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Re: Carbo Plus
Paul Sears wrote:
> The first problem is that the anode isn't eroded anywhere near as
> fast as it should be according to the current draw and the claimed CO2
> production. The CO2 production also isn't consistent with the current draw.
> It isn't an electrolytic device, as far as I can see.
I found some specs and performance data apparently from the manufacturer
at:
http://www.trueaquariumplants.com/articles/carboplus.asp
I think their specs are pretty consistent with it being an electrolytic
device. At maximum settings they claim a production of 0.29 grams of
CO2/hour at 20 V with a power consumption of 22.5 watts.
0.29 grams of CO2/hour is 1.83E-6 moles of CO2/second.
Producing one mole of CO2 requires 4 moles of electrons, so the maximum
CO2 production rate would require 7.32E-6 moles of electrons/second or
4.41E+18 electrons per second. An ampere is a flow of 1 coulomb of
charge/second, or 6.24E+18 electrons/second, so the peak CO2 production
would pull 0.7 amps.
At 20V working potential, the device would be using 14 watts of power.
Compared to the rated maximum power consumption of 22.5 watts, that
would indicate an efficiency of 62%. In my experience, 62% is pretty
good efficiency for a small electrical device.
The manufacturer's description seems to indicate that the carbon block
is eroded. They say that after the rated 2 months of use it may be
brittle and crumble into small pieces.
> > The unit works by maintaining a voltage between the anode and cathode that
> > is sufficient to break down water.
> >
> > The half reaction at the cathode is
> >
> > 4H2O + 4e- -> 4OH- + 2H2
>
> Oe usually writes: H+ + 2e- -> H2
>
> H2O <-> H+ + OH- is fast enough to make the two essentially
> the same.
Well, I wrote the half reaction the way it was written in the several
sources I had before me. Perhaps in equilibrium chemistry terms these
reactions would be equivalent, but in kinetic terms I don't think
there's any comparison. The reactant (water) in the first case is
present at a concentration of about 55 moles/liter. The reactant in the
second case (hydrogen ion) is present at a concentration of 1E-8 to 1E-6
moles/liter.
> > The half reaction at the carbon anode is
> >
> > 2H2O + C -> CO2 + 4H+ + 4e-
>
> From the point of view of electrolytic chemistry, this makes no
> sense. Carbon is very inert. If you _could_ run this electrode reaction,
> the unit would be doing something fuel cell makers would love to do.
As I pointed out in my second letter, this is actually two reactions;
oxidation of water and then oxidation of carbon. The oxidation of
carbon may not work all that well, in which case the device will bubble
O2.
> > Both the hydrogen and the CO2 should be produced as gas. The reaction
> > produces 2 molecules of hydrogen gas for each molecule of CO2 produced.
> > Of the visible bubbles produced, only 1/3 are CO2 bubbles. The rest are
> > hydrogen gas.
>
> The amounts of hydrogen produced would be large enough to be
> obvious and a definite hazard.
According to the diagram and description at the link I provided above
the production of hydrogen gas *is* obvious. The visible bubbles are
hydrogen gas. The CO2 bubbles dissolve so quickly that they aren't
observable. Of course, if the carbon reaction isn't working then some
of the visible bubbles will also be oxygen.
I think the hydrogen would be a hazard only if you can get it pretty
well concentrated. Otherwise, it just gradually reacts with atmospheric
oxygen to make water vapor.
> > The hydrogen gas produced by the unit should have some effects of its own.
> > Dissolved hydrogen gas is like candy to anaerobic bacteria, so a tank with
> > a CarboPlus unit may support some real active anaerobic activity.
>
> Hydrogen is essentially insoluble in water. It will leave.
According to the data I have, hydrogen gas has a solubility of about
8.2E-4 moles/l/atm under standard conditions. That's less soluble then
oxygen (1.4E-3 moles/l/atm) but more soluble than nitrogen (6.8E-4
moles/l/atm). No doubt some will leave before it dissolves. The
manufacturer of the device even says it leaves.
Despite our differences here, we do have common ground. In my original
letter I wrote:
> Ideally, the 4 hydroxide ions (OH-) and the 4 hydrogen ions (H+) would
> combine to form 4 water molecules. However, the two ions are not produced
> in the same location, so there won't be a 100% recombination to form
> water. Instead, some of the OH- will react with bicarbonate to form
> carbonate. In most water that will cause a calcium carbonate crust to
> form on or near the cathode. An equivalent amount of the hydrogen ion
> produced must also react with bicarbonate. The effect of both reactions
> is to lower the GH and the alkalinity of the water, with a larger decline
> observed in the alkalinity than in the GH. The actual amount of decline
> will be highly variable.
The two secondary reactions are:
at the cathode Ca++ + HCO3- + OH- -> CaCO3 + H2O
at the anode H+ + HCO3- -> CO2 + H2O.
the net reaction from combining these is the same reaction that Paul
describes:
Ca++ +2HCO3- -> CaCO3 + CO2 + H2O
The manufacturer says that a "whitish limescale" coating will
precipitate on the stainless steel cathode, so I think we can agree that
this reaction is happening. We differ over degrees.
If the device is working right then it should generate CO2 off the
carbon block. Some CO2 will also be generated by precipitating calcite,
with a resulting drop in alkalinity and GH. The amount of CO2 generated
from calcite precipitation will be variable, depending on GH and
alkalinity in the water, the amount of H+ and OH- that gets neutralized
before the calcite is precipitated and the amount of calcite that gets
redissolved after it's initially formed.
If the device is not working properly then electrolysis will still occur
and the device will still evolve bubbles of H2 and O2, but most of the
CO2 production will come from the precipitation of calcite. The amount
of CO2 generated and the amount of calcite precipitated will be
variable. If you generate much CO2 at all when the device isn't working
right then the alkalinity in the tank will crash. That sounds like what
happened in Ellen O'Connell's case.
It's also possible for the thing to bubble merrily along and produce no
CO2 at all. That would be the case when the carbon block isn't reacting
and there is little or no precipitation of calcite, for example, a
malfunctioning device in a soft water tank.
From all the symptoms I've read, I have to suspect that some of the
problems reported for the Carbo Plus might come from having a bad
electrical connection to the carbon block, probably due to oxidation of
the contact plate.
Roger Miller