Quote:
Originally Posted by rat156
I doubt that it was the IPA dulling your Aluminium. Freshly cut Sodium reacts slowly with IPA, that's what we use to get rid of excess Sodium without too much of an exotherm. Probably dissolved Oxygen in the solvent. AlOxide coatings on fresh aluminium are essentially impervious to liquids, though gas molecules are small enough to permeate slowly. Alkaline or acidic solutions will slowly dissolve the AlOxide and dull the Aluminium surface, which is why you shouldn't leave your mirrors wet. Also remember that most aluminium available to us is an alloy, not pure aluminium as would be vacuum deposited.
Cheers
Stuart
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Stuart both were analytical grades (eg pure Al we use for experiments not an alloy). Like you we also get rid of excess Na metal in 2-propanol for the same reasons and yes the reaction is very slow. I cleaned the Al surface with emery then steelwool and plunged it straight into the alcohol. The sheen of the Al dulled mildly after about 10 mins. Obviously acid or hydroxide will damage the coating with ease (the hydroxide will etch the glass of the mirror as well if it is of a high enough molarity) so best to keep these well away. As for the mirror coating being AlO3 I am unsure. From what I have read on mirror coating it is only possible to achieve the quality required in a vacuum chamber which is evacuated then the Al atomised to form the coat. In any case I would not put alcohols near the mirror sticking to de-ionised water and a mild detergent in the worst cases. Like you my initial thought on this was the 2-propanol must have been contaminated but I am not so sure after mucking around today. I am not excluding dissolved oxygen but I have never seen this happen using good old detergent and de-ionised water and think it's better to be safe then sorry.
Mark