Does CO2 injection work?

On Fri, 27 Jun 2003 00:38:52 UTC, "floguru" wrote:

Forgive me for being controversial but I have drawn the following
conclusions on CO2 injection.

What's wrong with controversy?

There have been a number of replies to this, but I'm going to toss in one
more, concentrating on what really happens to CO2 and carbonates in water,
which is tricky and misleading unless you look at it right, which no
aquarium book I've ever seen tells how to do.

I, of course, have it exactly right. Ahem.


CO2 injection I can only summise has one achievement, to increase the
acidity of an aquarium ...

If you want to hypersaturate your aquarium with CO2 a readily available
solution would be to pour in a bottle of soda water which is just water
hypersaturated with CO2 gas. The only thing is that pH would be extremely
low (never measured it but probably less than 4).

Yes, around 4. Methyl orange, which switches at 3.7, is used as an
indicator when you don't want to see the effects of carbonic acid, but
just stronger acids.

But "hypersaturate" sounds like a misconception of what's happening when
CO2 is injected. See below.


I haven't done the experiments (but I might) having an interest in creating
huge ocean algal blooms in the ocean to suck up some of the excess CO2 we
have injected into our environment.
I would be interested in wheither anyone has actually measured an increase
in dissolved CO2 before and after injection and the corresponding effect on
pH.

It's a challenge that probably no one will meet *directly*. I don't even
know how you'd go about directly measuring dissolved CO2, though I've done
a lot of carbonate measurements, as well as direct measurement of CO2 in
air. Fortunately, you can leave the direct measurements to the chemists
who create tables of chemical constants, and get the dissolved CO2 level
by simple calculations.

The research chemists have worked out the relations of all the forms of
"total carbonate" listed here. (This may be obvious, but bear with me.)

Carbon dioxide, CO2, dissolved in water (what you want, and what our
plants want)

Carbonic acid, H2CO3, in water

Bicarbonate ion, HCO3-, in water

Carbonate ion, CO3--, in water.


All of these are present if any of them is. The _relative_ amounts depend
on the pH in a simple way -- except for the ratio of dissolved CO2 to
H2CO3, which is a constant.

If you know the pH, then you can calculate the relative amounts of all
these forms of carbonate. A simple matter of simultaneous equations. If
you know the total amount, then you can figure out how much of each one.
If you change the pH by a small amount, they all shift.

So this is what we folks with CO2 injection do. Rather, here's what I do
with the water that comes out of my tap; others get different water and
handle it in different ways. I take tap water with a pH in the high 7's
and around 60 parts per million of total carbonate in all forms. I can
calculate how much free CO2 there is, with the available tables; 2 parts
per million or less, and not enough for good plant growth. I bubble CO2
into it till it's down to pH 7; and I add some kind of carbonate till
there are 100 parts per million of total carbonate, and bubble CO2 in to
hold it to pH 7. (A couple of teaspoons of baking soda in a 55-gallon
tank, as it happens.) I look up the amount of free CO2 in the tables,
and wow, I have 14+ ppm of dissolved CO2, which is good for plant growth
(Actually, growth is very good even at 4/5 that level.)

I could do the whole calculation using the good presentation at
http://www.chem.usu.edu/faculty/sbia...ate/Carbonic%2
0Acid.html
But it's easier to let others do it, and read the color-coded chart at
http://www.sfbaaps.com/reference/table_01.shtml
which has total carbonate in units of about 20 ppm, for historical
reasons.

This should make it fairly clear why you don't need soda water to make the
plants grow fast. Very definitely, if you start with very soft water and
bubble lots of CO2 into it to get a high level of dissolved CO2, you're
going to have acid water and unhappy fish, or dead ones; but carbonates
have buffering properties that take care of the problem if you know what
to do.

Now, about that hypersaturation:

Yes, there's too much CO2 in the water this way, far more than will stay
there in contact with the air. And you need contact with the air, because
the fish need oxygen. So, the CO2 slowly leaks out.

Most planted-tank people work hard to keep the air contact down to the
minimum needed to keep the fish population happy; and they dribble CO2
into the tank to replace what goes away. (Some CO2, of course, goes away
by becoming plant tissue; some by leakage.) I admit that I allow lots of
air contact in my system, and I just have to spend more on CO2 to replace
what gets away into the air.

--
Dan Drake

http://www.dandrake.com

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