EC5 - Sodium Hypochlorite (Bleach) chemistry

It’s been a little while since the last entry in the Everyday Compounds series, so today’s post takes a look at Sodium Hypochlorite. This chemical is likely to be found in several cleaning products in your kitchen, and additionally is one of the main compounds used to chlorinate the water in swimming pools. Here’s a look at the chemistry behind these uses, and the potential dangers.

Sodium hypochlorite is a solid white powder, but is more commonly used dissolved in water. Solutions of sodium hypochlorite are commonly referred to as bleach, although household bleach also contains small amounts of several other compounds, including sodium hydroxide and calcium hypochlorite. Sodium hypochlorite generally makes up 3-8% of the volume; dissolved in water, it has a strongly alkaline pH, which can irritate the skin. The idea of strong acids causing burns is common knowledge, but in fact, strong alkalis can be just as dangerous, and concentrated bleach is at a high enough pH to cause burns to the skin on contact.

Sodium hypochlorite dissolved in water also forms hypochlorous acid, HOCl, a weak acid but strong oxidising agent which is responsible for bleach’s bleaching effect. Hypochlorous acid can react with dyes in clothes, breaking bonds and preventing the molecule from absorbing visible light. It also has antimicrobial activity, as it can react with proteins and DNA of bacteria, as well as breaking down their cell membranes. A very low concentration of hypochlorous acid is required to achieve this effect.

You’re probably aware that it’s common advice not to mix household cleaning products, due to the potentially dangerous reactions that can take place. As an example, some toilet cleaners may contain hydrochloric acid. If these are mixed with bleach, it can react with sodium hypochlorite, and form toxic chlorine gas. Even mixing small amounts of these cleaners can result in the production of a volume of chlorine gas, a lung irritant, above safe levels. Ammonia containing cleaning products can also react with bleach to produce toxic chloroamines.

As a disinfectant, sodium hypochlorite also finds use in swimming pools. Although you might quite logically assume that chlorine is used to chlorinate swimming pools, it’s not a very practical choice as it’s difficult to handle and toxic. Therefore, sodium hypochlorite (or calcium hypochlorite) is commonly used instead. It forms hypochlorous acid, which as previously noted has antimicrobial properties. Its presence also has some interesting chemical consequences as a result of any swimmers who decide to relieve their bladders in the pool, an action which even Michael Phelps has happily confessed to.

As it turns out, the uric acid in human urea can react with chlorinated water, and via a series of reactions can produce cyanogen chloride (CNCl) and trichloroamine (NCl3). Both of these chemicals, particularly cyanogen chloride, are pretty toxic, with effects of exposure including coughing, convulsions, and vomiting. However, before you vow never to set foot in a public swimming pool again, it’s worth examining the production and concentration of these chemicals in a little more detail.

The author of the 2014 study in which these findings were published freely admits that the levels they detected in swimming pool water are nowhere near the levels at which they would be deadly, or even at a level that could be conclusively considered harmful. To emphasise that there’s little cause for concern, Ars Technica ran an article in which they considered just how much urine would need to be added to a pool in order to produce deadly concentrations of cyanogen chloride. They state:

“As it turns out, the concentration of uric acid in pee is, to our calculation, about 112 times that of the uric acid concentration used in the experiment. If we could assume a proportional yield of cyanogen chloride just from using more uric acid, we could actually achieve toxic levels of cyanogen chloride for an Olympic pool of 10mg/L chlorinated water… for an equivalent quantity of urine. That means if each person is peeing 0.8L of the highly concentrated urine, their entire day’s yield, into this pool, you’d need about three million people peeing in that pool. If you could get at that pool without dying of either suffocation or drowning in other people’s urine, you could probably pull off death by cyanogen chloride poisoning or at least a pretty good coma.”

They also go on to conclude that, actually, for all of the uric acid in all of that urine to be converted into cyanogen chloride, you’d need a much higher level of chlorination in the water – the effective equivalent of half a litre of chlorine per litre of water. So, if you plan on going swimming in a pool so chlorinated it’d probably kill you from that alone, and there are also three million people lining up to urinate in it, then yes, urinating in the pool is certainly going to be harmful. However, in your local pool, you can probably rest easy.



