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Re: Carbo Plus

> From: "Roger S. Miller" <rgrmill at rt66_com>
> Subject: Re: Carbo Plus
> I found some specs and performance data apparently from the manufacturer
> at:
> http://www.trueaquariumplants.com/articles/carboplus.asp

	I'll take a look at those.
> 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.

	The voltage is a _lot_ higher than one would expect for electrolysis
> 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.

	Yes, but the _current_ efficiency of electrolysis devices is usually
close to 100%.  One loses on the overvoltages.
> 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 correspondence I had indicated that the block wasn't noticeably
eroded, and by my calculations it should _all_ be gone in about three months.
> >         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.

	Don't forget that every water molecule has two potential hydrogen
ions, and that "hydrogen ions" are themselves hydrated.  The exchange reactions
are extremely fast, and water is a lot more complicated than out nice simple
formulae make out.  When there is a "hydrogen ion" in solution, which proton
is the "ion"???  With hydrogen bonding thrown in, one would be hard put to tell. 
> > > 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.

	The problem is that C doesn't do much with O2 unless you set fire to
it.  I'm doing some digging, and I haven't seen anything yet to support
this.  I would _really_ like the electrolytic reaction to go!!!!!!

> > > 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.

	The desriptions I was sent indicated that there _wasn't_ obvious
bubbling, just a sort of haze in the water.

	I suspect if either of us had one in our hands we would get to
the bottom of this reasonably quickly, but I'm not going to buy one.
At $20 for four years, my cylinder takes some beating.
> 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 lid of a tank could easily trap enough to cause problems.
> > 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.

If there is electrolysis going on then I agree with this.
The current carriers in the bulk solution will be Ca++ and HCO3-, since
there is so much more of them than of H+ and OH- (in spite of the higher
mobilities of these).  The actual ions discharged will be H+ (cathode)
and OH- (anode), effectively making OH- in the vicinity of the anode
and H+ near the cathode (as you said).  These will migrate into the
bulk solution and do just what you said.
> If the device is working right then it should generate CO2 off the
> carbon block.

	How about running one in a solution of (say) sodium sulphate?
Neither ion will be discharged, and it should still make CO2, which
could be measured.

	The other think I would like to know is the resistance of the device
_before_ it goes into the water.

Paul Sears        Ottawa, Canada

A sceptical chemist who would really like someone to show him a way to
do electrochemical reactions with carbon in aqueous media.
(I'm in the energy business)