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Non-Carbonate Experiment (Clarifications)
Just to clarify. I didn't want to get too long winded but I have received
some fantastic replies!
Paul Krombholz wrote:
>>R/O water was used to fill each tank, sodium bi-carbonate
>>and trace elements were used to make GH=3dH and KH=3dH
>I don't see how adding sodium bicarbonate could raise the GH
Kent R/O Right was used to get GH=3 (this is what I meant by "trace elements").
Sorry I wasn't very clear there.
Roger S. Miller wrote:
>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 (!!).
Wow, I wish I could do that. I'm an EE so I'm limited to circuits and
stuff. But to respond: the various types of chemicals in my tap water are
total unknowns. Maybe there are other chemicals which screw with the pH,
not just phosphate. I hear that Fort Worth lets the industries dump all
kinds of junk in the water.
>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.
Hmm, good point. That would pretty much flush my results since I assumed
that the CO2 concentration of the two tanks was equal.
Bill Warner wrote:
>please keep in mind that a system like this will never
>actually reach equilibrium. Depending on how constant your rates of CO2
>addition/loss are, you may reach a steady state condition but this is
>technically not the same as equilibrium.In particular, since the reaction
> CO2 + H2O --> H2CO3
>is kinetically slow, it is *possible* for the steady state concentration of
>CO2 to be significantly different than would be predicted by simple
This is another variable to put into the equation. The pH of my big tank
responds to increases in KH like a 3rd order differential equation.
When KH is increased suddenly, the pH jumps and my controller keeps my
CO2 on for 2 days straight. Eventually the pH will fall and then rise again.
This continues for several days until a steady state value is achieved.
Kinda like a really slow RLC transient response!
Thanks for your comments!