Made up some NaOH for titration practice a lil while ago (an entire litre of the stuff). Used distilled/deionized water, tested it for pH before mixing, used cleaned standard flask, bottles, stirrers, the whole shebang.
Then I went to actually do one of these titrations. Pippetted 20cc into a conical flask, added Universal Indicator, went to get a sheet of paper (for a background), came back and WTF!!! My nice purple pH 12 coloured NaOH was a nasty green pH 7.
So i assumed i had done something wrong and remade everything using equipment triple-rinsed in deionized water. Same result.
Any ideas? I think the NaOH pellets might be off, but it only neutralises when in contact with the air - I put some in an airtight test tube with some UI and nothing happened. Also repeated all with Methyl Orange and phenylthalein, same result.
THEN I tested some of the other NaOH. They were from a variety of storage cupboards, all sealed containers, mostly standard 1.0mol.dm^3 bench solutions used in practical tests, some 2.0mol.dm^3. Same problem. Now, we didn't have this problem last year, all the NaOH was fine. No other chemicals have been affected.