Adding CO2 does NOT change Alkalinity (which is _really_ what you are
talking about when you use the term "KH".

Many people (and MANY websites, even commercial ones) confuse water hardness
and alkalinity and the terms they use to describe them only add to the
confusion. Both are normally given in ppm CaCO3. When water falls as rain,
the falling raindrops absorb CO2 from the atmosphere. Some of this dissolved
CO2 will form Carbonic Acid, giving the raindrop a slightly acid pH. Calcium
bearing rock is very common all over the world and when the rain percolates
thru the soil, it dissolves some of the CaCO3 contained in the soil.

When a mineral like CaCO3 dissolves in water, the molecule breaks apart,
into a Ca++ ion and a CO3-- ion. The Ca++ is the ion responsible for water
hardness and the CO3-- ion is responsible for alkalinity (KH if you prefer).
In _most_ natural bodies of water, the alkalinity is all due to "Carbonate
Hardness" (the German word for this is where "KH" comes from). So, when you
measure "hardness", you are getting the measure of the Ca++ ions in the
water (along with other ions with a ++ charge, like Magnesium). When you
measure "alkalinity" (KH), you are getting the measure of the CO3-- ions.
Note that the bicarbonate ion (HCO3-) is a "form" of the bicarbonate ion -
see below.

Water not only has the ability to dissolve most minerals to some extent,
breaking the molecules apart into the constituent ions, it also breaks apart
itself, to some extent (not much - most of the molecules in a glass of water
are still H2O). When water disassociates, it becomes H+ and OH- ions. The H+
ions don't really exist in solution as naked protons (which is what you are
left with when an atom of Hydrogen loses its single electron) - they join
together loosely with water molecules forming H3O+. As a quick shorthand, H+
= H3O+.

Some of those "naked protons" (the H+ ions) can react with carbonate ions
(the CO3-- ions), forming bicarbonate ions (HCO3-). There is a direct
equilibrium relationship between dissolved CO2, pH, HCO3- and CO3--. If you
add CO2 the ratio of HCO3- to CO3-- will change, but the total amount of
[(HCO3-) + (CO3--)] doesn't change. The pH of the water will change as the
number of H+ ions varies (pH is the negative log of [H+]).

When you add CO2 to an aquarium, the pH will drop. How FAR it will drop
depends upon the Alkalinity of the water. In a water supply that has a
moderate Alkalinity, the pH will be held pretty constant, only dropping
slightly. If there was no Alkalinity in the water, the addition of CO2 would
cause the pH to crash, endangering your fish. You can increase your water's
Alkalinity by adding Baking Soda (NaHCO3) if necessary.

In a planted aquarium, it isn't necessary to shoot for a "specific" pH. I
see many questions from people worrying about how to get the pH "down" to
some specific number. This really isn't needed. Depending upon your water's
Alkalinity, adding CO2 can depress the pH - but it isn't the drop in pH
which is important. What is important for the plants is the increase in
dissolved CO2. You can use the charts on the KRIB to see how much CO2 is in
your water at varying alkalinity (KH) levels and pH values.

This is the Carbonate-Bicarbonate buffer system and is the primary buffering
system in most natural bodies of water. Other chemicals _can_ influence the
pH of the water. The one most frequently encountered by aquarists is
Phosphates. This is what is contained in products like "pH Down". Phospate
buffers can lower the pH but they will have absolutely NO EFFECT on the
level of CO2 in the water. You can't use it as a short cut or a substitute
for adding CO2. Excess phosphates can also contribute to problems with
algae.

Enough chemistry?

James Purchase
Toronto

"snark@boojum" wrote in message
...

Iain Miller wrote in message
...

CO2 does NOT affect KH.

Beg to differ. This is quoted from RedSea:

"CO2 gas dissolved in water can take the form of bicarbonate or carbonate.
These three forms are in chemical equilibrium. Which form is present in
what
amount depends on pH.

Dissolved CO2 = bicarbonate ion = carbonate ion

In the pH range favorable to water plants, pH 6.4 - 7.2, a percentage will
be present as dissolved CO2 and the rest as bicarbonate ions.
From pH 8.0 to pH 8.8 there will be almost no dissolved CO2, a large
amount
of bicarbonate and a small amount of carbonate."

From which I conclude that carbonate hardness can be reduced by lowering
either CO2 or pH.