Sciencemadness Discussion Board

Reaction mechanism? Anodic oxidation of chloride

woelen - 22-9-2010 at 07:08

At the moment I am experimenting with a miniature chlorate cell, using MMO anodes and titanium cathodes and this works quite well. A write-up on constructing a nice indoor-use chlorate cell for making research-quantities of KClO3 (e.g. in the order of magnitude of 30 gram per day) will follow soon and will be posted here.

From a practical point of view I am quite happy with the result, but I also did some theoretical reading about the subject and found some peculiar things.

In the ideal world, for each mol of KClO3 produced, 6 mols of electrons are needed. Chlorine goes from oxidation state -1 to oxidation state +5 and this is a change of 6.

The idealized reaction mechanism is:

At cathode: 6H2O + 6e --> H2 + 6OH(-)
At anode: 6Cl(-) - 6e --> 3Cl2

In solution (bulk), due to mixing and heating up of cell:
3Cl2 + 6OH(-) --> 3Cl(-) + 3ClO(-) + 3H2O
ClO(-) + H2O <--> HOCl + OH(-)
2ClO(-) + HOCl --> 2Cl(-) + ClO3(-) + H(+)
This set of reactions works best at pH around 6.

At high pH, there hardly is any HOCl and then you only get ClO(-) in the solution. With ClO(-) only, the formation of chlorate in solution is very slow and this is not the primary reaction under these conditions. At high pH (which occurs naturally in a cell, due to initial loss of Cl2 to the atmosphere, while OH(-) from the cathode region remains in solution) it is said that chlorate is formed directly at the anode:

6ClO(-) + 3H2O - 6e --> 2ClO3(-) + 4Cl(-) + 6H(+) + 3[O], where 2[O] combines to gaseous O2.
So, 6 electrons are used to make 3[O] atoms, or 1.5 O2 molecules. One chlorate ion simply is derived from 3ClO(-) ions. So, all electrons, used in this reaction can be considered as a loss.
For making 6ClO(-), 12 electrons are needed. So, in total 18 electrons are used, while only 12 electrons make useful chlorate, the rest is used for making oxygen. In this way never more than 2/3 current efficiency (66.7%) can be reached.

What is the mechanism behind the reaction, given above? It does not look like oxidation of the chlorine at all, there still simply is disproportionation of hypochlorite to chlorate and chloride and the reaction looks as a combination of two independent ones:

Disproportionation: 6ClO(-) --> 2ClO3(-) + 4Cl(-)
Oxidation of water: 3H2O - 6e --> 6H(+) + 3[O] --> 6H(+) + 3/2 O2

I am really wondering what is the mechanistic pathway for the total reaction. I could not find any info about this. There might be info about this, but usually paid-by-subscription and I have no access to those sites.

This is just an academic question. The answer does not affect the results of my experiments, but it is just out of scientific curiousity that I would like to understand this reaction.


[Edited on 22-9-10 by woelen]

dann2 - 22-9-2010 at 13:03

Hello Wolen,

There is some reading at this address which may be useful.
http://www.oxidizing.110mb.com/chlorate/further/bedtime.html

Perhaps you seen it before.

Dann2

kmno4 - 25-9-2010 at 06:30

This reaction:

6ClO(-) + 3H2O - 6e --> 2ClO3(-) + 4Cl(-) + 6H(+) + 3/2O2 (A)

looks not very good and I think it is useful only for explanation of
lower current efficiency caused by evolution of oxygen.
Woelen has noticed interesting thing - this equation may be a sum of another equations :
6ClO(-) --> 2ClO3(-) + 4Cl(-) (B)
3H2O - 6e --> --> 6H(+) + 3/2 O2 (C)
Then A = B+C
But equally good I can write another equations which are linear combination of B and C.
I can also incorporate another equation:
HClO -> H(+) + Cl(-) +1/2O2 D
and I can make another equation(s), let us say "W"
W = xC + yC + zD
It looks that loss of efficiency is caused - summa sumarum - by electrochemical reaction C .
If it is true - I would not bet, because spliting A into B and C is not so obvious. But
B and C look so familiar and how nicely they can be combined in A ;)

12AX7 - 25-9-2010 at 08:23

If you want elemental electrode reactions, you'll have to look a lot closer than the overall result. For starters, I would definitely look at what ions become radicals the best, since electron addition and subtraction is ultimately an individual operation.

For instance, are there any transient chlorate radicals which would explain the apparent six electron, single step chlorate reaction?

Tim

woelen - 25-9-2010 at 09:49

I have done a little research myself also and found snippets of information scattered on the Internet. The reaction mechanism seems to involve discharge of hypochlorite ions, so one gets ClO radicals in solution, which react with water.

