[Zerm]

I was originally thinking you could get a qualitative idea of purity by comparing the reaction rate of the iron and chlorine in the tubes.  However, a quanitative method is perhaps called for.  

One could perhaps test the purity by liquid volume.  After capturing a fixed volume of gas in each tube, carefully seal the tubes.  Submerge all three tubes in an dry ice / acetone cooling bath.  This will condense the chlorine but leave oxygen in the gas phase.  Once the gasses have condensed, mark the height of the liquid on the side of the tube.  Remove the tubes outside and unseal them allowing all gasses to disperse.  Afterwards, fill the tubes up to the lines with water.  Measure the amount of water it takes to fill the line to get the volume of chlorine gas in each tube.  

This experiment would require test tubes that can withstand a bit of vaccum pressure.  It also operates under the assumption that the catalyzed tubes represent pure forms of their respective gasses.  If there were impurities that would also be condensed by the cooling bath, however, they would likely be present in the oxygen tube as well.  You could measure the volume of these condensed impurity gasses to get a rough idea of how much to offset your chlorine volume measurements by.  

Use density to get the mass of chlorine and then the number of moles.  To get the amount of oxygen in the electrolysis tube, simply take the extra volume of the tube and use the gas laws to calculate the number of moles of oxygen in that volume of gas at the temperature of the cooling bath.  This process may be involved but I think it would yield fairly definitive results for a given electrolytic cell.

[vmelkon]

Thanks for all the replies

As for these links posted by billnotgatez
http://electrochem.cwru.edu/encycl/art-b01-brine.htm
http://en.wikipedia.org/wiki/Chloralkali_process
http://scienceaid.co.uk/chemistry/applied/electrolysis.html

I don't see anything about the voltage to use. Am I suppose to use 0.5 V or 1 million?

Although it seems like common knowledge that you get H2 and Cl2, this is incorrect as Grundalizer mentioned several posts above.
Here is my youtube video
http://www.youtube.com/watch?v=6WwytVbSKUI

[zaphraud]

Since you've got graphite rods on hand anyways, why not use a large one scooped out as the bottom of the vessel and negative electrode, top electrode put in below the solid salt crystals.. then heat the bottom one with a largely independent circuit providing lots of power, until the salt within it melts?

                                   (+ side of AC-DC adapter)
                                   |
                                   |
(AC)---------\___________________/-----------(AC-AC transformer)
                                   |
                                  (-)

You'll also get sodium metal. Be careful.

[vmelkon]

That wouldn't work since salt melts at 801 °C and sodium will float to the top and evaporate and burn.

Besides, the point of this thread is electrolysis of a salt solution and what you get at the anode.
There is a level of crapiness in chemistry textbooks, online material and TV shows when it comes to this subject and I wanted to clear things up.

[fledarmus]

The problem with chlorine generation at the anode is the presence of hydroxide ions. In the presence of hydroxide ions, chlorine oxides are formed at the anode. On an industrial level, membranes are used to separate the anode and cathode solutions so chlorine will not be mixing with sodium hydroxide.

[aarcain]

No matter how hard you try, you cannot get chlorine gas from the electrolysis of aqueous sodium chloride (NaCl).  You will get Hydrogen and Oxygen gas.  

If you want O2 gas it's much easier to just take apart an alkaline battery and extract the black manganese dioxide, then add store bought hydrogen peroxide.  Oxygen gas will bubble out which you can then use to make things burn more brightly, which is pretty cool.

Why do you want chlorine gas anyway?  Breathing that stuff in will instantly destroy your lungs

It does produce chlorine gas if the NaCl does not have iodine in it. . I have used this to produce chlorine gas in the lab.