KH test kits technical info - The Planted Tank Forum
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post #1 of 3 (permalink) Old 01-28-2004, 08:54 PM Thread Starter
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Do they measure the [HCO3-] or [CO3--] or both? ("[]" = concentration)

Do different brands measure differently?

This is very important to know since you can get extremely different results if one KH test measures e.g. [HCO3-] but not [CO3--] when compared to another KH test kit that does vise versa. While KH stands for "carbonate harness," this can be misleading since by adding CO2, there is practically no CO3-- and the buffer exists as HCO3-.
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post #2 of 3 (permalink) Old 01-29-2004, 10:43 AM Thread Starter
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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.
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post #3 of 3 (permalink) Old 01-29-2004, 05:03 PM Thread Starter
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Great! Everything makes perfect sense now. I knew I was missing something b/c today I was thinking over the matter again and finally figured out that without a pH >> 8 that HCO3- would be the primary buffer. Anyhow thanks for you help. And goes to show how some techies really can give a rip about their customers.
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