[D45]

Hi all

I've recently been attempting to plan an experiment in which it is necessary to track the rate of reaction as a small piece of aluminium foil is anodised using approx. 2M sulphuric acid. My first thought was to measure the volume of hydrogen produced, but when I tried this using a Büchner flask the wires got in the way and the seal at the top of the flask was not air tight, meaning that the hydrogen escaped here and subsequently wasn't collected in the syringe (see image).

I've also tried recording the current through the system over 20 minutes, although the change is negligible for the majority of the process, but when the current does change it rises! I expected it to decrease due to the high resistivity of the new oxide layer, and so I'd be very grateful if anybody could explain this.

Other options could be to measure the change in mass, although by my calculations the total change would only be 1/100 of a gram...

My question is:
Can anybody think of a way to follow the rate of the reaction during anodising? Ideally I would like to do this by collecting the gas, and so is there a way that I could create a more gas-tight system without the wires to the power source being a problem?

Thanks in advance
David

Edit: before sealing the aluminium I also tried to dye it using cresol red at 70 degrees celsius. The plan is to use congo red for this in the main investigation, but I couldn't get cresol red to dye the aluminium at all. Out of interest, what may the reasons be for this?

[Arkcon]

Unless you can get the wires into the system air tight, you won't be able to use hydrogen produced quantitatively. I think that the mass change may be all you can look forward to.  Question:  if you have measured cc of hydrogen, what would have been your next calculation?

[Enthalpy]

Maybe you only degraded the quality of the pre-existing oxide layer, in which case the current may increase.

Sensitive scales can weigh the oxide layer. 2*5cm² and 0.2µm weigh 0.5mg.

2*5cm² and 0.2µm alumina would correspond to 0.04cm³ H₂, that's not much for a measure.

If you expect a thickness like 0.2µm, the optical effect is measureable.

[D45]

Thanks so much for the replies so far.
I have since repeated the anodising using aluminium that had been thoroughly de-greased is ith propanone and reacted with warm 2M NaOH before, and strangely the current is still smoothly increasing.

Arkcon: I've tried the change in mass and it seems that it could be a promising way to go. There may not even be any calculations afterwards; the plan is to compare the success of the anodising under different conditions (i.e. Voltages and temperatures), and so one dependant variable would be all that I need to plot the necessary graphs for analysing this. It may even be that the increasing current is sufficient for this, although I'd prefer to understand why that is happening!

Enthalpy: Would you mind please running me through your calculation for the volume of gas produced? Using the same figures I reach 1.4cm^3 by:
Calculating the volume of the layer
Using a density of 3.95g/cm^3 to calculate its mass
Finding the number of moles using mr=101.96
Multiplying this by 3 to find the no. Moles of hydrogen
Assuming 1 mol H2 occupies 24dm^3 at r.t.p

Thank you again guys!

[Enthalpy]

Well, I botched it, shame. Presently I'm at 0.56cm³ H₂: we converge.

0.78*10^-6 kg Al₂O₃
23µmol H₂