Question

How does the xylenol orange indicator work ? The colour of the solution either side of the end point when it is reacted with a solution of metal eluents( nickel) and edta and titrated with zinc

Answer 1

Xylenol Orange is a big molecule containing carboxylic acid groups --COOH.

Don't try to remember its structure let's just say it is RCOOH

In water it ionises as below and we have an EQUILIBRIUM [ie Reversble Reaction]

RCOOH + H₂O   à   RCOO^- + H₃O⁺

Now RCOOH is RED and RCOO^- is YELLOW

If the solution is acidic [ i.e contains a lot of H₃O⁺ ions] the equilibrium is pushed to the Left

[i.e contains a lot of RCOOH] and this is RED. If solution is alkaline the equilibrium moves to right

[i.e contains a lot of RCOO^-] and the solution is YELLOW.

All Acid -Base Indicators work in this way.