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Re: Nitric & Sulfuric acid
> From: spush at saudan_HAC.COM
> Date: Thu, 06 Feb 1997 16:25:04 PST
>
> I was wondering if nitric acid is a biproduct of the nitrification
> cycle or other chemical processes in the aquarium?
Yes, the nitrification process is equivalent to titrating with nitric
acid. The result is hydrogen ions (H+) and nitrate (NO3--) (and other
stuff, of course). Carbonate hardness is slowly used up in the
process, reducing buffering and pH. When the KH is completely
depleted, you will find that the pH will stabilize around pH 4.4
(given typical natural conditions).
Here's the chemistry involved (not precise, but good enough for a
general discussion; I've been told there are a few electrons missing
here and there).
| From:
| Water Chemistry in Closed System Aquariums
| Alfred J. Gianascol, 1987
|
| -----------------------------------------------------------------
| Prior to nitrification, ammonification of protein occurs as follows
| using an emprical formula for algal protoplasm:
|
| C H O N P + 118 O --> 106 CO + 65 H O + 16 NH + H PO
| 106 181 45 16 2 2 2 3 3 4
|
| Sulfur is omitted because of its low concentration. Note the production
| of both ammonia and phosphate.
|
| Nitrification follows. C H NO represents the composition of nitrifying
| bacteria. 5 7 2
|
| + -
| (NH ) + 1.83 O + 1.98 (HCO ) -->
| 4 2 3
| -
| 0.021 C H NO + 1.041 H O + 0.98 (NO ) + 1.88 H CO
| 5 7 2 2 3 2 3
|
|
| Omiting the negligible effect of the growth of the bacteria:
|
| + - -
| (NH ) + 2 O + 2 (HCO ) --> (NO ) + H CO + H O
| 4 2 3 3 2 3 2
|
| Essentially, nitrification has the same effect as titrating water with
| nitric acid (bicarbonate is used up and nitrate is generated).
|
| -------------------------------------------------------
| Denitrification:
|
| This requires an organic carbon source such as methanol, but the
| organics in the water from carbohydrate and fat decomposition may
| serve as the carbon source.
|
| -
| (NO ) + 1.08 CH OH + 0.24 H CO -->
| 3 3 2 3
| -
| 0.056 C H NO + 0.47 N + 1.68 H O + 0.056 (HCO )
| 5 7 2 2 2 3
|
| Omitting bacterial synthesis:
|
| - -
| (NO ) + 0.833 CH OH + 0.167 H CO --> 0.5 N + 1.33 H O + (HCO )
| 3 3 2 3 2 2 3
|
|
| Note that in nitrification, nearly 2 mEq of bicarbonate are consumed
| as 1 mEq of ammonium is oxidized to 1 mEq of nitrate while in
| nitrification, only 1 mEq of bicarbonate is produced by reducing 1 mEq
| of nitrate. On balance, a progressive loss of alkali and an increase
| in carbon dioxide will result unless some means are taken to correct
| this.
> The pH in my aquariums is quite low; lower than can be accounted for
> by CO2 injection. I wondered if this was due to humic acids or from
> the nitrification process. A much stronger acid, sulfuric acid, could
> that also result from the decay of organic material?
I would vote for nitrification. I don't think the prcesses in the
aquarium equate to titrating with sulphuric acid. Any sulphates that
are produced are generally benign. Note that Gianascol ignores
sulphur due to its low concentration. This is true unless your
system is seriously screwed up.
> How does one obtain long term pH stability in those aquarium systems
> where water is not changed regularly?
The first suggestion is to change water regularly.
The next suggestion is to monitor KH and replace that which is used up
by nitrification. A good target is 3 dKH (~50 mg/l CaCO3 equivalent).
George in Loveland, Colorado
- where you should always look *both* ways when crossing a one-way street