KH means carbonate hardness, a measure of the carbonate and bicarbonate concentration. Just because a test kit, labeled as a KH test, measures something else doesn't change the definition of KH.
I agree but disagree. kH measures the alkalinity associated with the carbonate buffer system.
Here is an example:
If you have 3 drop checkers, one with 0.00142 molar NaOH, one with 0.00142 molar NaHCO3 and one with .000703 molar Na2CO3 and place them into your "CO2 excess" tank, they all will eventually be the same color and in fact the same pH because they are are all at the same kH. I've done this and I use NaHCO3 and NaOH interchangeably in my systems.
Actually, kH is only partially a function of the amount of HCO3- or CO3-2 present. It depends (as I said) on the alkalinity associated with the carbonate buffer system.
If your curious how to test for this here is how it is done;
First take a known amount of your sample and titrate it to pH 4.5 with a standardized HCl solution. Boil your sample to remove any residue CO2 then retitrate back to pH 4.5. This will give the total alkalinity of the sample and will eliminate any CO2 from your sample.
Now back titrate your sample with a standardized NaOH to a pH of 8.5. This will give the alkalinity of the sample not associated with CO2. On a molar basis subtract one result from the other and do the math and you have the kH of the sample. (Now who understood that? PM me and I help you with the chemistry.)
Now if your sample doesn't have much PO4-3 (say less than 100ppm) Your non specific alkalinity will be a very small part of the total alkalinity and the difference will be small and the kH will be the same as your total alkalinity.
This is what the typical kH test kit does. I test my tank water both ways and though doing the long way is tedious and more accurate, the short way using the kH test kit is OK for ordinary work.
I have found in my work using soft acid water that the standard kH test kits give reliably good results even when using phosphate fertilizers.
in my systems.