While doing research and thinking about a post in this forum I decided to write down what I know about alkalinity and KH. That helps me remember it. Now, to supplement my memory I'm posting it here so I can look it up later if I want to.
KH is not a measure of hardness, but a measure of alkalinity. Alkalinity is the capacity of an aqueous solution to neutralize an acid. So, it is a measure of the concentration of negative ions in the water. Those negative ions are carbonate (CO3), bicarbonate (HCO3), borate (BO3), phosphate (PO4), etc. When the pH is below about 8.4, more than 90% of the alkalinity in natural water is made up of bicarbonates. For that reason KH in our aquarium water is almost always a very close measurement of the concentration of bicarbonate in the water.
“The main sources for natural alkalinity are rocks which contain carbonate, bicarbonate, and hydroxide compounds. Borates, silicates, and phosphates also may contribute to alkalinity.” (Alkalinity and Stream Water Quality
KH is not a buffer against changes in pH. But, the combination of carbonic acid, a minor part of dissolved CO2 in water, and bicarbonates, is a buffer against changes in pH for small additions of weak acids. Buffers maintain the pH at a fixed value, and that value is a function of the relative concentrations of CO2 and bicarbonates in the water - more CO2 lowers the pH being maintained, and more carbonate raises the pH.
The concentration of bicarbonates in water is not affected by the amount of CO2 in the water. (See The Principle of Conservation of Alkalinity by Pankow - http://tinyurl.com/j7vrj7t
) Or, “The Principle of Conservation of Alkalinity by Pankow … shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. …” (Chemistry and the Aquarium: What is Alkalinity? ? Advanced Aquarist | Aquarist Magazine and Blog
In the unusual case of water which has a very small concentration of bicarbonates in it, usually surface water, the water company that supplies that water will add chemicals to the water to raise the pH enough to prevent erosion of copper and lead in the plumbing where the water is used. Some of these chemicals may raise the alkalinity - hydroxide or phosphate compounds, for example. It may be possible to have a relatively high KH, but very low concentration of bicarbonates, in that water.