in APha 2350e use of KI, starch, Na2S2O3, H2SO4, then say use of KI trap and send bubble to KI, then in 10 min or any time ( lower time is more speed of measurement). then mix a 10ml of H2SO4 with KI and now color is like brown. then say titrate with Na2S2O3 until "the yellow iodine color almost disappears". and then add 1-2 ml of starch to change color to blue. now I can't see yellow color is disappear. in some of paper say "until KI is colorless and I can't reach to white color".
Hi are you referring that you can't observe the colour change from brown to yellow? Are you using trap other than water ? End point of titration should give you a transparent solution (Brown -> Yellow -> Add starch turns purplish blue -> Colourless)
The aim of sodium thiosulfate titration before adding starch indicator is to remove interference faced of binding starch with iodine when presence of too much iodine in solution.
If you add too much KI solution, you need a lot of sodium thiosulfate solution for your titration. Did you check the concentration of sodium thiosulfate and KI solution you used ?
Even though sulfuric acid was used in APHA, I more prefer HCl as sulfuric acid is an oxidizing acid which may contribute to your final result.
Perhaps you can upload your titration pictures, or video for a clearer image of the problem you faced.
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