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We had a malfunction with the chlorination system at our pool. There was a reaction between the Calcium Hypochlorite tablets and either $\ce{CO2}$, Carbonic Acid, or Muriatic Acid. There was a definite malfunction that led undetermined amounts of $\ce{CO2}$ (or a mixture of that gas dissolved into water, carbonic acid?), into the tank where hypochlorite tablets are dissolved. The tablets became very warm, a yellow/greenish gas was observed, and there was an extreme chlorine type smell that was easily observable from at a hundred feet away. We are talking $\pu{150 lbs}$ of calcium hypochlorite tablets. Is it possible for the $\ce{CO2}$ to react this violently with the hypochlorite? Or, is it more likely someone poured some muriatic acid in the water tank or on the tablets?

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  • $\begingroup$ Just for more clarification... this was at a public pool. The two operators were out of town. The tablets (not powder) are in a hopper. The hopper sits on top of a tank which sprays a small amount of water up at the tablets to dissolve them. There is supposed to be 50 lbs of tablets in the hopper but there was more like 150 (lots of fuel). The "yellow/green" vapor that escaped from the lid when opened was witnessed by several people (not me) The FD was eventually called as the fumes continued to increase. Next morning the tablets were warm and the plastic tray they sit in was deformed. $\endgroup$ – user82195 Aug 28 '19 at 4:00
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It is almost certain that the gas you saw was chlorine (Cl2). It is a VERY TOXIC gas, so stay away!!

Hypochlorite releases chorine gas when pH is too low. That is why bleach must not be mixed with acid. Never!

In this case it is probably Muratic (hydrochloric) acid that has been added to the hypochlorite in excess. The acid is used to adjust the pH in the pool water, because hypochlorite in itself will raise pH to an unpleasant level. It is unfortunately a fairly common accident seen in chlorination systems.

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  • $\begingroup$ Thanks for this explanation. $\endgroup$ – user82195 Aug 28 '19 at 4:00
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Note that chlorine can be created just by adding water.

$$\ce{Cl- + ClO- + H2O <=> Cl2 ^ + 2 OH-}$$


Enough $\ce{CO2}$ dissolved in wet tablets of calcium hypochlorite ( mixed calcium hypochlorite, chloride and hydroxide) would speed up the above, as it would release hypochlorous acid $\ce{HClO}$, which is weaker than carbonic acid.

$\ce{HClO}$, then oxidizes chlorides to free chlorine. The summary reaction is like this:

$$\ce{ CO2 + Ca^2+ + Cl- + ClO- -> Cl2 ^ + CaCO3 v}$$


The reaction with hydrochloric ( obsoletely muriatic) acid $\ce{HCl}$, would be the most dramatic, if it was present :

$$\ce{ 2 H+ + Cl- + ClO- -> Cl2 ^ + H2O}$$

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  • $\begingroup$ Thanks for this explanation. $\endgroup$ – user82195 Aug 28 '19 at 4:00
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Calcium hypochlorite is supposed to be added to water. If a little water is added to the hypochlorite, it can start to oxidize and produce stronger chloric acid. The SDS says to avoid water and moisture. To test this, try adding just a few drops of water to a pile of powder or a pill and see if a reaction begins.

Unless you have a vandal, it is not likely that someone would be pouring acid into your hypochlorite. There might be a possibility of moisture getting in thru a small splash of rain or condensation from temperature changes and high humidity; tight closure of containers is advised.

Here's a link to a video where other additives are added. https://www.youtube.com/watch?v=RZOl48Ppanw

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  • $\begingroup$ Thank you, interesting video. $\endgroup$ – user82195 Aug 28 '19 at 4:01

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