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> Date: Wed, 8 Apr 1998 22:32:10 -0700 (PDT)
> From: "A. Inniss" <andrewi at u_washington.edu>
> So, I try to use "KH" or "Buffering Capacity," neither of which is
> all that accurate, (the first because more than carbonates are involoved
If you go by the definition in the Tetra "Water Chemistry and Fish Diseases"
handbook (cheap but very useful), KH is defined as the concentration of HCO3-
and CO3-- ions. So if you accept that definition, KH is apply named.
> BTW, when using the pH/KH table, if one has high phosphates
> (from lots of fish food, not from phosphate buffers being added) in the
> tank water being tested, will such phosphate also affect the accuracy of
> the table?
The table is based on CO2/carbonate/pH relationships, not CO2/alkalinity/pH,
which is the reason the term "KH" is useful to use. Unfortunately, you cannot
measure carbonates directly (according to Roger Miller). The Tetra "KH" test kit
actually measures total alkalinity, to which phosphate contributes. I believe
other "KH" test kits are the same.
So if your phosphates levels are very low relative to carbonates, the tables
will work well but not perfectly (good enough for our purposes). I suspect this
is the case with normal feeding and reasonable plant growth (i.e., phosphate
uptake). Heavy feeding or the use of phosphate buffers will throw the KH
reading off and will render the tables relatively useless. Note that high
phosphates will even cause incorrect high readings in LaMotte CO2 test kits.
>And what about the sulfates from the MgSO4? Will they affect
> the accuracy?
I don't think sulphates contribute to alkalinity (but I'm not sure). I do think
sulphates will make water alkaline (but I'm not sure about that either). :-)
I still hold the position that the term KH is better for our purposes even
though you can't measure it and it is not recognized by any "professional"
chemists. At least now there is one other person who agrees :-).