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Re: pH and the uptake of CO2

Steve Amor wrote:

> I can't get 15ppm CO2 in my tank when it's got a pH of 8.2.  Not according
> to the ph vs KH tables anyhow.

That's probably because most pH-KH-CO2 charts and tables have been limited
to what their author regarded as a reasonable range.  You can use formulas
(there's at least one at the Krib) to get beyond that range.  A pH of 8.2
and 15 ppm of CO2 implies 260 degrees alkalinity.  Not likely to happen.

> >  CO2 concentrations
> >are controlled mostly by the amounts of CO2 moving into and out of the
> >aquarium and the amounts being used up or generated in the aquarium.
> >
> > Normally,
> >though the alkalinity isn't that high and a pH of 8.4 implies a pretty
> >low CO2 concentration.
> These last two paragraphs seem to contradict each other.

The first paragraph says that aquariums (this could be stated more
accurately to refer to the water in the aquariums) are open systems with
respect to CO2, so CO2 is usually controlled by environmental conditions.
The second paragraph says that if alkalinity is in the normal range and
the pH is 8.4 then the CO2 is low.  These are not contradictory

> If CO2
> concentration is controlled by the amounts of CO2 moving into and around
> my aquarium, then why does my CO2 concentration go up when I lower the pH,
> when my CO2 injection rate stays the same?

This can happen in a short term because the acid you use to lower the pH
also converts bicarbonate ion to CO2.  That effect stops as soon as the
tank equilibrates.

There are a couple other possibilities...

As I recall, your initial determination that CO2 was low was based on a
CO2 indicator.  I don't really know how those work but I suspect they are
of qualitative value only and that they will produce results only within a
specific range of conditions.  You may well have been outside that range
of conditions.  I know that with alkalinity up to 7 or 8 degrees and the
precision normally available from a pH test kit that you really can't
determine CO2 with much accuracy.

Your second determination was based on results after treating the water
with peat and using water amendments with organic acids.  If you used
enough of those that you reduced your alkalinity to some low value then
(as you might get from Steve Pushak's post in this digest) your alkalinity
reading may not be useful for CO2 determinations.  That's because the
organic acids are measured as alkalinity and are treated in the
determination as if they were bicarbonate.  That creates an erroneously
high result for CO2.

A similar problem effects results if your CO2 test is based on a test for
"acidity", as the organic acids may be incorrectly read as CO2.

> >
> >
> >At low CO2 concentrations and with other things being conducive, some
> >plants will start using bicarbonate as their carbon source, and some will
> >not.  I don't think there is any pH where that change occurs, and
> >certainly not one that's applicable to all plants.
> >
> At a pH of 8.2 my plants do not bubble and generally die back.  When I
> lower the pH (and carbonate hardness as a result of the acids in the water
> conditioner) the plants pick up and stream O2.

I have tanks where healthy plants bubble merrily with the pH well over 8.
In fact, the more they bubble, the higher the pH climbs.  That is normal

> Thus, I can only conclude that the CO2 is in my aquarium all along, but in
> some for that the plants cannot make use of.

Possibly for some plants high pH decreases the availability of some
nutrient in your tank, or the low level of hydrogen ion in solution has an
indirect effect on uptake rates or more directly reduces enzyme
activities.  Any of those could reduce photosynthesis rates to the point
where bubbling isn't evident.  I imagine there are lots of other possible

If you check the archives I think you'll find that there are people on
this list with tap water pH over 8 who can get good CO2 concentrations and
desirably low pH values without adding anything additional to their water.

Roger Miller