garage chemist - 11-7-2005 at 13:06
I've built a perchlorate cell with platinum wire as an anode, and I'm planning to use dichromate as a catalyst to improve current
efficiency.
The problem is that the evolved hydrogen creates a fine spray mist that contains dichromate.
Dichromate dust is very highly carcinogenic.
The cell is built absolutely air- tight with a single vent pipe for the off-gases to escape through.
Now I need to make a filter that removes every single particle, as fine as it may be, from the off-gases.
I thought of several methods:
-a filter with tightly stuffed cotton wool or better glass wool
-a sintered-glass bubbler washing bottle filled with water (not sure if this removes every particle)
-a sintered glass filter with porosity Nr.3 (I have one of them, they are nifty for filtering very aggressive solutions)- not sure if this is fine
enough
-putting a venting pipe through the window to blow the gasses outside (worst idea)
How would one construct a gas filter capable of removing even nanosized particles from the gas stream?
12AX7 - 11-7-2005 at 17:34
Ok, you've got what one whole gram of Cr2O7 in solution? Ok maybe a bit more. Now assuming half the water sprays off, and 99% of those
particulates are cought by the vent pipe (you must be running 300A worth of hydrogen through a 2mm tube if any less than 99% of the dust is being
caught), that's 10mg that might escape. Dilute this by an average room of 15 x 15 x 8 feet = 51 m^3 and, for a one week run time, you get
200ug/m^3 at the end of the week, in a closed room, with no ventilation whatsoever. Now, listed PEL is 0.2mg/m^3, not only a paranoid worst-case
day-in-day-out-for-three-decades-no-symptoms rating, but there aren't many rooms on earth that have zero ventilation. I'd guess as little
as 2 to 20ug/m^3.
Tim
Dichromate particles
MadHatter - 11-7-2005 at 23:20
How much current are you running to kick up that much spray ? The worst problem I ever
had was it running up the cathode and drying out. You might want to look into using
fluorides or persulphates if it's a major concern. So far the fluorides have worked the
best.
garage chemist - 12-7-2005 at 04:15
I'll only use 1 ampere of current, to minimize the ohmic heating of the electrolyte (perchlorate cells like it cool, as opposed to chlorate cells
which like it hot) and also because my Pt wire is only 0,3mm thick and I don't want to go over 500mA/cm^2 current density at the anode.
So you think that I don't need to worry about dichromate spray?
There was some website about electroplating chromium from dichromate solution and they had drastic warnings about the dichromate spray from the
hydrogen. They added a drop of detergent to the solution to cover it with foam and make spray droplet formation impossible.
I don't want to use detergent (it would foam over!).
I think I'll use a combination of a cotton filter and a washing bottle.
Did you know that rain water contains sea algae, because of the spray that happens in the ocean with every wave?
About fluorides or persulfates: I am under the impression that they are only effective in cells with a lead dioxide anode (dichromate has a
detrimental effect on efficiency with PbO2 anodes, therefore this substitution).
It would be good if they also worked with Pt...
12AX7 - 12-7-2005 at 05:32
As I recall, F-, S2O8-- and Cr2O7-- all work comparably, with Cr2O7-- being a bad idea for PbO2.
Tim
garage chemist - 12-7-2005 at 07:47
Wouldn't the persulfate attack my copper cathode (it's used for etching copper, after all) or is it cathodically protected?
Then I would have to always keep current flowing through the cell, because corrosion would start as soon as the power is switched off. Not so
convenient.
Fluoride has the disadvantage of forming insoluble CaF2 as soon as calcium ions are added (even tap water contains enough Ca2+ for precipitation to
occur).
I also don't have sodium fluoride. Could I just add a ml of 40% hydrofluoric acid instead? Would I have to adjust the pH with NaOH or is the HF
weak enough to not cause problems?
Dichromate still seems to be the most convenient additive.
It can be very easily removed from the spent electrolyte by gassing with SO2 and making the pH slightly alkaline, Cr2O3 precipitates.
12AX7 - 12-7-2005 at 08:03
I don't know if persulfate would eat the copper, I doubt it as there's so little.
I don't know about ClO4 but chlorate cells like it slightly acidic (5.5-6.5). I wouldn't worry about it, unless it does move the pH too far
off the ideal range.
HF <--> H+ + F-, the proton would just as well escape as a chloride or chlorate, being weaker than HF or HClO4 (IIRC). You can always
neutralize the HF before adding it, which...would also have the exact same effect as adding the acid and neutralizing the cell pH. 
Tim
[Edited on 7-12-2005 by 12AX7]
They like acid
Lambda - 12-7-2005 at 11:47
Indeed Tim, they like it to be slightly acidic. A way to do this, is to buble CO2 gas through the electrocell, this works out just fine.
garage chemist - 13-7-2005 at 06:06
The filter problem is solved, I used the washing bottle and the glass frit, this seems to work fine with the rather slow gas stream I'm dealing
with. I used potassium dichromate, only 0,7g of which colored the 250ml of cell liquid bright orange.
Pics of the perchlorate cell soon to come.
On Wouters page, there is no info to the recommended pH of a perchlorate cell.
Chlorate cells like it slightly acidic, CO2 is of great help with them.
With perchlorate cells, pH doesn't seem to matter a lot, none of the configurations I've seen use pH control.
A perchlorate cell evolves absolutely NO chlorine as I've noticed (not the slightest hint of Cl2 smell, I'm using commercial NaClO3 of high
purity), so the pH can't go up too fast.
[Edited on 13-7-2005 by garage chemist]
garage chemist - 13-7-2005 at 13:13
Here a pic of my platinum anode (25cm 0,3mm Pt wire coiled around a glass pipe, the end is molten into the glass tip and the electrical connection
inside).
garage chemist - 13-7-2005 at 13:14
Here the entire perchlorate cell, with copper cathode and venting pipe.
garage chemist - 13-7-2005 at 13:23
Here a closeup of anode + cathode.
The cell has a capacity of 250ml 45% NaClO3 solution (density= 1,38, and free of chloride -very important when using Pt as the anode!). It has to run
4 days (96h) with 1 Ampere (at 5 Volt).
Theoretically, one can get 200g of KClO4 from it per run.
The residual chlorate will be destroyed with SO2, this also reduces the dichromate to Cr(III) which precipitates on basifying.
The cell only contains 0,38g of Pt with an anode surface of about 2 square cm.
Mixing of the electrolyte is done by a magnetic stirrer connected to a timer which switches it on for 5 seconds every minute (to save energy and
decrease the wear on the stirrer).
It has been running for about 10 hours until now and looks fine, the anode shows absolutely no signs of corrosion.
Plenty of H2 is produced at the cathode, on the anode, only a very slight oxygen evolution takes place (from splitting of the water).