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References & Further Reading

13 CommentsClose Comments


  • Под тягой
    Posted July 6, 2014 at 8:20 pm 0Likes

    The thing about that cyanogen article is the following: the authors (probably, to make their study more important) say that the swimming pool is a badly mixed system. Due to this fact, high local exposure to dangerous chemicals can occur. As for me, I don’t take this seriously (I’m a chemist, yep) but I didn’t like their emphasis.

    • Compound Interest
      Posted July 6, 2014 at 11:24 pm 0Likes

      I’ll have to look further into the studies that they reference in that section. For me, even if it’s the case that they aren’t well mixed systems, I’m still not convinced that that the concentration of the products in certain areas would be affected to such an extent that they exceed the exposure limits. Surely they’d need to carry out further experimental work on that somehow in order to prove the claim.

      So I’m going to have to join you in your cynicism and say I’m positive that they over-emphasised the potential effects in order to get their study more exposure 🙂

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  • Enrico Uva
    Posted June 27, 2015 at 10:26 am 0Likes

    Even in small concentrations, nitrogen trichloride can cause problems. In trying to fight chemophobia, we can’t fall into the trap of constantly arguing that dilution dispels all concerns. See

    • Compound Interest
      Posted June 27, 2015 at 9:36 pm 0Likes

      Completely agree that we shouldn’t understate any risks. However it’s worth noting that the study you linked relied on self-reported data, and the only statistically significant correlation was found between pool workers and increased fungal infections. Does this conclusively link it to them experiencing increased concentrations of chlorination byproducts? I’m not too sure it does.

      With that said, there’s an interesting study on respiratory symptoms as a consequence of chloroamine exposure here (, though again it’s self-reported data from pool workers. However, it does seem to show some symptoms were significantly correlated with chloroamine levels.

      The WHO has stated a recommended airborne value for trichloroamine of 0.5mg/m3. Most studies that report links to respiratory symptoms seem to do so at levels above this, so I suppose then the concern turns to how we can ensure that trichloroamine levels remain below the recommended guidelines.

      • Enrico Uva
        Posted June 27, 2015 at 10:15 pm 0Likes

        Any odds ratio above 1.4 is statistically significant, as the authors pointed out, and if you look at the data, it wasn’t just the fungal infections that were above 1.4. Also,those who were both pool attendants and instructors experienced more problems, which is not likely a coincidence, given that through their dual role they would be more exposed to NCl3. And keep in mind that it was also self-reported data from nurse’s diets that led to the trans fat ban several years ago. Finally, recommended guidelines from WHO or EPA or not always based on rigorous research, so they could be either too stringent or, as in this case, possibly too lax since the average of 0.66 mg/m^3 they measured is pretty close to the WHO limit.

        • Compound Interest
          Posted June 27, 2015 at 11:15 pm 0Likes

          Ah, apologies, I clearly misread the initial paragraph which I first thought implied that it was the only statistically significant overall adverse effect. I see now it’s referring just to cutaneous symptoms at that point.

          If using p values to determine statistical significance, many have a p value of over 0.05. Mycosis, voice loss, runny nose and cold are the ones they found to be statistically significant, unless I’m mistaken?

          I’d agree that the WHO limit seems a little high based on the studies out there. I wonder if they’ve any plans to reconsider it in the near future?

  • John Monahan
    Posted August 15, 2015 at 4:53 pm 0Likes

    In swimming pools it is urea rather than uric acid that comes from people peeing in the pool. As “in the uric acid in human urea” should be “urea in human urine”. Human urine contains lot of urea and very little uric acid. Uric acid is very insoluble (It’s uric acid crystals that cause gout).

    • Compound Interest
      Posted August 16, 2015 at 12:22 pm 0Likes

      Whilst you are, of course, correct that the level of urea in urine is much higher than that of uric acid, uric acid is still present at low levels in urine. The study referenced in the text refers to chlorination of uric acid, so this isn’t a mistake. The study in question can be found here:

      • John Monahan
        Posted August 26, 2015 at 3:47 am 0Likes

        wow, I stand corrected. I didn’t expect uric acid to break down to cyanogen chloride in pools!
        I was thinking about the pungent pool smell which is mostly from urea breaking down to produce trichloroamine.

        However, there is still a typo in the article “the uric acid in human urea” should be “the uric acid in urine”

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