Potassium bitartrate is the salt of a weak biprotic acid.
Acc. this source:
http://pubs.acs.org/doi/abs/10.1021/ac60010a017
... a 0.03 M solution of potassium bitartrate in water at 25 C has a pH of about 3.6, significantly lower than 7.
So drain of your supernatant liquid and wash your crystals with cold deionised water, then recrystallize the presumed KCl. If the new supernatant
liquid has a pH significantly below 7 it probably still contains potassium bitartrate.
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