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Question Phenolphthalein going clear by itself.

Nov 27, 2019
I work in a lab and each day we have to analyse a sample for percentage caustic. We add phenolphthalein and titrate with 5M HCl. Today I got a sample, added the phenolphthalein, it went pink as normal, but then within 20 seconds it had returned clear, no matter how much phenolphthalein I added. Then I added the ACID to the clear mixture of sample + phenolphthalein and it turned pink and remained pink whilst also heating up and emitting a vapour.appvalley tutuapp tweakbox

Does anyone have any idea what the sample was? The pH was 12.3.
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