I am getting confused now! I'm not saying this is an unusual condition for me, but.....

The equation for CO2 in water is: CO2(ppm)=3*KH*10exp(7-pH)

I am assuming that this equation says that if you dissolve CO2 into pure water there will be an equillibrium reached between the pH and the KH, and the equillibrium values of each depend on the ppm of CO2 dissolved. Is this correct?

Assuming it is: How can this equation work if the water isn't pure - if it has a non-zero KH when there is no CO2 dissolved in the water? The only way the equation gives a zero value for ppm of CO2 is if the KH is zero.

So, how can this equation give a meaningful answer if the water has, for example a KH of 10, due to sodium bicarbonate for example, with no dissolved CO2 in it?

My lack of knowledge in this area is annoying me!