Don’t you ever get that feeling of frustration at having a huge chunk of some unknown (but probably valuable) metal and not knowing exactly what it is? I didn’t think it would be an emotion I would ever experience.
Over half term I got the chance to do a week of work experience at a minor metals traders in Walton-on-Thames, and apart from doing research for them I had the chance to sort through all of their samples and “dispose” of the unwanted stuff. This included a massive, toilet seat-sized block of what I was told might be Titanium. Some teachers doubted its identity; after all, it weighed around 2 Kg.
In order to quench my fears, myself and a few other students got together on a cold Monday along with one of our chemistry teachers. He had devised an ingenious plan – but it takes a little bit of explaining…
Anodising is a process where a metal is deposited with very thin layers of its own oxide because of an applied voltage and the presence of an oxygen-liberating (mineral/inorganic/carboxylic) acid. According to the voltage applied, different thicknesses of layer may be built up, and due to the constructive and destructive interferences caused by different phase changes, a spectrum of colours can be generated.
Constructive and destructive interference can be likened to two people standing at opposite ends of a narrow swimming pool with paddles. Suppose that one person generates ripples and then a short while later the other person does the same. Now focus on the middle of the swimming pool, where unfortunately the crest of one wave might collide with the trough of another. Result? Flat water. The displacements of the waves at each position have been summed. In constructive interference, I’m sure you can see how things work: two waves collide head on and they superimpose. The phases of the two paddles determine what one sees in the middle of the pool. In the same way, the two reflected rays in the diagram above will be more or less out of phase depending on the thickness of the oxide layer (and other factors, which were constant.)
To turn the ‘titanium’ some pretty colours we did indeed dip it in acid and apply a voltage. We only had a voltage of up to 15V, which we were instructed might not be enough for certain thicknesses, but we thought it wise to attempt the experiment anyway. We used 1M phosphoric acid in a plastic tray for our oxygen source and a piece of aluminium as our negative electrode.
As can be seen, a blue-violet colour has appeared on the submersed end of the metal. originally there was a band of yellow but as we left the experiment running and increased the voltage, the band changed. The reason, I discovered, that higher voltages cause thicker films is simply due to the fact that Titanium Dioxide is an electrical insulator: as the thickness of the layer builds up, more voltage is required to continue to build it up. the voltage required to deposit the oxide on the metal remains the same, however with a greater resistance more voltage is needed to deposit the layer.
So, how thick is this lustrous layer of lavender? Below I attempt to find out with the knowledge that if we as observers are seeing blue/violet, that all of the other wavelengths have interfered destructively or “cancelled out”. All that reaches our eyes are the wavelengths which aren’t cancelled out.
Overall I’m really happy with this experiment: only a few metals are known to be anodisable, so we genuinely have narrowed this slab’s identity down to about six elements. Even if this adventure wasn’t conclusive, however, it certainly was fun. To think that the colour in the picture below is not even real; no dye, no paint, just the interactions of light around an oxide layer…is mind blowing…