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Ammonia vs. Ammonium being Ph dependent

> Date: Thu, 10 Dec 1998 19:37:10 -0500
> From: Romano <romano at pathway1_pathcom.com>
> Subject: Ammonia vs. Ammonium being Ph dependent
> Karen Randall has taken exception to my book based understanding of the
> nitrogen cycle, whereby I have understood ammonium to be the form that
> nitrogen takes in an aquarium when the Ph is under 7 and that it exists as
> more toxic ammonia in a Ph over 7. When cycling a new tank in the past I
> have strived for a lower PH for just that reason. 

That's a noble purpose, and none here would probably disagree.

> If this information is
> erroneous I once again invite the chemists to comment. I have encountered
> this information from more than one book and the writers have been doing us
> all a great diservice should this be proven wrong.

The amount of hogwash in the aquarium literature is utterly mind
boggling! Many still warn us about the iodides in salt, if you can
believe it! [AFAIK, none point out the silicate problem in most table
salt, whether iodized or not.]

Either they, or you, are greatly oversimplifying the chemistry, here. 

The amount of ammonia becomes vanishingly small at low pH. True. 

At higher pH, there is still usually *mostly* ammonium, but the
highly-reactive un-ionized ammonia proportion is just greater. For
example, 64% will still be ammonium at 20C and pH=9! If the total
ammonia/um is 0.03 ppm, that 36% can be quite lethal in a Tanganican or
reef tank. It wouldn't even be noticeable at pH=6.

Note that your test kits are essentially worthless at such levels. 0.01
ppm ammonia is known to be irreversibly harmful to fish gills (see
Spotte, _Fish and Invertibrate Culture_).

Lifting a few numbers from Spotte (1970 ed., p 104), the percentage as
ammonia at 20C is:

 pH            %ammonia
7.0              0.5
7.5              1.5
8.0              4.7
8.5             13.7
9.0             35.8
9.5             62.1

I'll leave it to you to draw the curve and fill in the missing regions.
The ammonium is just the above %s subtracted from 100. Note that 50:50
(where ammonia becomes greater than ammonium) is well above pH=9 and
*not* at pH=7. That doesn't mean that pH=7 isn't about 10 times safer
than pH=8, tho. Look at the numbers.


PS. These numbers are all stable equilibria. I suspect the more-reactive
ammonia is preferentially grabbed by the plants, so the presence of
plants might tilt things into an even safer (for fish) direction,
depending on time constants. IDK if this has been documented. Good
science-fair project, if not!

Wright Huntley, Fremont CA, USA, 510 494-8679  huntley1 at home dot com
          *********   Eschew obfuscation   **********