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I have some unopened Clorox bleach bought over a year ago (at $8.25~\%\ \ce{NaOCl}$) and read on Wikipedia that bleach naturally decomposes according to

$$\ce{3 NaOCl (aq) -> NaClO3 (aq) + 2 NaCl(aq)}$$

Now, while there potentially might be some further decomposition as

$$\ce{2 NaClO3 (aq) -> 2 NaCl (aq) + 3 O2 (g)}$$

Would I be at risk of filling the air with toxic chlorate intermediates after using this bleach to clean up an ultrasonic ("cool mist") humidifier?

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    $\begingroup$ And why do you think chlorate would be any more dangerous then NaClO ? $\endgroup$
    – Mithoron
    Commented Sep 22, 2017 at 0:47
  • $\begingroup$ Because its detection threshold may be above safe exposure limits $\endgroup$
    – ManRow
    Commented Sep 22, 2017 at 1:11
  • $\begingroup$ Here's another interesting link to consider blogs.edf.org/health/2017/02/03/perchlorate-risks-from-bleach $\endgroup$
    – ManRow
    Commented Sep 22, 2017 at 1:17
  • $\begingroup$ That's perchlorate( $\ce{ClO4-}$ ), the article is talking about. You are interested in chlorate($\ce{ClO3-}$). $\endgroup$
    – Nilay Ghosh
    Commented Sep 22, 2017 at 2:56
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    $\begingroup$ To put it simply: old bleach does not contain NaClO, hence it is not useful for cleaning. Do not use it. $\endgroup$
    – vapid
    Commented Sep 22, 2017 at 10:47

2 Answers 2

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You have outlined the principal decomposition path of hypochlorite in the question. Chlorate in itself is rather stable under ambient conditions and would probably not decompose.

The greatest danger is if the bleach solution is no longer alkaline. If acidified, both hypochlorite and chloride as well as chlorate and chloride can comproportionate to give chlorine gas according to equations $(1)$ and $(2)$.

$$\begin{align}\ce{Cl- + OCl- + 2 H+ &-> Cl2 + H2O}\tag{1}\\ \ce{ClO3- + 5 Cl- + 6 H+ &-> 3 Cl2 + 3 H2O}\tag{2}\end{align}$$

This can happen if carbon dioxide is absorbed from the air by hydroxide according to $(3)$.

$$\ce{CO2 + OH- -> HCO3-}\tag{3}$$

However, you should be able to smell chlorine early enough if it starts to get dangerous. If upon opening the chlorine smell is stronger than usual, open windows immediately and leave the room.

All other decomposition products should not cause any great harm. However, chlorate’s oxidative ability is less than hypochlorite’s to the best of my knowledge, so you might find that the partially decomposed bleach does not clean as well as the fresh one.

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Bleach decomposition can proceed with two pathways i.e. one is forming chlorate and other one is forming chloride.

$$ \begin{align} \ce{3NaOCl -> 2NaCl + NaClO3} & \tag{R1}\\ \ce{2NaOCl -> 2NaCl + O2} & \tag{R2}\\ \end{align} $$

R1 is dominant although both reaction requires heat and pH control.

These two decomposition reactions of $\ce{NaClO}$ solutions are maximized at pH around 6. The chlorate-producing reaction predominates at pH above 6, while the oxygen one becomes significant below that. For example, at 80 °C, with $\ce{NaOCl}$ and $\ce{NaCl}$ concentrations of 80 mM, and pH 6–6.5, the chlorate is produced with ∼95% efficiency. The oxygen pathway predominates at pH 10. This decomposition is affected by light and metal ion catalysts such as copper, nickel, cobalt, and iridium. Catalysts like sodium dichromate $\ce{Na2Cr2O7}$ and sodium molybdate $\ce{Na2MoO4}$ may be added industrially to reduce the oxygen pathway, but a report claims that only the latter is effective.

Point to note that bleach can react with carbon dioxide and moisture if it left untouched for a long time. Hypochlorous acid that is created is somewhat unstable and it might decompose to give chlorine fumes or further react with hypochlorite to form chlorate.

$$\ce{OCl- + 2HOCl -> ClO3- + 2HCl}$$

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