Re: Oxalic acid

[Paul Sears <psears at nrn1_NRCan.gc.ca>]

> From: Vahe Ganapetyan <vahe at hotbox_ru>
> Subject: Oxalic acid
> 
> I would like to return do discussion of alternatice CO2 production
> method that was discussed here a while ago, through electrolysis of
> Oxalic acid. This method seems extremely attractive to me for several
> reasons: high yield (theoretically, 88g of CO2 should be produced from
> 90g of Oxalic acid), low cost (Oxalic acid is used in industrial
> quantities in dieing, bleaching, etc. and is quite cheap), and easy
> electronic control over the process (since CO2 production is directly
> dependent on electric current). Among the advantages should be listed,
> perhaps, that this method is used commercially by the greenhouses,
> which means it should be quite efficient. The disadvantages are, of
> cource, toxicity and corrosive effect of the substance, but if proper
> care is taken, its safe use can be provided.

	Sounds good...
> 
> To start with, I purchaced 2lb. of Oxalic acid (it costed me less than
> $2) and made a simple elecroliser from plastic coffee can. I took two
> stainless steel 

	The problem is that stainless steel isn't immune to corrosion
effects.  In an electrolytic situation, almost nothing is.

> Afterswitching on connecting the current the electrodes immediately
> started to bubble, but it seemed a little strange to me as the most 
> active bubbling was on the outer side of one plate (not the side
> facing the other electrode, but looking at the jar). There was also
> some gas production on the inner part of both electrodes, but less.

	Gas production isn't the only electrode reaction possible..

> I
> let the system run for an hour or two to see how it would behave. Now,
> the strange part begins. The liquid has become brown after a few
> minutes of operation and with time, it was turning darker and darker.
> After two hours it was almost black, so I couldn't see the electrodes
> through it.

	I'm not too surprised.  At the positive electrode, you will get,
among other reactions:

	Fe  ->  Fe++  +  2e-  and probably Fe+++ too.

Other elements from the stainless steel may enter solution as well.

> When I disconnected the current and took off the cap, the
> outer part of the electrode, on which most bubbles were produced was
> covered by bright lemon-yellow staff (apparently, it was Iron Oxalate
> as it looked exactly like the staff I produce by reacting Iron Sulfate
> with Oxalic acid

	The iron ions precipitate with the oxalate ions.

> - btw., by heating Iron Oxalate to about 600F
> atomized Iron is produced, which can also be used for the plants). The
> inner part of the electrode was also lightly covered with iron oxalate
> and some black and much dencer staff (did not clean off easily) -
> I could not figure out what it was.

	Fe+++ oxalate, another metal oxalate, or possibly an oxide
or hydroxide.

> The other electrode was clean,
> covered by a thin layer of whiteish staff, perhaps calcium carbonate
> from the water.

	About all that can go on there is:

	2H+  +  2e-  ->  H2

	There will be half as much H2 as CO2, which could present a danger.

> Now, my only knowledge of Chemistry is from general education school,
> and I need help to find out what to do next. I don't understand why
> most of the iron oxalate was built on the outer side of the steel
> plate, I wouldn't be that much surprised to see it between the
> electrodes. I don't understand what is the black-ish deposit on the
> inner side of the same electrode, and what colored the liquid into
> dark brown - judging from color, it must be some Fe (III) salt, but it
> is dilute, or colloid at worst, since it didn't set down at all by the
> next date. All I could think of would be Fe (III) oxalate, but I know
> nothing about its color or solubility of water, and I have no other
> ideas.

	Fe(III) and other metal complexes in the liquid, I suspect.
> 
> If there are people knowledgeable in electrochemistry, please give me
> some advice on how this research could be continued. Apparently, the
> method has no practical uses at this stage. First, would the addition
> of sulfuric acid change the situation at all?

	I rather doubt it.

> Second, what material
> could be used for the electrode to prevent formation of oxalate? Coal?
> (it is hard to find a coal electrode of decent size to provide
> sufficient current density). 

	Graphite would be the obvious choice, as it is pretty inert.

> Would silver be of any use?

	Not for the positive electrode:

	Ag  ->  Ag+  +  e-

> (I cannot use
> gold or platinum for this project :) Any advise or explanations are
> very welcome.

	I would get graphite electrodes, or one graphite electrode to
act as the positive one.  I would probably cut the voltage somewhat
and stir the cell contents.

	Keep up the good work!

-- 
Paul Sears        Ottawa, Canada