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Re: KH-pH-CO2 tables

Rachel wrote:

> > I am using the pH/KH table at
> > http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html to determine the
> > level of CO2 in my tank. I notice, though, that it says "The CO2-KH-pH
> > equilibrium should be correct
> > unless you are taking extreme measures to adjust pH, such as adding
> > lots of phosphate buffers like pH-UP or pH-DOWN or using sulfuric
> > acid." Now I do add some phosphate to my tank - maybe a BB-sized ball of
> > KH2PO4 weekly. Will this change my readings?

to which Doug replied:

> In a word yes.

In word, no.  While there are conditions that mess with the KH-pH-CO2
table, the amount of phosphate Rachel is adding to her tank will not
cause a problem. That is, unless the phosphate level builds up to 10's
of ppm or more and the KH is very low to start with.

> The table assumes that the only buffer system in the water
> is the carbonate buffer system, and adding another buffer will throw off the
> calculations.  I hadn't seen the "extreme measures" clause, but I wish Eric
> hadn't put that in there.  In my experience, *every* single time the table
> is checked against readings from a higher end CO2 kit, such as the LaMotte
> kit, the table comes out too high, sometimes ridiculously so.

The CO2 tests - even the "high end" ones - are just acidity titrations
and they are subject to the same sort of interference that can effect
the KH-pH-CO2 chart. If you have a difference between a CO2 kit result
and the KH-pH-CO2 table you should not automatically assume that it's
the CO2 kit that's right.

> If that table
> were accurate, either my fish should be dead, or they're all some mutant
> strain that can handle CO2 concentrations of up to 400 ppm with only a DIY
> bubbler. Somehow I don't think so. Plants produce all sorts of changes to
> water chemistry which I'd be willing to wager also throw off the table.
> Now, one very important caveat is that my tanks are often below ph6, where
> the table is very, very sensitive to small errors in pH measurement.
> perhaps the table's more useful around or above a pH of 7, but even then,
> you'd really need to have a good pH measure.

Doug's tank seems to have a pretty extreme case.

Late last summer I had a long off-list exchange with another list member
over just this subject.  He had the equipment, the patience and the
technical ability to walk through a series of tests that I prescribed. 
The upshot was that he managed to establish that in fact his tank did
have a non-phosphate interference that invalidated the KH-pH-CO2 table. 
Under conditions in his tank the CO2 kit did give better results.

In the course of those tests we also found that the two-pH test that I
have touted as approximate but interference-free is subject to
interference from an acid that associates between the two measured pH
> IMHE, the table's a good tool to get people to think about measuring CO2,
> but it doesn't provide a useful function except as a kind of teaching tool.
> This should of course be read as a call for testimonials of the fantastic
> value of the table!

Obviously I can't be very demonstrative about the value of the table.  I
wonder if we might be able to generalize about the conditions where the
table's results are suspect.  In the cases I can recall there were two
(or maybe just one) common elements.  I all the cases I can recall the
tank pH was consistently and substantially below 7.  In at least some
cases the tank also contained wood.  Doug, if your tank contains
ornamental wood then I think that will make it 100%.

My reasoning is that the low tank pH favors fungal activity.  The fungus
may not be evident either because it is growing mostly inside the wood
or because it's removed from the outside of the wood by rasping or
surface-grazing animals.  At least some varieties of fungus that
decompose wood in purely terrestrial settings produce acids in fairly
large quantities.  Perhaps the same thing happens under water. Low pH
leads to predominantly fungal decomposition of the wood, the fungal
decomposition produces weak, low equivalent weight acids and the acids
build up to a sufficient concentration to effect the KH titration. 
Thus, the KH-pH-CO2 table is not valid.

Peat can also produce acids that invalidate the table.

If true, then the combination of low pH and wood in the tank may be a
condition in which the KH-pH-CO2 relationship is consistently
undependable.  I'd like to hear some additional experience either way.

Roger Miller