Does chlorine evaporate or 'gas-off' after 24 hours?

Does chlorine evaporate or gas-off over a 24 hour period?

Chlorine is essentially a gas, so yes it is correct to assume that chlorine will evaporate or 'gas off' if left to sit for a period of 24 hours. This is a technique that is often mentioned by fish keepers when it comes to topping-up a pond whereby they will leave top-up water in a bucket to sit overnight rather than use a hose.

However, tap water nowadays commonly contains both chlorine and chloramine, both used to kill bacteria and harmful to organics. Chloramine will not evaporate and can only be neutralized by using a de-chlorinating product, or tap safe water treatment.

It is recommended that all pond or aquariums that are topped-up or filled using tap water are dechlorinated using a dechlorinator treatment product to remove both chlorine and chloramine.

chlorine in tap water

What is chlorine?

Chlorine is a chemical element with the symbol Cl and atomic number 17.

Chlorine is used as the principle water-cleaning product in swimming pools, and as a disinfectant in sewage processing and tap water treatment. The most common compound of chlorine is sodium chloride, or simple rock salt.

Boiling tap water will remove chlorine, but this is certainly not a method to use if then adding to an aquarium or pond for obvious reasons.

Is chlorine the same as bleach?

Yes, bleach is chlorine. However, supermarket bleach in bottles and the tablets added to pools are not made with the same concentrations of chlorine. The concentration of chlorine in tap water is very low, but enough to keep tap water free from bacteria and algae.

Is both Chlorine and Chloramine present in my tap water?

The most direct way to determine if both chlorine and chloramine is in your tap water is to contact your water company and ask what they use to treat the municipal water supply. By law, they must make the composition of your tap water available to you.

To browse our range of dechlorinators for ponds please click here.

To browse our range of dechlorinating tap safe for aquariums please click here.

Photo credit(s): Canva Pro Licence / Bing Chat AI

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