So, the reaction 6ClO(-) + 3H2O - 6e --> 2ClO3(-) + 4Cl(-) + 6H(+) + 1½ O2 indeed is a precise description and not a linear combination of two simpler reactions, as I first though. The reaction goes through the ClO radicals, but the precise mechanism still seems to be understood incompletely.

12AX7 - 26-9-2010 at 21:32

Could perhaps be a zipper of radical additions? ClO. + ClO(-) ==> ClO2. + Cl(-), ClO2. + ClO(-) ==> ClO3(-) + Cl., the assumption being perhaps ClO2(-) is less stable than the transient radical.

Or maybe ClO2 is somewhat indifferent, and tends to dissociate to ClO again, explaining the lack of chlorite production in this process. It might be interesting to electrolyze a chlorite solution and see if hypochlorite is produced at the anode, as the result of radical reactions.

For the majority pathway, it seems likely that HOCl has something to do with it, since the reaction proceeds best at a nominal pH, not strongly acidic (which favors Cl2(g)) or basic (which favors ClO-).

Tim

woelen - 27-9-2010 at 04:30

There is one main problem with your proposed solution. The reaction also creates oxygen and your proposed mechanism produces chlorate from hypochlorite through the radical discharge mechanism. In some way, the mechanism should yield three oxygen atoms (which combine to 1.5 molecule of O2).

12AX7 - 27-9-2010 at 07:02

Something with superoxide, perhaps? That's a notorious radical...

Perhaps it would form by the reaction of various ClOx radicals with water?

Tim

dann2 - 29-12-2010 at 13:47
Quote:
I am really wondering what is the mechanistic pathway for the total reaction. I could not find any info about this. There might be info about this, but usually paid-by-subscription and I have no access to those sites.

This is just an academic question. The answer does not affect the results of my experiments, but it is just out of scientific curiousity that I would like to understand this reaction.


If you Google 'anodic chlorate formation reactions' quite a few papers come up that can be had in the references section (supplied by the kind folks there). The DOI's are available from google.
I am totally bandwidth limited myself and cannot download the DOI's etc as I would request the papers myself if I were no so bandwidth limited.

Dann2

dann2 - 25-1-2011 at 14:03

Hello,

Some refs. on Anodic Chlorate formation thanks to the good dude's over in ref. section.
Wanted References and Needed Translations(6) 23 Jan 2011
Four papers to be had if anyone is interested. I was going to post them here but
thought it's a bit wasteful of space. Posted one that I though was interesting (not so high fulutent).


Some (bit off topic) info. on Cathodic reduction from Electroehlmica Acta, 1964, Vol. 9, pp 1 to 16. (page 4) (attached below)


The action of the chromate is
explained by the formation on the cathode surface of a thin film, probably consisting
of chromium oxides, which reduces the effective cathode surface area and thus the
reduction of hypochlorite. The same effect can be attained in chromate-free electrolytes
by using a chromium-plated cathode.

There is a nice graph giving a idea of Cathodic reduction via Cathode currend density. It appears that
higher concentrations of NaCl or/and Chlorate lead to less Cathodic reduction (~ 4% less @ 60mA per square cm Cathode cd). Lot's more at lower Cathode cd's.


Anodic Formation of Chlorate (Anodic Oxidation of Chloride) is an UNWANTED reaction in pH controlled cells.
It's the way all Chlorate gets made in the average (non pH controlled) amateur cell.

Dann2

Attachment: eca_1964.pdf (672kB)
This file has been downloaded 1065 times

[Edited on 25-1-2011 by dann2]

blogfast25 - 26-1-2011 at 12:25

Woelen@

Probably a silly question but what are 'MMO anodes'?

DJF90 - 26-1-2011 at 12:36

Mixed metal oxides.

dann2 - 28-1-2011 at 11:59


Hello,

Would anyone know what would be the approx. Hypochlorite concentration in a Chlorate cell that has a high pH (not pH controlled)?
Figures given for industrial Na Chlorate cells (pH controlled) give figures for Hypochlorite (I presume they mean actual Sodium Hypochlorite) of 2 to 5 grams per liter (from Kirk Othmer or Ullman Ency.).
Cathodic reduction is strongly influenced by Hypochlorite concentration.

Dann2

BaCON - 29-1-2011 at 03:24


I don't think there is an definitive explanation but I found a thesis by Lynda Nylen at KTH Chemical Science and Engineering in Stockholm titled "Critical Potential and Oxygen Evolution of the Chlorate Anode" to be very informative.
It is available in PDF at the link below

http://www.dissertations.se/dissertation/349c17e5ae/