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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 Sep 22 '17 at 0:47
  • $\begingroup$ Because its detection threshold may be above safe exposure limits $\endgroup$ – ManRow Sep 22 '17 at 1:11
  • $\begingroup$ Here's another interesting link to consider blogs.edf.org/health/2017/02/03/perchlorate-risks-from-bleach $\endgroup$ – ManRow Sep 22 '17 at 1:17
  • $\begingroup$ That's perchlorate( $\ce{ClO4-}$ ), the article is talking about. You are interested in chlorate($\ce{ClO3-}$). $\endgroup$ – Nilay Ghosh Sep 22 '17 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 Sep 22 '17 at 10:47
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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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There is a old paper which discuss the mechanism of decomposition of bleach. It says the major product is chlorate($\ce{ClO3-}$)

The decomposition of sodiun hypochlorite has been re-examined. The results show that Foerster and Dolch's mechanism of the decomposition to chlorate and chloride is correct; they postulated a slow bimolecular reaction to chlorite and chloride, followed by a faster reaction of chlorite with more hypochlorite. [...] The rates are such that at 40 C. A solution of sodium hypochlorite will contain about lyo as much chlorite as hypochlorite.[...]. No signs of specific catalytic effects of sodium chloride, hydroxide, or carbonate could be observed, and it seems probable that earlier reports of this were due to variations in ionic strength. The decomposition to chloride and oxygen has been measured and is a unimolecular reaction, which is possibly, but not certainly, uncatalyzed.[...]

$$\ce{2NaOCl -> NaClO2 + NaCl}$$ $$\ce{NaOCl + NaClO2 -> + NaClO3 + NaCl}$$ $$\ce{2OCl- -> 2Cl- + O2}$$

But since this paper is in the 50's, most of the information would be outdated or misleading. So, here is the result.

Wikipedia says:

  • It decomposes (autoxidizes) when heated, forming sodium chlorate and sodium chloride: $$\ce{3 NaOCl(aq) → NaClO3(aq) + 2 NaCl(aq)}$$
  • Sodium hypochlorite decomposes with increasing temperature and under the influence of light and such metals as copper, nickel, or cobalt: $$\ce{2 OCl⁻(aq) → 2 Cl⁻(aq) + O2(g)}$$

There are two pathways of decomposition of bleach. One is forming chlorate and other one is forming chloride. But the 1st reaction is dominant one i.e chlorate is the major product. But the formation of chlorate actually requires heat(to be precise 303-323 K).

The 2nd reaction also requires heat(343 K) and also some catalysts:

Typical metals for nickel and copper are in the range of 50 ppb or less when sub-micron filtration is provided but many times, levels in the range of 300-400 ppb for nickel and 50-60 ppm for copper can be in the product which will create $\ce{O2}$ and increased decomposition.(source)

So, the conclustion is that the formation of chlorate or chloride do require heat. But there are 2 reaction that indeed happens when bleach is remained untouched for a long time i.e reaction with carbon dioxide and moisture.

  • If not properly stored in airtight containers, sodium hypochlorite reacts with carbon dioxide to form sodium carbonate:

    $$\ce{2 NaOCl(aq) + CO2(g) → Na2CO3(aq) + Cl2(g)}$$

  • $$\ce{NaClO + CO2 + H2O → NaHCO3 + HClO}$$

    Sodium hypochlorite react with carbon dioxide and water (moisture) to produce sodium bicarbonate and hypochlorous acid. Sodium hypochlorite - saturated solution. The reaction proceeds at room temperature.(chemiday)

Hypochlorous acid is somewhat unstable so it might decompose to give chlorine fumes or further react with hypochlorite to form chlorate.

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

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    $\begingroup$ ‘But since this paper is in the 50's, most of the information would be outdated or misleading. So, here is the result.’ — scientific papers automatically getting outdated or misleading is a new concept to me. I believe scientific papers only have an expiration date if they are proven wrong by subsequent papers or formally retracted. $\endgroup$ – Jan Sep 22 '17 at 15:39

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