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# Re: why CO2 doesn't affect alkalinity, only pH

• To: Aquatic-Plants at actwin_com
• Subject: Re: why CO2 doesn't affect alkalinity, only pH
• From: "Roger S. Miller" <rgrmill at rt66_com>
• Date: Wed, 17 Feb 1999 11:22:28 -0700 (MST)
• In-Reply-To: <199902170848.DAA23474 at acme_actwin.com>

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On Wed, 17 Feb 1999, A. Inniss wrote:

> Subject: Why doesn't CO2 affect alkalinity, only pH?
>
[snip]
> 	Someone care to re-post dave's explanation or re-explain it?

Hmm.  I don't remember Dr. Dave's explanation.  Maybe he'll be kind enough
to explain it, but I'll take a shot, anyway.

The short answer is that adding CO2 does change alkalinity.  But the
change is proportionatly so small that there isn't much reason to think
about it.

Take the example of a 100 milligrams of CO2 reacting in 250 liters of
water that starts out with a pH of 7 and 4 degrees of alkalinity.  The
carbon dioxide combines with 41 milligrams of water and forms 2.3
milligrams of hydrogen ion and 139 milligrams of bicarbonate.  In 250
liters of water, the reaction adds 0.0092 milligrams/liter (mg/l) of
hydrogen ions (which makes pH) and 0.556 mg/l of bicarbonate (which makes
alkalinity).

The water starts out at pH 7, so it contains about 0.0001 mg/liter of
hydrogen ion; adding 0.0092 mg/l is an enormous change and the pH drops to
about 5.

The water starts out with 4 degrees of alkalinity - about 87 mg/l of
bicarbonate.  Adding 0.556 mg/l is a really small change.  The result is a
0.6% rise in bicarbonate concentration and the alkalinity stays at about 4
degrees of alkalinity.

So the change in the alkalinity (0.556 mg/l bicarbonate) is *way* more
than the change in hydrogen ion content (0.0092 mg/l), but when these
changes are compared to what is normally there in water, the change in
alkalinity disappears into the background and the change in the hydrogen
ion content translates to a big change in pH.

Roger Miller

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