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Standardisation of Sodium Thiosulphate with Potassium Dichromate watch

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    Does anyone have (or can find) an example of the colours the standardisation goes through? The method details a pale greenish-gold colour, at which point you add starch indicator and titrate from a dark blue-black colour until it reaches a clear light blue colour.

    I'm performing this titration soon and would like to have a good idea of what I'm looking for.

    Thanks in advance.
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    (Original post by Forceflow1)
    Does anyone have (or can find) an example of the colours the standardisation goes through? The method details a pale greenish-gold colour, at which point you add starch indicator and titrate from a dark blue-black colour until it reaches a clear light blue colour.

    I'm performing this titration soon and would like to have a good idea of what I'm looking for.

    Thanks in advance.
    If you are using starch as an indicator there must be some iodine involved.

    Maybe you add a little potassium iodide to the sodium thiosulphate giving a colourless mixture.

    If you then run potassium dichromate in the iodine is liberated and immediately reacts with the thiosulfate until all of the thiosulfate is used up.

    In this case you are adding an orange solution to a colourless solution so you will have a pale yellow solution that reacts forming chromium(III) ions which are green.

    As they are dilute then the colour will be very pale green. When there is no more thiosulfate left to absorb the iodine it will appear as a pale yellow solution, mixed with the pale green chromium (III). Addition of starch solution before this point would turn the iodine into a blue-black complex, which could mark the end point...

    ... all conjecture as I don't have a copy of Vogel handy.
 
 
 
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