This was written in response to a question over in Water Works regarding the effect of CO2 supplimentation on KH. This afternoon I realized it would be a good idea to put it here too.
To answer the question "Is KH affected by adding CO2 to an aquarium?"
KH is the measurement of Carbonate (CO3 2-) in the water and that measurement isn't going to be affected by the amount of CO2 in the water. Your KH remains stable as long as the CO3 molecules aren't broken down, such as happens during Biogenic Decalcification, which will actually _raise_ the KH.
Carbonate (CO3 2-)alone cannot be used for Biogenic Decalcification as it contains no Calcium, only Carbon and Oxygen. Biogenic Decalcification only occurs in environments high in Calcium Carbonate (CaCO3) and severly deficient in Carbon (organic or inorganic). The plants will absorb the CaCO3 and break the molecule down to get at the Carbon atom. The result is Calcium deposits on the leaves and Oxygen. In a system where Carbon is available readily (CO2 gas or organic carbon in the environment) the plants will use that instead. Biogenic Decalcification is much much more energy costly than breaking the O-C bond found in CO2.
Yes, your wrong! c02 has a big impact on the kh and ph.
Not quite. CO2 along with KH have an impact on pH. More on that later.
If your c02 unit is pumping to much c02 into your tank it is causes a reaction kh and the kh drops!
Aquarium with very low kh are subject to rapid ph shifts, and this is what kills fish.
Yes and no. Stress caused by large pH shifts can lead to disease which can kill fish. The drop in pH is caused by increased amounts of Carbonic Acid in the water. That Carbonic Acid is dissolved CO2. Increasing the concentration of an acid in solution lowers the pH. Therefore, as the pH lowers, the concentration of dissolved CO2 increases which will suffocate your fish.
On a kh test kit you will see on the chart its 1-16 dkh and from 17.9-286.4 ppm kh.
KH and dKH are two totally different things. KH is Carbonate Hardness and is commonly referred to as the Alkalinity (opposite of Acidity) of the water. dKH is what most Europeans use to refer to what we in North America call General or Total Hardness (GH). GH is the measurement of dissolved Calcium and Magnesium in an aquarium. It's very important to not get those two mixed up.
Check this link out for more information:
Here's a discussion of water chemistry regarding the relationships between CO2, CO3, pH, and KH. This is the chemistry of an aquarium where CaCO3 and CO2 are supplimented and are in excess for purposes of chemical reactivity.
Don't forget the states:
(g)=gas, (s)=solid, (l)=liquid, (aq)=aequeous solution
-->/<-- denotes chemical equilibrium
CO2 is added to water creating Carbonic Acid:
CO2(g) + H2O(l) --> H2CO3(aq)
Being an acid H2CO3 will dissociate and release a Proton(H+, a Hydrogen atom without the electron) into the solution forming Bicarbonate (HCO3-)
Here's the dissociation equation:
H2CO3(aq) --> HCO3-(aq) + H+(aq)
CaCO3 is added:
CaCO3(s) + H2O(l) --> Ca+2(aq) + CO3 2-(aq)
We now have in solution an acidic Proton (H+), Carbonic Acid (H2CO3), Calcium (Ca), and Carbonate (CO3 2-). These four substances will react to form and aqueous solution containing Hydronium Ions, Calcium ions, Calcium Bicarbonate, and Carbonate.
H2CO3(aq) + CaCO3(aq) -->/<-- H+(aq) +Ca 2+ (aq) + Ca(HCO3)2(aq) + CO3 2-(aq)
This actually results in the increase in the concentration of Carbonate in solution for a moment. In this system Carbonate is the conjugate base of Carbonic Acid. Those free acidic protons are quick to react with the free Carbonate to form Bicarbonate and Carbonic Acid again.
H+(aq) + CO3 2-(aq) --> HCO3-(aq)
2H+(aq) + CO3 2-(aq) --> H2CO3(aq)
Therefore, we have a buffered solution, which is why we use CO3 to buffer our pH. Keep in mind, this is nearly instanteous so there will be no change in KH measurable by our hobby test kits.
What we have here is a weak acid-weak base titration which will continue as long as we keep adding our acid. As we add more CO2 to our artificial environments the concentration of Carbonic Acid increases until the concentration of CO3 2- in solution (KH) is no longer able to compensate and the pH goes down.
The relationship is lograthmic, here's a link to a great visual of the correlations.
I hope that's answered some questions.