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Re: other forms of alkalinity
On Tue, 19 Sep 2000, Dave Gomberg wrote:
> At 03:48 PM 9/18/00 -0400, George Booth discussing KH wrote:
> > If carbonates are the main form of alkalinity, this will produce useful
> > results. If
> >other forms of alkalinity are also present (such as phosphates), the results
> >will be garbage.
> George, try to follow me on this and tell me if I am way off base. If
> there is 100ppm HCO3- and its relatives, how could .1-1ppm of HPO4 and its
> relatives screw up the measurement MUCH?????
Just getting my two bits in here...
Phosphate at the levels we usually see it won't mess with the alkalinity
test. The problems comes up when someone adds a non-carbonate buffer.
If someone has enough phosphate in their water (e.g., enough to alter the
pH) then it definitely will mess with the alkalinity measurement.
> 1ppm of HPO4 is only about
> 2/3 the molarity of the same weight of HCO3, so I would think the
> interference (which is surely present) would be quite small. Same for
> tannic acids with huge molecular weights. Where am I missing it?????
I've seen several problem descriptions that make little or no sense unless
there is a significant role for dissolved organic acids. I think that the
descriptions come from people who's water comes from private wells,
usually in swampy or previously swampy, low-lying areas. I suppose that I
could chalk the problems up to inaccurate observations, but I've covered
the details with them enough to be reasonably confident that their
observations make sense.
Tannic acids are hardly the only acids potentially present. The carboxyl
group (-C00H) provides the acidity for most organic acids, and it weighs
in at 44 mg per equivalent when dissociated. That makes it potentially
more effective than bicarb, which comes in at 61 mg per equivalent. An
organic acid based on carboxyl will weigh more than 45 mg per equivalent,
and most of them will be a *lot* more than that. As a result, fairly
large concentrations of an organic acid would usually be necessary to
alter the alkalinity. But there are instances - particularly in water
with very low bicarbonate concentrations - where the measurable alkalinity
might be significantly effected by dissolved organics.
Personally, I'd love to see some data on the organics dissolved in water
from planted aquaria. My guess is that relatively light organics -- some
of them acids -- abound in our tanks. Moreover, I suspect we might find a
correlation between dissolved organics in the water with at least some
kinds of algae outbreaks (Following the usage common in much of the
ecological literature, my use of the term "algae" implicitly includes
Dissolved organics are the big unknown in aquarium water chemistry.