Re: NANFA-- Neutralizing Chlorine

Bruce Stallsmith (
Fri, 01 Jun 2001 12:00:16 -0400

>In a message dated 6/1/01 11:13:18 AM Eastern Daylight Time,
> writes:
><< Most of them arise in acidic waters, in which the
> mass balance will shift to nasty NH3 from the relatively benign NH4+ >>
>Bruce I think you got that backwards, NH4 occures in acidic water and NH3
>alkaline. Many years of marine Aquariums has taught me this.
I was in doubt for a second, this kind of buffer chemistry can be
bewildering. But looking at the ugly numbers, I remembered correctly about
more ammonia at lower pH:

There's always confusion about the relationship between ammonia and
ammonium and the pH. The equilibrium that interests us
NH4+ <-> NH3 + H+

which has an equlibrium constant (Ka for the ammonium ion) of
about 5.89 x 10^-10. i.e., if the concentrations of everything
are expressed in moles/L, then:

[NH3][H+]/[NH4+] = 5.89 x 10^-10

in water at 25C

[something] = concentration of something

It follows from this that the concentrations of ammonia and
ammonium are equal when the hydrogen ion concentration is equal to
the Ka, or that the pH is equal to the pKa (take the log of the number
and change the sign). The pKa is 9.23, so the pH for equal ammonia
and ammonium is 9.23. The ratio of ammonia to ammonium will change
by a factor of 10 with every unit change in the pH. The lower the pH,
the more ammonium there will be.
To find the ratio of ammonium to ammonia, subtract the pH from
9.23, and get the inverse log of the result. As an example: at pH 7.23
the result of the subtraction is 2.0, and the ratio is thus 10^2.0,
which is 100.
There is _no_ magic cutoff number at either end, though of
course at pH 3.23, the ratio is 1000000!
(with thanks to Paul Sears of Ottawa...)

--Bruce Stallsmith
Huntsville, AL

Get your FREE download of MSN Explorer at

/"Unless stated otherwise, comments made on this list do not necessarily
/ reflect the beliefs or goals of the North American Native Fishes
/ Association"
/ This is the discussion list of the North American Native Fishes Association
/ To subscribe, unsubscribe, or get help, send the word
/ subscribe, unsubscribe, or help in the body (not subject) of an email to
/ For a digest version, send the command to
/ instead.
/ For more information about NANFA, visit our web page,