posted by ~christina~ .
I'm going to do the Baeyer test (Pottasium permanganate)
Instructions say to disslove 25mg of unknown or 2 drops of liquid unknown in 2ml of 95% ethanol. Slowly add 1% aqueous solution (weight/volume) of potassium permanganate, drop by drop with shaking, to the unknown.
In a possitive test, the purple color of the reagent is discharged, and a brown precipitate of manganese dioxide forms, usually within one minute.
My question is, what do they mean by discharge? Do they mean it disappears and the manganese dioxide forms or does it stay and the manganese dioxide forms?
It means the purple color fades and goes away. The MnO2 forms as a ppt. I suppose you COULD add enough permanganate so that you have an excess of permanganate in which case all of the unknown would be used up (in a positive test) and excess permanganate would then color the solution purple. But the intent of the test is to add the permanganate drop by drop, as your instructions indicate. Then the disappearance of the purple color and the formation of the brown/black MnO2 is what is called a positive test.
Okay. The reason I was confused was that in another test they just plain stated "when the color disappears...positive test result" but then they said discharge in this test.