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> Unfortunately, even the term alkalinity has a lot of potential for
> linguistic confusion. I can't tell you how many times I've heard someone
> use "alkalinity" when they are talking about how alkaline (basic) their
> tap water is (ie how high the tap water's pH is).
You're right. It is necessary to distinguish between "alkaline" (pH >
7.0) and "alkalinity". Alkalinity measures the concentration of chemicals
that tend to make or keep the water more alkaline.
> I've also used "alkalinity" (KH if you will) while
> buffering capacity, and confused people who think I'm talking about a high
> pH. So, I try to use "KH" or "Buffering Capacity," neither of which is
> all that accurate, (the first because more than carbonates are involoved,
> the second because I would imagine there is both acid buffering capacity
> and base buffering capacity?), but they get the point across. I've also
> used acid binding capacity to describe the tanks ability to resist
> downward shifts in pH. This term works okay.
"Buffering capacity" works just fine for me. Buffers work to buffer both
increases and decreases in pH. You can think of alkalinity as a measure
of the buffer effect against decreases in pH. "Acidity" measures the
buffer effect against increases in pH.
For what its worth "acidity" is a little-used measure that in most waters
can be treated as a direct measure of CO2 concentrations. It is
determined by titrating a sample up to a pH of 8.3. Interpreting the
acidity as a measure of CO2 concentration suffers the same sorts of
potential problems we see with pH-alkalinity curves.
> 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? And what about the sulfates from the MgSO4? Will they affect
> the accuracy?
The alkalinity-pH tables assume that the measured alkalinity is due to
bicarbonate ion. Many different acids can be read as part of the
alkalinity and if they are present in significant quantities then they
will effect the accuracy of the table. Significance might be hard to
Phosphate can effect the pH-alkalinity curve, but not usually at the level
you get just from fish food. Lets say that you have 3 mg/l of phosphate
in your water, and the pH is 6.8. At that pH, the phosphate is present
mostly as H2PO4-, which has an equivlent weight of 97 mg/meq. 3 mg/l of
phosphates would be about 0.03 meq/l, which is only 1.5 ppm alkalinity, or
0.09 degrees. If your pH is higher (say, 7.6) the phosphate would be
present as HPO4-2 and the same amount of phosphate would provide twice as
much alkalinity. Either way, the effect from that much alkalinity would
be swallowed up in the precision of the test - you would never see its
effect. If your kit reports P instead of phosphate, then the buffer
capacity from the same amount of phosphate would be about 3 times higher.
If you have high pH and moderate or low overall alkalinity, then 3 mg/l of
P might be significant.
A similar assessment can be made for humic acids, but here I have to
speculate on their equivalent weight. Soil organic acids have fairly high
equivalent weights, and I think it would be reasonable to estimate a
value of about 1000 mg/meq at near-neutral pH. At that weight, it would
take 178 mg/l of the acid to make a 0.5 degrees of alkalinity. I can't
say for sure, but I think that's probably a higher concentration
than we usually see.
There may be some low equivalent weight organic acids that if they were
present could effect alkalinity measurements at relatively low
concentrations. EDTA might be in that category.
Sulfates (and chlorides and nitrates for that matter) have no effect on
the pH-alkalinity curve because the acids don't associate above a pH of