dann2
International Hazard
Posts: 1523
Registered: 31-1-2007
Member Is Offline
Mood: No Mood
|
|
Reaction in Chlorate cells
Hello,
This subject has been beaten to death in a variety of places...... but lets beat it some more!!!
The first thing that happens is Chlorine gets made at the anode and Hydrogen gets made at the Cathode.
An anode material with properties that favour Chlorine discharge at the anode as opposed to Oxygen is wanted.
Next Chlorine reacts with water to form Hypochlorous acid (HClO) as per:
Cl<sub>2</sub> + H<sub>2</sub>O --> HClO + H<SUP>+</sup> + Cl<sup>-</sup>
AND Chlorine reacts with water to form Hypochlorite ions (ClO<SUP>-</SUP> as per:
Cl<SUB>2</SUB> + 2OH<SUP>-</SUP> --> ClO<SUP>-</SUP> + H<SUB>2</SUB>O + Cl<sup>-</sup>
The next reaction takes place in the bulk of the solution. (ASSUMING WE ARE CONTROLLING pH)
2HClO + ClO<SUP>-</SUP> --> ClO<sub>3</sub> <sup>-</sup> + 2H<SUP>+</SUP> +
2Cl<sup>-</sup>
Since we are not controlling the pH the species 2HClO + ClO<SUP>-</SUP> do not exist (as per the graph at bottom) therefor the reaction
above cannot take place. Our cells are approx. pH 9 or 10.
We are therefor dependent on the reaction below to make the Chlorate:
6ClO<sup>-</sup> + 3H<SUB>2</SUB>O - 6e ---> 2ClO<sub>3</sub><SUP>-</SUP>
+4Cl<sup>-</sup> + 6H<SUP>+</sup> + 3/2 O<sub>2</sub>
What I wanted to ask is:
It the last reaction stated here a 'difficult' reaction for an anode material to perform?
Or to put it a different way. Will anode materials get it 'easier' or 'more difficult' to get this reaction to happen as compared to making Chlorine
in the first place?
This never gets discussed in journals etc as ALL industrial setups use pH control. They would be foolish not to as it costs alot of power to make
Chlorate by this electrical route as opposed to getting the reactions to happen in the solution bulk (pH approx. 6.8).
The max. possible current efficiency with the non pH controlled cell is 66%.
[Edited on 11-3-2008 by dann2]
[Edited on 11-3-2008 by dann2]
[Edited on 11-3-2008 by dann2]
|
|
|
garage chemist
chemical wizard
Posts: 1803
Registered: 16-8-2004
Location: Germany
Member Is Offline
Mood: No Mood
|
|
Have you considered that the cathode makes OH- and the pH should ideally stay completely constant if no chlorine escapes?
The hypochlorite ions can also be oxidised to chlorate at the anode instead of disproportionating in the bulk of solution.
This oxidation requires more energy than the disproportionation to make the same amount of chlorate, but current efficiency is not so important to the
amateur as it is to industry.
The anodic oxidation is favored if the temperature is low and the pH high.
|
|
|
12AX7
Post Harlot
Posts: 4803
Registered: 8-3-2005
Location: oscillating
Member Is Offline
Mood: informative
|
|
OCl- is rather basic (pKa ~ 7.3 in the picture), and a substantial amount of chloride will be converted to hypochlorite (10%?). The pH will rise even
in a sealed anode chamber.
Tim
|
|
|
dann2
International Hazard
Posts: 1523
Registered: 31-1-2007
Member Is Offline
Mood: No Mood
|
|
Hello,
Thanks for replys.
When pH is low ( when you start cell up) there is always a strong smell of Chlorine.
If pH is held at 6.8 (by adding acid) over a period of time will Chlorine escaping from cell remain constant or will it slow down as the cell gets
'low' in Chlorine?
I asked about the reaction whereby Hypochlorite ion is converted into Chlorate ion at the anode (high pH cell) as I am getting very poor current
efficiency with an LD anode. I thought my LD may not be good at making the reaction (Hypochlorite to Chlorate)happen. It makes Chlorine OK AFAIK.
Perhaps I have a poor LD type surface for the job in hand.
I was thinking of setting up a syringe pump with HCl going to the cell.
I have read a figure of 14KG of (100%)HCl was needed per ton Chlorate produced in a particular set up. They got 94% current efficiency.
Not too sure exactly what they mean by 100% HCl? Would that mean you need three times that amount (42kg) if you were using 33.3% Hydrochloric acid?
They probably did not use 33% HCl (or 100%!) in practive, it's just the way they stated the figure when reporting there findings?
I will be using approx. 6% HCl when/if I get a syringe pump.
DANN2
|
|
|
Rosco Bodine
Banned
Posts: 6370
Registered: 29-9-2004
Member Is Offline
Mood: analytical
|
|
One thing you should consider is that the graph will vary
according to the catalytic effect of certain anode coatings .
For example just as cobalt favors chlorine evolution , it also catalyzes disproportionation of sodium hypochlorite
to sodium chlorate and sodium chloride , at much lower temperature , ionic concentration , and at a higher pH
than would normally apply to a sodium hypochlorite solution disproportionation absent that catalytic effect .
So things like cobalt , nickel , and copper will have bearing
on what is the most efficient operating pH , but that may also be at a higher pH than is allowable because of the erosion of the anode . The
operating pH which is found to be most useful is probably then a comprimise governed by
the economics .
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
