The USP Pharmacopeia still contains the classical wet chemistry tests for many ions. It was supposed to replace them with instrumental tests -- for example flame AA or even ICP, which come in tabletop models these days. However, that;s been pushed back to 2018. Therefore:
In neutral, concentrated or moderately concentrated solutions of potassium salts (depending upon the solubility and the potassium content), sodium bitartrate TS produces a white crystalline precipitate that is soluble in 6 N ammonium hydroxide and in solutions of alkali hydroxides and carbonates. The formation of the precipitate, which is usually slow, is accelerated by stirring or rubbing the inside of the test tube with a glass rod. The addition of a small amount of glacial acetic acid or alcohol also promotes the precipitation.
I've done this a number of times. I suppose if someone sold me sodium sorbate instead of potassium sorbate, then the test would fail. But really, this test won't help with a minor impurity, and a major contamination would alter the results of other, quantitative tests, so why do it at all?