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Re: Non-carbonate buffer experiment
> Date: Fri, 11 Apr 1997 20:47:53 -0500
> From: Chunx <joannaj at why_net>
> Subject: Non-carbonate buffer experiment
Gee, I love experiments !
> 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
> 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.
That's too much of an increase in KH to explain with a small amount of PO4.
Buffer apparently added from non-RO source is 5 KH - 3 KH, or 2 KH. 2 KH
is 35.8 mg/l (ppm) as CaCO3. CaCO3 is 50 mg/milliequivalent, so 35.8 ppm
as CaCO3 is 0.716 meq/l. Phosphate is 48 mg/meq as HPO4. In order to
provide a 2 KH buffer with HPO4, you would need to add at least
0.716*48=34.4 mg/l HPO4. This would work only at pH well over 7.2. At a
pH of 7.2 the phosphate would be present as both HPO4 and H2PO4 in roughly
equal amounts. The actual phosphate concentration necessary to get the
2 KH buffer would be twice what was calculated above, or nearly 69 mg/l
That's more than a small amount of phosphate.
> 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
Oops. Adding CO2 at a constant rate doesn't produce a constant
concentration of CO2. At a constant rate of addition, the CO2
concentration will be determined by the rate of CO2 loss through
circulation, aeration, photosynthesis, etc.
> 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 ?
Stick with the charts. A simple explanation for your observation is that
the Fort Worth tap water contains something that stimulates photosynthesis
and is unavailable in your original mix. You already know that there's
phosphate (required for plant growth) in it. There may well be any number
of other components that will allow your plants to increase their rate of
If the rate of photosynthesis increase, the rate of CO2 consumption by
the plants increases. But you're adding CO2 at a constant rate, so the
net effect is that the CO2 concentration drops. CO2 drops from 14 ppm to
9 ppm, KH rises from 3 to 5 as the H2CO3:HCO3 ratio shifts to HCO3, and
the pH increases from 6.8 to 7.2.
> Regan Nantz - please don't flame me, I'm just a poor guy with a
> laptop and a fish tank.
And two variably fertilized plant tanks....