[habbababba]

I would like to know why starch is added near the end point and not at the beginning of a redox titration. Also, does thiosulfate react with I₂ or with I₃^-?

Thanks.

[Vidya]

https://en.wikipedia.org/wiki/Starch_indicator

[habbababba]

It is stated that starch is added near the end point because the starch-iodine complex is insoluble. But the insoluble complex is also formed when starch is added near the end-point. Is the solubility of the complex higher when the concentration of I₃^- near the end point is low?

[habbababba]

What about the actual redox reaction? Anyone cares to state whether it's I₂ or I₃^- that reacts with thiosulfate?

[Borek]

I never heard about the insolubility of the starch complex.

http://www.titrations.info/iodometric-titration-end-point-detection

I₂ and I₃^- are in equilibrium in the solution, doesn't matter much which reacts. I would guess it is I₂, as the reaction doesn't require presence of iodides.

[habbababba]

Thanks Borek.

I would guess it is I₂, as the reaction doesn't require presence of iodides.

Interesting. I used to think that Iodide is required during titration because 1) iodine is insoluble in water in which thiosulfate is bathing and 2) iodine is volatile.
It seems to me that the addition of iodide has the benefit of dissolving iodine but this same process changes iodine to triiodide and now I can't really tell if iodine or triiodide oxidizes thiosulfate.

[AWK]

1) iodine is insoluble in water in which thiosulfate is bathing and

Solubility of iodine in water is 0.28 g and 0.35 g in 1000 g of water at temperature 18 C and 25 C , respectively. Usually we titrate very small amounts of iodine and work with a perfect solution.

[Borek]

I used to think that Iodide is required during titration because 1) iodine is insoluble in water in which thiosulfate is bathing and 2) iodine is volatile.

Presence of iodides definitely helps dissolving more iodine and keeping it in the solution, but working with just a saturated 0.001 M solution sounds doable (although requires much better technique).

[habbababba]

Solubility of iodine in water is 0.28 g and 0.35 g in 1000 g of water at temperature 18 C and 25 C , respectively. Usually we titrate very small amounts of iodine and work with a perfect solution.

I'm interested in knowing what indicator you would use when titrating iodine without iodide.

[AWK]

Exactly the same. During titration NaI or KI is formed increasing solubility of iodine.