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in titration, why does the solution go colourless after 30 secs?

basically during a titration experiment when you get a faint colour that persists for around 30 seconds than the end point has been reached. but after 30 seconds the solution becomes colourless why is this? im not really sure how to answer this. my assumption know is that it reverts to acid and has something to do with equilibrium. thanks to anyone that answers.

Update:

my bad phenolphthalein was the indicator

8 Answers

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  • 1 decade ago
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    The closer the titration is to the endpoint the longer the pink color will persist, but that generally will be only for a few seconds depending on the rate at which you are swirling the flask. I'm assuming that you are seeing the color fade after you have reached the endpoint and the flask is sitting still.

    Carbon dioxide from the air is often cited as a reason for the change in color of phenolphthalein from pink to colorless. But the catch is that the solution is often quite basic. For the color to change due to a change in pH, the solution would need a lot of CO2 dissolved in it.

    There is a second reason for the change of phenolphthalein from pink to colorless in solutions with high pH. There is a third form of the molecule which is colorless in solutions with high pH. The transition from pink to colorless is slow, and may account for what you are observing. In fact, it lends itself well to a kinetics experiment. I've been doing this as a kinetics experiment with my AP chemistry students for a number of years. You simply follow the color change as a function of time using a spectrophotometer or a colorimeter.

    Here is some info on the kinetics lab, and it has a nice section on the three forms of phenolphthalein and the slow transition from pink to colorless.

  • 1 decade ago

    Titration is done normally between an acid and a base.and an indicator is used which gives different colours with bases and acids. for example phenopthalein is pink with bases and colourless with acid. when a base is titrated with an acid the solution will be pink at the beginning and it would indicate the solution as base, when adding the acid drop by drop the base is neutralised. at a point all the bases will be neutralised and the next drop of the acid will turn the solution acidic so the phenopthalein will indicate the acid by turning colourless!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  • 1 decade ago

    If you have the faintest color possible then you are as close to the end point as possible using, I assume, phenolphthalein. Carbon dioxide from the air continues to dissolve into your solution making it slightly acidic again and the color goes away.

  • 7 years ago

    Acid base titrations are neutralisation titrations so when end point reaches then phenolphthalein indicator get changed structurally. i.e from benzenoid to quinonoid so the colour changes from pink to colourless.remain fairly persistant.if you are talking about the strong acid & strong base titration.the salt formed is irreversible i.e not under going further hydrolysis.

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  • 1 decade ago

    Oxidation by the air - I assume you're using phenolphthalein.

  • 1 decade ago

    It has to do with the rate of reaction. At 'Null point' the colour persists for quite sometime.

  • 1 decade ago

    ABOUT WHICH TYPE OF TITRATION YOU ARE TALKING I CAN NOT UNDERSTAND

    IF THE END IS ACHIEVED THEN THE COLOUR WILL NOT FAINT

    IF THE COLOUR FAINTS IT MEANS THE END POINT HAS NOT ARRIVED

    i. e. THE REACTION IS NOT COMPLETED THEREFORE YOU HAVE TO ADD

    MORE SOLUTION FROM BURETTE. TO COMPLETE THE REACTION.

    ONCE THE END IS ARRIVED THE COLOUR WILL NOT FAINT FOR EVER.

  • 1 decade ago

    due to comlition of the reactionin the present of suitable indicator

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