The glassy carbon Anode: First test in a chlorate cell

Glassy carbon is a unique form of pure carbon with outstanding properties that set it apart from graphite.
While graphite is grey, soft and rubs off easily, glassy carbon is shiny, black, hard and does not leave a line when rubbed on paper.
It is also extremely chemically resistant (no acid, base, or oxidising melt attacks it) and oxidises about ten times slower than graphite in air at high temperature.
It is used for making crucibles for high-end analytical equipment and for melting noble and reactive metals. Glassy carbon has remarkable nonwetting properties- molten metals, salts and even glasses do not wet it.

Glassy carbon crucibles find use in dentistry for melting precious metal alloys without any fluxes.

The idea of using glassy carbon as an inert anode has been brought up before: http://www.sciencemadness.org/talk/viewthread.php?tid=4411#p...
but I haven't heard or read of anyone trying it in a chlorate cell, so I assembled a test cell using a glassy carbon melting crucible.

This is the glassy carbon crucible I'm using, 10cm long and 2,4cm outside diameter, trade name SIGRADUR.


This is how it's attached to the cell lid using a piece of PP pipe and hotmelt glue. The cathodes are strips of 1mm titanium sheet, 2cm wide, and sealed to the lid using RTV silicone caulk. Also note the offgas pipe.


The anode is pressed against the cell lid by a bent piece of Ti sheet. This is meant to prevent the anode falling off when the hotmelt glue is softening or being attacked.


The electrical connections. The anode is contacted by a thin strip of titanium running down it.


The setup- the offgas passes through a washing bottle with 10% NaOH to absorb the chlorine and prevent it from stinking up the house (I'm running the cell in my living room).


The cell, filled with 500ml of saturated NaCl solution (no additives, no pH control) and started at 500mA.


The cell running at 4A. It clouded up rapidly from the hydrogen bubbles after this picture was taken.


The cathodes are running at 62 mA/cm2, with unprotected backside. The anode is running at about the same current density.
Shortly after turning on, the headspace above the solution was colored green from the evolving chlorine. After about 20 minutes had passed, the color had disappeared, and now, after 1 hour, practically no more chlorine is being evolved, as evidenced by lack of smell when the plastic hose to the washing bottle is disconnected.
Instead, the offgas from the washing bottle has started to smell of ozone (!). This is reminiscent of my platinum anode perchlorate cell, which also produces ozone. Perhaps glassy carbon might even make perchlorate- but for now, I'm trying to find out how it holds up in a sodium chlorate cell without pH control and high anodic current density.

Since there is no pH control, 9 electrons will be needed to make 1 mole of chlorate, and I'm going to let it run for the time that is theoretically needed to bring the NaCl content down to 100g/L.


[Edited on 23-1-2011 by garage chemist]

Picture of the cell after 50 hours; 200Ah have flown.
The liquid level has sunk by about 1cm, it was topped up with 50ml saturated NaCl solution.
The cell has been standing in a water bath for cooling, this keeps its temperature below 50°C.

There is no black sludge at all. I think this picture makes it clear to anyone who has ever run a graphite anode chlorate cell with no pH control and high anodic current density that glassy carbon does not shred like graphite and holds up much better in alkaline solution.

The surface of the anode is still glassy, but there are many dull spots where tiny shards have separated from the surface. These black shards can be seen at the bottom of the cell.




[Edited on 25-1-2011 by garage chemist]

Results after the cell has run for 88h and 350Ah have passed through it:
The anode has suffered visible erosion, but it looks nothing like graphite. The surface is still glassy, but there are many shallow holes where pieces have spalled off. Diameter reduction of the anode is very small, only 0,1mm at most, and the anode would most likely survive tens of additional runs if erosion continues at this rate.

I haven't analyzed the cell liquor yet- I have to decide on a method first. I was thinking of boiling down a neutralized sample, drying the residue at 150°C in an oven, weighing out 10g with an accuracy of 0,01g and heating it to total decomposition to measure weight loss. From this, a chlorate content of the dry salt can be calculated.
The only problem I see with this is the fact that heating chlorate to release oxygen always forms some perchlorate, which needs a temperature of at least 550°C to decompose, dangerously close to the softening point of borosilicate glass.
I don't want to heat it in a crucible due to the potential for loss of material through spattering.
Any ideas for a good method to determine chlorate in cell liquor? I don't want to use iodometric titration since this would require lengthy and laborious preparation and standardization of sodium thiosulfate solution- I would have to make and purify potassium iodate for use as a reference substance first.

