Ask A Scientist General Science Archive

name         Chen
status       student
age          20s

Question -   Dear sir,
I am told that CHLORINE and CHLORAMINES are present in the tap water in my area. Unfortunately 
these chemicals are not kind to aquarium fish. Many aquarium keepers (myself included) resort to 
the use of chemical dechlorinators (cf. physical removal) such as sodium thiosulphate.

I have read that a by-product of the dechlorination is ammonia (also bad for fish). In order to 
understand what happens precisely, I would like to know the exact formula for the chemical 
reaction that occurs when sodium thiosulphate is used to rid water of chlorine and chloramines.

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The chemical reaction of thiosulfate and chlorine is:
2S2O3(-2) + Cl2 ---> 2Cl(-1) + S4O6(-2).
There are other reactions going on also, because the CL2 is in equilibrium with OCl(-1) the 
hypochlorite anion. The chloramines result from the reaction of ammonia and chlorine and/or 
hypochlorite. They will be formed from ammonia generated in the fish tank
that come from waste products from the fish. Chloramines are very toxic, that is why there are 
warnings on bottles of household ammonia and/or chlorine bleach not to mix the two.
Be aware that dechlorinating chemicals sold by pool supply stores use sodium sulfite (Na2SO3) as 
the active ingredient the reaction product is sulfate [SO4](-2).
A simple and effective way to remove chlorine and chloramines from tap water is to boil it for 5 
- 10 min. No chemicals needed. Be sure to let the water cool and stir vigorously because the 
boiling will also deplete oxygen from the boiled water.
Invest in a chlorine monitoring kit available at any pool supply store. It is sensitive and 
reliable.
If you want to monitor the chlorine present in your tank and/or water reserve.

Vince Calder
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