Originally Posted by otherwise
KH normally measures [HCO3-] instead of [CO3--]. There isn't any significant [CO3--] until pH >>> 8.
I made the exact same assumption, but later tought about it and chemistry-wise, it would be the illogical for KH kits to measure [HCO3-]. I called Red Sea tech support and asked the same question. The guy there was pretty rude and arrogant but said their KH test kits measure [CO3--]. This does makes sense and hereís how:
[HCO3-] <---> [CO3--] + [H+]
This is the equilibrium equation for our typical tap water buffer.
Think of it as a see-saw. As [H+] increase, the right side of the equation becomes too heavy and upsets the balance. Therefore, CO3-- reacts the with H+ to form HCO3- so the left side becomes heavier and the see-saw is again balanced.
We witnessed pH trying to decrease (As [H+] increases, pH decreases). We also know that lower pH means lower KH and vise versa. Therefore it would make sense that between the two ions, CO3-- and HCO3-, that KH measures the one that is deceasing and the correct answer is CO3--.
I however have read in various sources and as you suggest, that KH measures HCO3- and could have made a fault somewhere in my reasoning. I too question the Red Sea guys validity since he said GH kits measure HCO3-, which is just plainly wrong since they measure CaCO3.
Did you see anywhere I went wrong? Do you have any good sources to explain what KH measures? Itís very strange that I have never heard of KH and GH in my chem courses. They must be associated with fish keeping only.