Re: NANFA-L-- Chlorine

Subject: Re: NANFA-L-- Chlorine
From: Joshua L Wiegert (
Date: Mon Nov 29 2004 - 11:56:02 CST

Gah, my telnet server is being cranky and giving me trouble sending out
e-mail... oh well. Most commercial dechlorinators are sodium thiosulfate.
In case you care, it is Na2S2O3. In water, Chlorine Gas reacts with water
to form HOCl, hypochloric acid, and HCl, hydrochloric acid. The
hypochloric acid is bleach and what does the killing -- its a heavy
reducing agent (meaning it wants to lose that O atom) an, as such, it
tends to gain electrons, removing them from other organisms and .. well,
killing them.

Itis essentially the same mechanism as how peroxide works on a cut or such
(though I believe peroxide is an oxidiser, so its the opposite, but...
still the same... honest!) In water, the tiosulfate ionizes to become
2Na+ and S2O3--. By picking up the HO from the hypochloric acid, the S2O3
becomes neutrally charged, and the Cl is liberated. Since the S2O3 has
two negative charges on it, it takes two HOCl's to neutralise it --
resulting in 2 Cls, which quickly bond to become Cl2, the original gas.
The remaining molecules in the tank are 2HO+ and S2O3, which I believe
restructure as 2HO+ + S2O3 + H2O = 2H2SO4 -- sulphuric acid. This is part
of the reason why tanks tend to acidify over time -- though organic
reactions do much more. In fact, if you ever test the pH of your tap
water, add dechlorinator and test it again, you'll find a pH drop --
though it would take a pin point meter to do it. From that change in acid
concentration in the water, you could theoretically figure out how much
chlorine is in your water to begin with -- probably easier to call the
water company and ask! :)

Now, I have a funny feeling I haven't actually answered any of your
questions..... The dose of dechlorinator that you add to your tank is
much, much higher than it needs to be. This is a CYA technique used by
the companiesmanufacturing their products..... If only 50% of your
chlorine was neutralised after use, and your fish died... they could be in
trouble. :) I've often gone on the cheap and used half or even quarter
doses of sodium thio.

Ideally, it doesn't matter where or how the thio is added to the water or
how dilute it is. The reaction is going ot take place. If you don't have
a lot of turbulance, it'll take time. Ideally, the water is well mixed
and the thio mixes through it -- this is another reason why you should
dose the bucket before you pour it into the tank oras you add the water.
However, in the end, it doesn't really matter too much.

Also, I must point out:
HOCl + HCl <--> H2O (Water) + Cl2 (Gas)

Essentially, when you bubble chlorine gas through water, which is what the
water company is doing,, you get those two products as explained above.
But this is a reversable reaction -- given time, the two products will
recombine to form water and Cl2 Gas, which bubbles out of the tank and
leaves. This is why you can leave a bucket of water overnight to remove
the chlorine. The caveat, though, is that without sufficient turbulence,
the rate of the reaction is slowed down -- that CL2 has to reach the
surfac e to leave, or it'll just react with more water and go back.
Boiling not only increases the turbulence, but increases the rate of the
reaction. Boiling water also has the advantage of causing water to lose
its temporary hardness..... It can soften it.

Joshua L. Wiegert
AIM UID: JoshuaWiegert ICQ UIN: 276060292
Feel free to contact me by any of the above means for any reason.
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... Had this been an actual emergency, we would have fled in terror, and
you would not have been informed.

On Mon, 29 Nov 2004, Hoover, Jan J ERDC-EL-MS wrote:

> Date: Mon, 29 Nov 2004 11:17:49 -0600
> From: "Hoover, Jan J ERDC-EL-MS" <>
> Reply-To:
> To: "''" <>
> Subject: NANFA-L-- Chlorine
> Last week, after topping off three large (> 100 gal) tanks with tap water,
> the fish in the first of the three tanks turned belly-up. Fish were saved
> by immediately transferring them to other tanks. While doing water changes,
> we noticed that water coming out of the tap had a stronger-than-usual odor
> of chlorine. The total volume of water added to the tanks was small (< 15
> gal), but we wondered if chlorine could have been the culprit. The incident
> reminded me that I never have learned how dechlorination works. I wondered:
> Are commercially-produced dechlorinators "sensitive" only to variation in
> water volume and not concentration of chlorine ? Is the recommended dosage
> (e.g., 1 drop per gallon, or 1 ml per gal) based on a presumed maximum
> concentration of chlorine used in most municipal water systems ?
> Do dechlorinators effectively neutralize chlorine when tap water is diluted
> with aged water? For example, when making a 20 gal water change to a 60 gal
> tank, should you add 20 gal worth of dechlorinator or 60 gal worth ?
> Please excuse what may be naive questions from a chemistry- and
> physics-impaired individual.

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: Fri Dec 31 2004 - 12:42:55 CST