Titration: colour observed to fade

[priyanka.jatan]

Hello,

I was conducting an investigation to determine the amount of acetylsalicylic acid in Aspirin. After titrating the titrant (NaOH) against the analyte (solution of ethanol + 2x relatively dissolved aspirin tablets and 5x drops of phenolphthalein indicator), a colour change from a cloudy-opaque to a weak pink was observed. This colour change seemed permanent at the time, considering the flask containing the solution was swirled afterwards and the same pink intensity stayed. However, after approximately 8 minutes, the solution was observed to fade back to its original, cloudy state.

Why did this occur? Was it due to the endpoint not clearly being reached? Were the reactions between the chemicals still occurring? I'd greatly appreciate any reasons as to why this had occurred.

Thanks.

[bambii]

An indicator are coloured acid-base conjugates in equilibrium

ie. Let HIn be the indicator

Then HIn <--> H+ + In-

If we say HIn is yellow and In- is red, then we can obviously use this as an indicator ie. adding acid causes reaction to shift left, thus causing more HIn to be produced, thus the soln becomes more yellow.

What happened to you was that you shifted the equilibrium of the indicator by titrating it, and by letting it sit there, the reaction moved in a direction so that equilibrium was re-established. In your situation, the reaction favoured the consumption of the coloured (purple) conjugate and production of the colourless one.