Quote:
Originally Posted by bojan
The water solution must always be fresh, after couple of days it stops being active.. so I suppose it must be oxidised (what else could happen to it?)
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Bojan, as Geoff said NaOH sucks up water from the atmosphere with the greatest of ease and it is this that makes it totally useless as a primary standard. Once wet it can react with many things e.g. forming insoluble solids with just about any metal outside of Na and K. It even reacts with glass if concentrated enough. Lucky there are plentiful supplies available under the name "caustic soda" found on many hardware and super market shelves. The trick to keeping it fresh is to keep it in a well sealed container and only open it in dry warm conditions to prevent moisture being absorbed.
Mark