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RE: Determining CO2 Concentrations in Natural Waters

> From: David A. Youngker
> Sent: Monday, January 28, 2002 6:43 PM

> Speaking of concentration, mine's beginning to fade right
> about now - been _very_ sick the last few days (sorry I
> was off-line, Teresa). And as this is rather detailed, I'll
> pause here momentarily to entertain any questions up
> to this point. We'll continue once this is established.

Up for a short break while my sweat-soaked bedding is changed out - a "water
change" for me ;-)

Back to David's arguments:

> External factors certainly play a point in this
> but to say that CO2 levels are "entirely" determined
> by external factors is just not a scientifically sound
> principal.

Not exactly certain of the point you're trying to make here. The factors
that influence the pH/KH/CO2 relationship are definitely external to the
solution involved, as they are products of the varying topology and geology
the water encounters along its flow. The topology affects the contact time
while the geology provides the range of compounds to which the water is

Ever take a look at the calculations in an Ecology text or site? They
usually provide some kind of qualifying disclaimer, such as "assume there is
no contact with carbonate-based compounds", "does not apply where the water
is in contact with carbonates", etc.. That's so as to provide an
instantaneous "snapshot" of the _current state_ of the water, as any
acid-bearing solution will cause the further dissolution of carbonates,
affecting the calculations. And any water that has carbon dioxide dissolved
into it (essentially anything that contacts the atmosphere) contains an
acid. So we make the current situation "static" in order to limit the
calculations to algebra and not have to deal with calculus (which _does_
describe the results of changes to a dynamic system).

Then there are sources of carbon dioxide to consider as well. Carbon dioxide
is the result of aerobic biological activity, so any water containing
sufficient nutrients - especially carbon-bearing ones - provide the food for
organic activity at the bacterial level if nothing else. Flora and fauna
respiration add to the overall content. Underground activities can be a rich
source, as the pH around a lot of springs will tell you.

> However I am not advocating adding CO2 to a system with
> sufficient CO2 present as can be found in a system with
> a low KH and pH. All my tanks have a 0 KH and I don!&t
> have any so-called pH bounce or crash. I just change my
> water regularly and dispose of the organic compounds that
> would begin to leech acids over time.

The pH "bounce" everyone mentions is normally a high KH phenomenon
associated with trying to change the chemical parameters to the low end -
the pH rarely "bounces" when adding the buffering agent itself. It is caused
by the rebalancing of the carbonic- to- bicarbonate ratios of the

When we add a mineral acid to high KH water, the hydrogen concentration
naturally rises along with the rather _immediate_ dissolution of the acid.
The shift from carbonic to bicarbonates (or vice- versa) takes considerably
more time along the same scale. Dumping a lot of hydrogen into the system is
like hitting a big pothole with your car- an immediate, large input is
dissipated across time through the action of the shock absorber. This gives
the absorber a chance to dissipate the energy through other mechanisms
rather than providing a "straight line jolt".

With that in mind, a large influx of hydrogen will initially show as a large
downward shift in pH because of the number of free hydrogen ions present -
the concentration has increased. But as the bicarbonates are driven into
absorbing hydrogen to become carbonic acid, hydrogen is removed from the
solution and into the solute. Now the pH shows as low because there is a
disproportionate amount of carbon dioxide in the water, directly affecting
the carbonic- to- bicarbonate ratio. Since the CO2 can usually outgas
quicker than it is dissolved, the excess bleeds off to the atmosphere (a
carbon dioxide sink). The result strikes a new balance in relation to
atmospheric levels of CO2 and the now lowered bicarbonate population.

A pH "crash" occurs when there is no more buffer to dissipate the influx of
hydrogen through out gassing. In such a case, the pH is tied directly to the
amount of hydrogen added to the system and we have a "straight count
population". In other words, a direct influence on pH.

As to why your tanks don't "crash" at such low pH/KH combinations, consider
some of the "organic compounds that would begin to leech acids over time".
If you filter with peat or Oak leaves, one of those "organic compounds" is
tannic acid. Hmmm...tannic acid. Acid. Complete with transferable hydronium.
But tannic acid is a much weaker acid that those produced by nitrification,
and is a pretty weak buffer. The consistent water changes will replenish the
buffer while depleting the nitrates, keeping everything on a pretty even

> Most importantly the plants themselves will do a
> great job of maintaining you pH. A healthy tank
> with good plant growth will have a higher pH. Why?
> Because the plants are using up the CO2 and thus
> there is less available to make carbonic acid.

This depends on a few more of those "external factors".

Light intensity is, of course, paramount. Light energy is the initial
impetus to the system, determining the metabolic rates of the plants. More
light or more plants increases the demand for carbon, and it does indeed
remove CO2 from the system. However, in a circulating body of water as small
as a tank, the carbon dioxide can often be maintained at atmospheric levels
just through contact. But the key here is that it _is_ replaced, although
the replenishment may not happen with sufficient speed to keep up with
depletion. Then your pH may indeed go _very_ high as the plants start
cracking bicarbonates to obtain CO2.

But since light energy _is_ the key, that means everything's tied to the
*diurnal cycle*. The plants will drain the CO2 during the day, and the
atmosphere and respiration will replenish it during the night. A 12-hour
"high-low" doesn't sound very stable to me, especially if it's fluctuating
by a couple of points - which is distinctly possible. Carbon dioxide
injection keeps the depletion and replenishment rates fairly consistent
across 24 hours.

> I have yet to see any natural bodies of water with
> CO2 injection...

You're a Floridian - ever go looking for Elosoma (dwarf sunfish)?
Particularly Elosoma evergladei? They frequent soft, acidic springs with
little DOCs and an excellent source of carbon - the gas that bubbles up in
the spring. Check out the types of plants that grow in those headwaters.

Natural CO2 injection...

Well, I've got a fresh set of sheets now, and I feel the bed calling - so
I'll be back in another few hours...


David A. Youngker

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