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My experiment involves titrating iodine solution against some sample solutions to find the amount of ascorbic acid in it. The indicator I'm using is 0.5% starch solution which will turn the solution to blue-black signaling the end point of titration.

However, the last time I conducted the experiment, I found that even 10 drops of the indicator was not enough to signal the endpoint of titration (no color change was observed). After I added the entire 100ml solution (containing 0.5g of starch + 95.5ml water), the color change could be observed.

But, I don't want to add 100ml of indicator for titration experiment in future. I was wondering if I could just add 0.5g (entire amount of starch in 100ml of starch solution) of starch powder directly instead of making a starch solution and then using it while titrating? If not, what can I do to make starch be able to detect the endpoint of titration without using too much of starch solution?

Thank you,

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  • $\begingroup$ In principle, you could perhaps use solid indicator. In practice, starch powder can take a long time to dissolve in water, so this approach might not be best in your specific case. How old is your indicator solution? In our teaching labs, we find starch indicator solution has a useful shelf life of 1-2 weeks. Water slowly hydrolyzes the starch. $\endgroup$
    – Ben Norris
    Nov 28, 2021 at 16:38
  • $\begingroup$ They sell spray starch in the laundry section of grocery stores. It worked well for the iodine indicator reaction and was convenient to use: just a short spritz was enough. Worth a try. $\endgroup$
    – Ed V
    Nov 28, 2021 at 17:06
  • $\begingroup$ Concentration of the indicator is one thing. But if the dilution of your analyte is too high (or: its concentration too low) then you do no recognize the moment when colours change. 1) Perform a series of check runs with solutions known in advance to contain $\pu{xy g/L}$ of analyte to learn how it should look like, and to check how well the results are accurate and precise (two different, though related concepts). 2) If your your eyes are your recorder, place a white paper (better: a white glazed tile) below the Erlenmeyer flask during the titration, this increases contrast. $\endgroup$
    – Buttonwood
    Nov 28, 2021 at 17:15
  • $\begingroup$ 3) If still not successful enough, you might resort to perform the titration with a UV-Vis spectrometer. Depending on the spectra of blank and reaction mixture, you might consider an interval of wavelengths instead of a single wavelength for higher sensitivity. $\endgroup$
    – Buttonwood
    Nov 28, 2021 at 17:17
  • $\begingroup$ Do you have hands on a classical quantitative analysis book? $\endgroup$
    – Alchimista
    Nov 28, 2021 at 19:02

1 Answer 1

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Starch solution cannot be kept more than a couple of days. After this tlme, starch will be hydrolized, transformed into glucose and not be able to react in blue with iodine.
On the other hand, if you put solid starch into the solution to be titrated, the reaction with iodine will only happen on the surface of the grains. It will be difficult to notice the very beginning of this blue color, if it only occurs on the surface of grains which are not in solution. They may fall on the ground of the container, and take time to react with iodine produced in the solution. The equivalence point is much more visible when starch is regularly distributed in the whole solution.

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