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Oxalic Acid to make CO2?

Contents:

  1. Carbon dioxide generation by "electrolysis?" of Oxalic acid (ethanedioic acid)
    by Denis Daly <dalymob/bigpond.com> (Tue, 08 Sep 1998)
  2. Oxalic acid
    by "Richard J. Sexton" <richard/aquaria.net> (Tue, 8 Sep 1998)
  3. Carbon dioxide generation by electrolysis of oxalic acid
    by Paul Sears <psears/nrn1.NRCan.gc.ca> (Wed, 9 Sep 1998)
  4. oxalic acid
    by "Roger S. Miller" <rgrmill/rt66.com> (Wed, 9 Sep 1998)

Carbon dioxide generation by "electrolysis?" of Oxalic acid (ethanedioic acid)

by Denis Daly <dalymob/bigpond.com>
Date: Tue, 08 Sep 1998

Dear list members

A friend seen a CO2 generator in a hydroponics shop that was
supposed to work on electrolysis of Oxalic acid. The price was
ridiculous. Could have been another con. So I did a little
exploration in a chemical encyclopaedia and internet search and
discovered that:-

When Oxalic acid absorbs oxygen it is converted to carbon dioxide
and water. Indeed lots of chemicals, light etc., make oxalic acid
break down to carbon dioxide and water. Thus the idea is probably
possible.

The salesman trying to sell my friend the commercial product
claimed that it could be turned on and off by turning on or off
the DC supply to the electrolysis cell.

Obviously the oxygen liberated at the anode as the water is
electrolysed into hydrogen and oxygen combines with oxalic acid
converting it to carbon dioxide and water. No doubt if conditions
are correct the oxygen molecules combine with the oxalic acid and
it appears that carbon dioxide is liberated at the anode. There is
a hydrogen outlet in the commercial unit.

There is a catalyst used. It is a liquid. My friend did not find
out what it was or what colour the solution was. Potassium
permanganate causes oxalic acid to break down into carbon dioxide
and water but that is no good. It can't be switched off.

There was sodium bicarbonate supplied with the pack but I assume
that that was to put in the hydrogen filter and carbon dioxide
filters (supposed to be in the pack) to wash any oxalic acid
vapour from the gas.

The real beauty of this unit is the switch on switch off. For
example using two pressure switches like those used in washing
machines to limit the quantity of water put in, one could make a
low pressure control that would suit application to a CO2 bell and
not waste CO2. It could be turned off at night etc., etc.

Has anyone got any ideas of what the catalyst could be, optimal
cell voltage (I could find that by using a variable power supply)
and whether stainless steel would be suitable for the anode and
cathode. I would rather not have to use noble metals such as Pt,
Au etc.

>From a list of incompatible chemicals I found on the internet I
think that Ag and Hg would cause the oxalic acid to rapidly break
down, probably dangerously so.

Any ideas, comments please. Replies to the list would seem
appropriate as many list members would probably be interested, but
I welcome any comments, even if you want to keep them private.

Regards
Denis Daly
Sydney, Australia
(Before you ask I am in the southern part of the city that has the
good clean water.)


Oxalic acid

by "Richard J. Sexton" <richard/aquaria.net>
Date: Tue, 8 Sep 1998

This stuff is *so* toxic, you have to sign the "poison register" at the
pharmacy if you buy some. I use it to clean rust off old car parts.


- --
Richard J. Sexton
richard-at-aquaria.net
Bannockburn, Ontario, Canada                       +1 (613) 473 1719


Carbon dioxide generation by electrolysis of oxalic acid

by Paul Sears <psears/nrn1.NRCan.gc.ca>
Date: Wed, 9 Sep 1998

> From: Denis Daly <dalymob-at-bigpond.com>
> Subject: Carbon dioxide generation by "electrolysis?" of Oxalic acid (ethanedioic acid)
> 
> 
> A friend seen a CO2 generator in a hydroponics shop that was
> supposed to work on electrolysis of Oxalic acid.

	In priciple, this could work O.K..

	-OOCCOO-  ->  2CO2 + 2e-     Anode reaction
	2H+  +  2e-  ->  H2          Cathode reaction

	I haven't been able to find the standard electrode potential
for the anode reaction, so I don't know how well it would proceed
in competition with the other possibility:

	4OH-  ->  2H2O  +  O2  +  4e-

	Keeping the pH low would help, as would keeping the oxalate
concentration high.

> The salesman trying to sell my friend the commercial product
> claimed that it could be turned on and off by turning on or off
> the DC supply to the electrolysis cell.

	That is how it would work.
> 
> Has anyone got any ideas of what the catalyst could be,

	I would start with sulphuric acid, to keep the pH low and 
hence the OH- concentration _very_ low.  Don't use hydrochloric acid -
you would get undesirable anode reactions.

> optimal
> cell voltage (I could find that by using a variable power supply)

	It will be only a very few volts.

> and whether stainless steel would be suitable for the anode and
> cathode.

	Probably.

	As mentioned by someone else, oxalic acid is very toxic, and
not to be handled carelessly.


- -- 
Paul Sears        Ottawa, Canada


oxalic acid

by "Roger S. Miller" <rgrmill/rt66.com>
Date: Wed, 9 Sep 1998

As Richard Sexton pointed out, oxalic acid is toxic.  It's also a fairly
strong acid, which causes safety problems for storage and handling.  On
the up side, it's available as a powder, which eases some of the storage
and handling problems when compared to some other acids.

Aside from that, it looks to me like electrolysis of oxalic acid could
make a practical CO2 generator.  For a while.  Then I suspect that
maintenance and upkeep costs will set in.

Oxalic acid is HOOC-COOH, and above a pH of 5 or so it's present in
solution as -OOC-COO-.  The negative charges are easily removed by either
chemical means (it reacts with tarnished metals) or by electrolysis.
Electrolysis rearranges -OOC-COO- into 2 CO2.  Pretty simple.  No oxygen
required.

On the downside, I speculate that the process will work only in dilute
solution (so the pH will stay high enough) which will limit the production
rate and require some device to maintain the solution concentration - a
pH contoller perhaps or maybe manual monitoring and adjustment. It will
also produce hydrogen gas as a by-product and this is quite dangerous
(remember the Hindenburg?).  Whatever mechanism is used to control the
hydrogen gas will also require attention and maintenance.  The electrodes
probably will be subject to corrosion and/or deposition so will require
periodic maintenance and replacement.  And of course there's the
electrical cost, whatever that is.

Maybe at the scale of a commercial greenhouse this sort of CO2 generator
makes some sense.  Compared to bottled gas there might be a cost savings
when generating large volumes of CO2 that offsets the upfront costs and
maintenance of the electrolysis unit.  I doubt that's true at a hobby
scale.

If I were looking into this system I'd want detailed answers to questions
about maintenance requirements, material costs, replacement costs, power
costs, dependability and operations - in particular I'd want to know if
the machine produces CO2 at a constant rate.  The sales person should be
ready with quick answers to those questions.

All in all, bottled gas is likely to give an aquarium hobbiest more
dependable results and lower long term cost.  And of course, yeast
generated CO2 is just simpler - especially if you have more than one
planted tank.


Roger Miller

in Albuquerque, where I'm wondering if George Booth hasn't already added
one of these marvels to his equipment collection.

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This page was last updated 29 October 1998