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Please if anyone kind enough,I would like a clear definition of what KH is and also the chemical equation when co2 reacts with whatever is reacting with and forming carbonic acid.Any other information relating KH with pH would be highly appreciated.I don't want to play brainiac I just want to start from the begining
Thanks a lot..
 

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The first part of your question deals with finding a clear definition of what KH is. This has been nicely summed up by http://www.thekrib.com/Plants/CO2/hardness-larryfrank.html, paraphrased below:

The Krib said:
All fresh water sources contain calcium and magnesium in varying quantities. These are cations with a 2+ charge. They form salts with anions which have a negative charge. The most important of these are bicarbonate (HCO3- ; carbonate CO32- ; and sulfate SO42-.

General Hardness (GH) measures the cations (+ charge); for calcium, magnesium.

*edit* GH is also a measure of Iron, strontium, aluminum, and manganese but these aren't very common in water samples so don't play a huge role in the final GH measurement.

Carbonate Hardness (KH) Refers to only the bicarbonate, and carbonate anions(-charge); it does not measures the sulfates and other anions.

Carbonate Hardness is a confusing term because it does refer to hardness, but rather to the alkalinity (the ability of a solution to resist a pH change with an addition of an acid.) from the carbonates and bicarbonates. Other anions (such as hydroxide, borates, silicates, and phosphates) can contribute to the alkalinity. To be absolutely correct, you should NEVER use the term 'KH'; however, this is often referred to in aquarium literature. It should be noted that it is the bicarbonate/carbonate buffering system which provides the majority of the alkalinity in aquariums plant aquariums.
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The second part of your question was about the equations for CO2 and H2O reacting.

The equation for CO2 reacting with water is:
CO2 + H2O <--> H2CO3 (carbonic acid) <--> H(+) + HCO3(-)

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Brief Explanation

These are all reversible reactions, the reactions happen and then sometime later unreact, bouncing back and forth between the three main forms shown above as the water molecules bump into and pull apart the above products. Under everyday conditions CO2 + H2O is most abundant, then HCO3(-) and finally H2CO3.

The last product in the series of reversible reactions above is a depiction of the H+ ions that have dissociated from the parent molecule. As H+ ions are essentially what acid is the last part of the above equation simply shows the acid coming off the molecule.

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*Extra Info*

Since carbonic acid is a polyprotic acid (has more than 1 hydrogen atom that can dissociate) it can undergo another reaction (below). But this reaction is - by far - not as common because the (2-) ionic state is a lot more unstable than HCO3(-).

H(+) + HCO3(-) <--> 2H(+) + CO3(2-)
 

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Zapins is correct. The Larry Frank article called "Water Hardness" is great. That's why I listed it first in my response.
 
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