See the picture for a close up of the used anode.

I think that glassy carbon is an interesting anode material which corrodes much less than graphite and does not turn the cell liquor black.
However, it is not the totally inert platinum equivalent anode that I had hoped it to be.
Whether it makes sense to use it in a chlorate cell depends on the price that it can be obtained for. It certainly has its advantages over graphite.

Experimentation concerning possible perchlorate formation on glassy carbon anode will be done when I have time.




[Edited on 1-2-2011 by garage chemist]

Quote: Originally posted by garage chemist

Any ideas for a good method to determine chlorate in cell liquor? I don't want to use iodometric titration since this would require lengthy and laborious preparation and standardization of sodium thiosulfate solution- I would have to make and purify potassium iodate for use as a reference substance first.

Quote: Originally posted by garage chemist

Glassy carbon is a unique form of pure carbon with outstanding properties that set it apart from graphite. While graphite is grey, soft and rubs off easily, glassy carbon is shiny, black, hard and does not leave a line when rubbed on paper. It is also extremely chemically resistant (no acid, base, or oxidising melt attacks it) and oxidises about ten times slower than graphite in air at high temperature. It is used for making crucibles for high-end analytical equipment and for melting noble and reactive metals. Glassy carbon has remarkable nonwetting properties- molten metals, salts and even glasses do not wet it. Glassy carbon crucibles find use in dentistry for melting precious metal alloys without any fluxes. The idea of using glassy carbon as an inert anode has been brought up before: http://www.sciencemadness.org/talk/viewthread.php?tid=4411#p... but I haven't heard or read of anyone trying it in a chlorate cell, so I assembled a test cell using a glassy carbon melting crucible. This is the glassy carbon crucible I'm using, 10cm long and 2,4cm outside diameter, trade name SIGRADUR. This is how it's attached to the cell lid using a piece of PP pipe and hotmelt glue. The cathodes are strips of 1mm titanium sheet, 2cm wide, and sealed to the lid using RTV silicone caulk. Also note the offgas pipe. The anode is pressed against the cell lid by a bent piece of Ti sheet. This is meant to prevent the anode falling off when the hotmelt glue is softening or being attacked. The electrical connections. The anode is contacted by a thin strip of titanium running down it. The setup- the offgas passes through a washing bottle with 10% NaOH to absorb the chlorine and prevent it from stinking up the house (I'm running the cell in my living room). The cell, filled with 500ml of saturated NaCl solution (no additives, no pH control) and started at 500mA. The cell running at 4A. It clouded up rapidly from the hydrogen bubbles after this picture was taken. The cathodes are running at 62 mA/cm2, with unprotected backside. The anode is running at about the same current density. Shortly after turning on, the headspace above the solution was colored green from the evolving chlorine. After about 20 minutes had passed, the color had disappeared, and now, after 1 hour, practically no more chlorine is being evolved, as evidenced by lack of smell when the plastic hose to the washing bottle is disconnected. Instead, the offgas from the washing bottle has started to smell of ozone (!). This is reminiscent of my platinum anode perchlorate cell, which also produces ozone. Perhaps glassy carbon might even make perchlorate- but for now, I'm trying to find out how it holds up in a sodium chlorate cell without pH control and high anodic current density. Since there is no pH control, 9 electrons will be needed to make 1 mole of chlorate, and I'm going to let it run for the time that is theoretically needed to bring the NaCl content down to 100g/L. [Edited on 23-1-2011 by garage chemist]

Simple Titrations

Quote: Originally posted by garage chemist

..................... Experimentation concerning possible perchlorate formation on glassy carbon anode will be done when I have time. [Edited on 1-2-2011 by garage chemist]

Addition of some MnO2 or KMnO4 to your chlorate might help reduce the temperature to set all oxygen free.
This procedure was used in old times labo to produce pure oxygen from dry chlorate with moderate heating.