KH, or carbonate hardness, is a misnomer. You're actually measuring alkalinity.
In fancy words alkalinity is "the sum of all strong bases in solution less the strong acids".
What this means in the practical sense is that bicarbonate, carbonate, and phosphate are the three main possible contributors to what we refer to as "KH". There are other compounds that may play a role as well, borates and silicates come to mind, but even these are in most circumstances going to be negligible besides the carbonates.
The thing to keep in mind with species contributing to KH is that we measure it by strong acid titration, meaning that the results we get tell us the total amount of H+ needed to completely neutralize the carbonates and phosphates.
What I mean:
Phosphate has 3 negative charges, so one equivalent of PO4 contributes 3 equivalents to KH
Carbonate has 2 negative charges, so each equivalent of CO3 contributes 2 to KH
Bicarbonate has 1, you get the picture.
KH = [bicarbonate] + 2x[carbonate] + 3x [PO4] - [H+]
[x] indicates the concentration of x
The pH-KH-CO2 relationship is tied to this, it assumes that [PO4] is negligible, but this is a little off topic.
In the case of well maintained tanks, PO4 isn't going to be much higher than 1 or 2, for the sake of argument, we can be liberal and even let it stray to 5ppm.
Which, if I'm doing my calculations correctly (have I ever mentioned that I hate "ppm"?
), works out to about 0.15milliequivalents of PO4, or roughly 8ppm KH. My test kit is 10ppm per drop, so yes, I figure that this is measurable, but then, my PO4 doesn't get close to 5ppm!
1ppm of PO4 should result in about 1.7ppm of alkalinity, say 2. Definitely not within the sensitivity of my test kit.
In fancy words alkalinity is "the sum of all strong bases in solution less the strong acids".
What this means in the practical sense is that bicarbonate, carbonate, and phosphate are the three main possible contributors to what we refer to as "KH". There are other compounds that may play a role as well, borates and silicates come to mind, but even these are in most circumstances going to be negligible besides the carbonates.
The thing to keep in mind with species contributing to KH is that we measure it by strong acid titration, meaning that the results we get tell us the total amount of H+ needed to completely neutralize the carbonates and phosphates.
What I mean:
Phosphate has 3 negative charges, so one equivalent of PO4 contributes 3 equivalents to KH
Carbonate has 2 negative charges, so each equivalent of CO3 contributes 2 to KH
Bicarbonate has 1, you get the picture.
KH = [bicarbonate] + 2x[carbonate] + 3x [PO4] - [H+]
[x] indicates the concentration of x
The pH-KH-CO2 relationship is tied to this, it assumes that [PO4] is negligible, but this is a little off topic.
In the case of well maintained tanks, PO4 isn't going to be much higher than 1 or 2, for the sake of argument, we can be liberal and even let it stray to 5ppm.
Which, if I'm doing my calculations correctly (have I ever mentioned that I hate "ppm"?
), works out to about 0.15milliequivalents of PO4, or roughly 8ppm KH. My test kit is 10ppm per drop, so yes, I figure that this is measurable, but then, my PO4 doesn't get close to 5ppm!1ppm of PO4 should result in about 1.7ppm of alkalinity, say 2. Definitely not within the sensitivity of my test kit.
