The way that carbonate affects pH is a tad complex and not always well
understood. It involves Bronstead acid-base theory, which 99.99% of the
world doesn't need to know. Lots of factors such as temperature, general
hardness, and other chemicals affect pH and KH. So I'll try to simplify and
give some basic generalizations as to how CO2 and carbonate affect pH. If
you want to better understand these principles find a college chemistry text
or e-mail me and I'll try to help.
CO2 when mixed with water becomes carbonic acid, H2CO3. Carbonic acid is
in simple language one acidic proton molecule with one basic bicarbonate
molecule. CO2 bubbles will slowly react with water given proper contact time
and depending on the carbonate levels all ready in the water. Soda water
(all carbonated drinks) have been supersaturated CO2 at high pressure to
make carbonic acid. At atmospheric pressure the excess carbonic acid likes
to disassociate into water and CO2 (the bubbles in your pop). Baking soda
(sodium bicarbonate or NaHCO3) is one sodium molecule, one proton, and one
carbonate molecule. Calcium carbonate, lime, limestone, aragonite, crushed
coral, shells, and kalkwasser (which is German for calcium carbonate) is
CaCO3. Adding carbonate in any of its many forms including CO2 (as carbonic
acid), baking soda, or limestone will raise the amount of carbonates in
water. dKH or KH stands for German degrees of carbonate hardness and is the
old method for measuring carbonates. Chemists today prefer to measure it in
parts per million or in molarity.
Here are some basic generalizations:
Injecting CO2 into water will lower the pH and raise the KH.
Adding baking soda to water will raise the pH up to about 8.3 and will raise
the KH.
Adding shells (or limestone) to water will very slowly raise the pH (to
about 8.3) and the KH. CaCO3 is slow to dissolve in most forms.
Carbonates usually help stabilize pH.
As plants absorb carbonates from the water the KH goes down and pH generally
goes up.
Water with a low KH will absorb CO2 from the air and will raise the KH. This
is how it happens in nature.
Water with a high KH will degas (expel) CO2 and lower the KH (the soda pop
effect). Some people will see small bubbles form on the sides of a glass
with fresh water if it is allowed to sit for awhile, this is the
degassing of CO2 and Nitrogen.
Fish tanks look better with lots of live plants.
Sorry for the chemistry lesson, but I wanted to share some of the knowledge
that I remembered from getting my chemistry degree.
Ross F.
"LeighMo" wrote in message
...
Many plants require a minimal level of KH (this is the CO2 level in your
water) to grow. If you want fast, beautiful growth, it is worth trying to
change. Unless you are injecting CO2, there isn't much point to changing
pH,
GH, and you'll have no control over KH.
I think you misunderstand the relationship between KH, pH, and CO2.
KH is not the CO2 level in your tank. It's the amount of carbonate in the
tank, which isn't the same thing. And injecting CO2 does not give you
control
over KH.
I do agree that plants need some KH. If your KH is zero, it's definitely
worth
trying to increase it. But you don't do that with CO2. You do it with
carbonate. Baking soda, calcium carbonate, etc.
Leigh
http://www.fortunecity.com/lavender/halloween/881/