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Such a simple topic - so many problems! The commonest are
- the colour change at the endpoint of a titration is often given the wrong way round;
- the colour change is often not specified;
- 'clear' does not mean 'colourless'.
Suggest why, in a titration involving potassium manganate(VII), this reagent is usually placed in the burette.
This simple question arose in June 1997; it was not well answered. The reagent is intensely purple, and when it reacts it turns colourless. The endpoint is when the titrating reagent is in excess, so the reaction mixture will (just) turn pink. The reason for placing it is the burette is simply that the colour change from colourless (usually) to pale pink is easier to see than the reverse, pink to colourless. The very common error was to give the colour change the wrong way round.
The exception (there are always some) is in the titration of nitrite ions; to prevent loss of nitrogen dioxide during the titration, acidified potassium manganate(VII) solution is placed in an evaporating basin and the solution of sodium nitrate run in. As soon as nitrous acid is formed it is oxidised.
Which indicator is used when iodine is titrated with sodium thiosulphate solution, and what colour change is seen? State with a reason at which stage of the titration it is added.
The indicator is starch solution, which changes from bluish-black to colourless.
The indicator is added when the solution has become a pale straw colour. If added too early, the indicator forms an insoluble complex with iodine, and the titre is too low.
The common error: to say that the indicator is 'added near the endpoint'. True - but I cannot know what the candidate means by this. I do not have a clear picture of what's in his head. How do you know you're 'near the endpoint'? Spell it out.