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Re: Aquatic Plants Digest V3 #215
Steve Pushak wrote:
> Carlos Munoz <cmunoz at crystal_cirrus.com> wrote:
> > Water filtered by peat contains pH buffers other than carbonates, so the
> > standard pH/KH/CO2 tables will *not* work. The same buffers will interfere
> > with direct CO2 tests, so forget about a C02 kit also.
>
> I know I have always been saying this but Roger Miller said he wasn't so
> sure. He thought that there would have to be a very high concentration
> of humic acid in the water in order for the contribution to alkalinity
> to be significant to affect the CO2 test kit titration because the
> organic molecules are very large. I don't have the figures he quoted at
> hand, maybe he does.
I think I originally put these numbers in a letter to Steve... don't
recall if they got aired here or not. FWIW, here's an excerpt:
: The method assumes that all of the alkalinity is due to bicarbonate. If
: a significant part of the alkalinity is due to organics (or anything
: else), then the method will not work. Its interesting to speculate on
: how much organic acid it might take to have a significant effect. For
: instance, if the water contained 4 degrees of alkalinity due to bicarb,
: it would contain 1.42 meq of bicarb. In order for organic acids to add
: even 10% to that value, you could easily need 80 mg/l of the organic
: acid (That's based on the acid providing 180 meq/100 grams - similar to
: the CEC of soil organics near neutral pH). I think this is an unusually
: high number, but not necessarily out of the question.
:
: If the organic acids are present but not in solution then they might
: effect the pH, but they won't effect your measurement of alkalinity. So
: the method is still good.
> It does occur to me that we could have fairly significant concentrations
> of humic acid in the water. After all, there is enough to make a
> significant yellowing of the water. Back in the days of the blue water,
> yellow water debate, somebody might recall what the concentration of
> humic acid might be when you begin to get a noticeable yellowing of the
> water. My water was very yellow in the peat tank for a month or more and
> even now I'm sure the yellow color is quite noticeable. You can easily
> see yellow in the water when its in a white bucket. Previously, you
> could see the yellow when you looked at the tank. BTW, I think it would
> be ok to use even less peat than I did if you just want a little organic
> matter to help the iron reduction process in the substrate.
There's some technical yadda-yadda after this, but what it comes to is
that if you have less than 4 degrees of alkalinity, then you might get
significant effects from organics. Otherwise, you probably won't.
I recall what it takes to get the yellow color in water, but I think it is
a really small concentration. The question of how much dissolved organics
constituted reasonable or high numbers sent me back to the environmental
literature. I didn't find anything that was aquarium specific, but I
found some numbers that might give a high-end estimate of effects.
First, organic concentrations in water samples are most often cited just
as dissolved or total organic carbon (DOC or TOC), and are the weight per
volume of carbon contained in the organic. The most common soluble
organics in fresh surface waters (fulvic acids, which are also strong
coloring agents) are somewhere around 20% to 40% carbon.
One article cited tile-drain effluent from domestic wastes at about 40
mg/l DOC (I hope our tanks are a lot cleaner than that!). Another
interesting article on a hypereutrophic lake in central Florida (where the
underlying soil was peat) found DOC levels in an adjoining wetland at
20-30 mg/l (even higher in the organic floc that was settling out in the
wetland).
From these numbers, I'll guess that DOC in aquariums probably doesn't go
much beyond 30 mg/l. If the organics in question were 20% carbon, that
would give a total concentration of 150 mg/l. This should be a high
estimate.
If the organics provided 180 meq/100 grams as I estimated earlier, then
the buffer capacity from the organics would be 0.27 meq/liter. That's an
alkalinity of 13.5 mg/l as CaCO3, or 0.8 degrees.
For most of us, this amount would be significant, but I think this value
is probably extreme. If half that is reasonable and your alkalinity is
low (say less than 4 degrees) you might see some problems from organics.
Otherwise, I doubt that its important.
> > Your best bet is to make an estimate based on your bubble rate and look
> > at your plants to see how they're doing. Even just a little extra CO2
> > helps a lot. Make sure you have good circulation _below_ the water surface
> > and that the water surface itself isn't disturbed too much.
>
> Another way to deduce the CO2 concentration is to estimate your
> carbonate hardness (from tap water analysis and by guessing how much you
> add) and then observing the pH change in a water sample when the CO2 is
> driven off overnight or by boiling. It's not really a good estimate
> because I think carbonate molecules get used up in some biological
> processes so the carbonate hardness will be less than what you calculate
> it to be from additions.
>
> Does this make sense Roger? At least we could put an upper and lower
> bound on the CO2 concentration by this method.
I'd probably start just by looking at the appearance, growth and behavior
of the plants (bubbling and all). If there's a problem then first I'd
look at morning-evening pH changes to get a qualitative sense of what was
going on: Big change means add more CO2, little change means either the
plants have more CO2 then they need or DIC's aren't controlling pH. If I
needed a CO2 number then I'd use the alkalinity measurements and the
pH/KH/CO2 tables as they are. This will give you a value that is (assuming
all else is accurate) equal to or maybe a little bigger than the real
number. You might be able to do a few other things, but everything else I
can think of involves using apparatus inside sealed containers and other
things that most of us don't want to mess with.
My tap water alkalinity is high enough that I can't get useful CO2 values
from normal pH test kits and alkalinity kits. So I don't. I figure that
as long as the plants are healthy and produce bubbles under moderate light
then I don't have a problem with CO2.
Roger Miller