{2 HClO2 → HClO + HClO3}$$

Also, here are some interesting comments on the decomposition of chlorous acid in "Kinetics and Mechanism of the Decomposition of Chlorous Acid" from J. Phys. Chem. A 2003, 107, pages 6966-6973, to quote:

Of the many mechanistic models tested, the one that fit best included the following reactive intermediates: HOCl, Cl2O2, Cl2O3, •ClO, •OH. The stoichiometric ratio of ClO2 produced to Cl(III) consumed varies with pH and [Cl-]. Reaction of Cl2O3 with Cl(III) yields chlorate exclusively. Reaction of Cl2O3 with Cl- favors ClO2 over chlorate, but does not entirely exclude chlorate, because it is produced by hydrolysis of Cl2O2. Invoking Cl2O3 explains the variation in stoichiometric ratio as well as the maximum observed in the initial rate of ClO2 formation as a function of pH. The kinetics of chlorous acid decomposition cannot be quantitatively fit through the last stages of the reaction without postulating a first-order decomposition. Scission of chlorous acid to give short-lived hydroxyl and chlorine-(II) monoxide is a plausible route for this process [...]

Several groups of investigators[5-7] have found that in the absence of chloride ion the stoichiometry of the decomposition of chlorous acid is given by reaction A:

$$\ce{4 HClO2 -> 2 ClO2 + ClO3- + Cl- + 2 H+ + H2O}$$ (A)

The stoichiometry of the decomposition of chlorous acid in the presence of chloride ion is given by reaction B:

$$\ce{5 HClO2 -> 4 ClO2 + Cl- + H+ + 2 H2O}$$ (B)

Also, to quote:

Earlier studies,[9,18] in agreement with our present results, have also found the formation of more chlorate than predicted from reaction A. Reaction C

$$\ce{3 HClO2 -> 2 ClO3- + Cl- + 3 H+}$$ (C)

also plays a role in determining the stoichiometry at higher $$\ce{HClO2}$$ concentrations."

So, several intermediate products and depending on chloride presence possibly $$\ce{ClO2}$$, which is a problematic explosive and toxic gas, along with the strength of the $$\ce{HClO2}$$, which can introduce chlorate at higher chlorous acid concentrations.

Not a particularly good path to an acid as you are also inquiring about. As a disinfectant, the transient creation of the powerful disinfecting HOCl and associated radicals, may actually make it weaker, albeit more stable (considering end products) than hypochlorous acid, in my opinion.

[EDIT] To answer a comment question, as to whether is ClO2 safe, here is a statement from the CDC, Public Health Statement for Chlorine Dioxide and Chlorite, to quote:

If you are exposed to chlorine dioxide or chlorite, many factors will determine whether you will be harmed. These factors include the dose (how much), the duration (how long), and how you come in contact with them. You must also consider any other chemicals you are exposed to and your age, sex, diet, family traits, lifestyle, and state of health.

Interestingly, ClO2, which is a stable free radical, has found commercial application as an odor removal agent, likely due to its interaction with organics to create volatile organic chlorides (I would suspect that the presence of light would be catalytic). These VOCs are much more of a long-term health danger (as in carcinogenic), in my opinion.