G’day I had to make a deionising plant for laboratories many years
ago, and I had the outgoing deionised water pass through a tube
containing pure platinum electrodes with a known surface area to
measure it’s purity. Good tap water had an electrical resistance of
56,000 ohms per square centimetre and the deionised water has a
resistance of 5 million ohms (meg ohms) per square centimetre. But it
still contained the organic volatiles which weren’t ionised. So for
very pure water, we put distilled water through the columns and the
outgoing water had a resistance of 5 million ohms, but no volatiles,
so it really was pure. But contrary to what one might expect, it
didn’t taste very nice at all; the taste can only be described as
’flat’. So is it really permanently pure? Only if contained in a
completely airtight, non reactive vessel. The instant this very
pure water is exposed to the atmosphere it begins to dissolve firstly,
carbon dioxide, which very quickly lowers the electrical resistance,
then it starts to absorb air. Not that this affects it for most
laboratory purposes, but it can no longer be regarded as
scientifically pure water. All a bit academic. I know, I know; but
it’s hard to shut me up. I sometimes got walloped for it at school;
but it obviously wasn’t any good. Nothing worked. Cheers, –
John Burgess