I recently purchased a set of MMO anodes with corresponding titanium cathodes. Now I find something very peculiar. I have a small set (2 cm width, 6
cm length) of MMO anode and similarly sized titanium cathode. I already used this set quite a few times in a small cell for making KClO3 and CsClO3.
Both electrodes kept up very well in that cell.
Now I made a somewhat larger cell, using a 250 ml Duran bottle and using the same set of electrodes. I filled the cell with concentrated solution of
NaCl and also have some solid NaCl on the bottom. I also added a pinch of Na2Cr2O7. My next goal is to make NaClO3, which is much harder to isolate
than CsClO3 and KClO3. The cell is running at a higher current than the 100 ml cell I used with KCl. The cell also becomes hot, I think well above 60
C. I only can stand touching it for a second or so.
After 18 hours of electrolysis, the anode still is in perfect condition, but the cathode is eroded, and I also see many grey particles on top of the
layer of solid NaCl. The cathode is still perfectly smooth at the side which is faced towards the anode. The other side of the cathode has become much
darker and its surface is somewhat rough.
I also noticed something else very peculiar. If I switch off the current, then production of gas immediately stops at the cathode, but a fairly strong
production of gas occurs at the MMO anode.
I took out the set of electrodes, rinsed them with water and then put them back in the cell with the still hot solution of NaCl/NaClO3. When I do
this, immediately some gas is formed at the MMO anode, without being connected to the power supply.
Could it be that the titanium cathode absorps hydrogen and that this makes the metal brittle? Why is only the side of the cathode, which is farthest
away from the anode corroded?hissingnoise - 14-11-2010 at 06:17
Quote:
Could it be that the titanium cathode absorps hydrogen and that this makes the metal brittle? Why is only the side of the cathode, which is farthest
away from the anode corroded?
It's certainly a puzzle, woelen!
Not that it'll help, but I found this . . .
How far apart were the electrodes separated in your old cell vs. the new one?smuv - 14-11-2010 at 14:41
This is so crazy, but could it be titanium hydride which gets oxidized by the chlorate? woelen - 14-11-2010 at 23:52
In my new cell the distance between the electrodes is 2 cm instead of almost 3 cm. This allows a higher current to flow. The cell is larger, so some
more current (and corresponding production of heat) is not bad at all.
I put the cell in a cold water bath and after that I just let the cell run for a longer time. It feels hot, but now I can keep it in my hands without
excessive pain.
A pitted titanium cathode is not as bad as an eroded anode. Cathodes can be made of many materials and can be replaced relatively easily. I might
order a set of titanium cathodes from 5bears, just to have some spares. But this whole mess remains a mystery to me.
Now, again 18 hours later, the dark specks of solid material have disappeared, but now there is a very fine flocculent turbidity in the liquid.
Apparently the dark specks are oxidized in the hot chlorate-containing liquid.
[Edited on 15-11-10 by woelen]watson.fawkes - 15-11-2010 at 06:38
In my new cell the distance between the electrodes is 2 cm instead of almost 3 cm. This allows a higher current to flow. The cell is larger, so some
more current (and corresponding production of heat) is not bad at all.
I would have to guess that the
current density on the back side of the electrode is significantly different than on the front side. Having the electrodes closer will increasing the
disparity in CD between front and back sides. If you replace the cathode, consider a mesh cathode or an array of rod cathodes, both of which will have
much less of this problem.woelen - 16-11-2010 at 00:52
Right now, the cathode also erodes on the side, which is pointed towards the anode. I again took out the anode and cathode and rinsed them with cold
water. Next, I put some hydrochloric acid on both electrodes to make them really clean. What I see is that the acidic rinse liquid from the anode is
clear and pale yellow, the liquid from the cathode is grey and turbid. Very fine solid material can be wiped away from the cathode. After treatment
with the acid, the cathode feels smooth again.
One thing which I really have learnt from this experiment is that cathodes also are more sensitive to erosion than I ever imagined. In a next cell,
I'll put the cathode plate perpendicular to the anode mesh, instead of in parallel. In that way, both sides of the cathode are equally treated.bquirky - 16-11-2010 at 01:56
A random stab in the dark from a diffrent angle.
Chould it be that the physical agitation from the bubbling on the cathode surface is removing a small amount of the protective oxide coating causing
the erosion ?
hissingnoise - 16-11-2010 at 02:32
Quote:
One thing which I really have learnt from this experiment is that cathodes also are more sensitive to erosion than I ever imagined.
I assume you've checked your rectifier for leakage but I mention it in case you missed it.
