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Non-carbonate buffer experiment
hey folks,
There have been some great discussions in this mailing list
about the validity of the KH/pH/CO2 tables and it is the
concensus that the table holds true in a purely
carbonate-bicarbonate system. But many water supplies contain
non-carbonate buffers, such as the water in Fort Worth. The
theory is that non-carbonate buffers will only affect the
readings from your standard KH test kits. With these buffers
present, your KH test kit will give you a higher reading than
the actual amount of carbonate hardness in your water. This
is because the kit measures total alkalinity and not just the
carbonate hardness (old news, right?). So I decided to perform
a simple experiment to determine the affects of adding such a buffer.
First, two 20 gallon tanks were setup with 60W of flourescents
on each one. Identical substrates and plants were put in each
tank. R/O water was used to fill each tank, sodium bi-carbonate
and trace elements were used to make GH=3dH and KH=3dH. CO2
is injected into each tank at a rate of 1 bubble/second. Pre-
established filters were installed on each tank. After a week,
the pH of each tank stabilized around 6.8. With a pH=6.8 and
KH=3dH in a purely carbonate system gives a CO2 concentration of
14ppm.
Next, I added 1 gallon of de-chlorinated tap water to tank A
(tank B is my control) and gave it 24 hours to stabilize. Note
that I can measure a small amount of PO4 in my tap water so I'm
assuming that there is some kind of phosphate buffer in it.
After the stabilization period I measured the KH and pH of both
tanks. Tank A measured KH=5 and pH=7.2 and tank B still measures
KH=3 and pH=6.8.
Now for the assumptions. The amount of non-carbonate buffer in my
tap water is unknown but we can assume that it is small compared
to the amount of carbonate buffer. Since the existence of the
non-carb buffer will make my KH readings larger we can assume that
the actual carbonate hardness of tank A is between 3-5dH. With
a pH=7.2 the table shows CO2 concentrations ranging from 6-9ppm.
Tank B still reads KH=3, pH=6.8 --> CO2=14ppm.
Since a given source of CO2 will continue to produce the same
concentration of dissolved CO2 in water no matter how the water
is buffered, both tanks should still have 14ppm of CO2. Even if
we assume that tank A has a carbonate hardness in the range of
3-5dH (not including the non-carb buffer) the chart says that
the pH should range from 6.8 to 7.0 (but the actual pH=7.2).
Maybe the existence of a non-carbonate buffer does a little more
than alter our KH test kit readings?
I don't mean to diss the chart, I love the chart, I worship it
every morning, I sleep with it every night, the chart is great!
Of course, none of this matters since we should only use carbonate
bicarbonate buffers in the first place.
Comments / feedback / corrections ?
Regan Nantz - please don't flame me, I'm just a poor guy with a
laptop and a fish tank.