Question

Explain each colour change or observation made throughout the iodometric titration experiment.

Redox 1: 2Cu+ + 2I- --> 2Cu+ + I2

Precipitation: Cu+ + I- (excess) --> CuI (yellowish)

Redox 2: I2 + 2S2O3 2- --> S2O3 2- + 2I-

Redox 1 + 2: 2Cu2+ + 2S2O3 2- + 2Cu+ + S2O3 2-

My observation:

bluish to yellowish - addition of KI to copper (II) sulfate solution results in a formation of CuI solution (Precipitation)?

yellowish - brownish/dark blue - after addition of KSCN deepens the colour of the solution?

brownish/dark blue to whitish pink - End point is the whitish pink but why? Iodine has been completely removed?

Answer 1

bluish to yellowish color change because In an iodometric titration, a starch solution is used as an indicator since it can absorb the I₂ that is released. This absorption will cause the solution to change its colour from dark blue to light yellow a colour when titrated with standardised thiosulfate solution.

Copper(I) iodide can be dissolved in acetonitrile, thats why the CuI is precipitate in salution.

The KSCN is added to displace the adsorbed I₂ making it accessible to the starch collodial particles, hence yielding a sharper endpoint

Starch is used as an indicator for iodometric titration because starch binds to the triiodide ion formed in the redox reaction of excess iodide with the oxidizing agent in the solution and it forms a starch-iodine complex which gives a dark blue color.

Near the endpoint, the concentration of I2 is very low and thus the color is pale and very low and hence starch comes to the rescue by providing a sharp endpoint.

The iodine formed in the reaction can then be titrated by means of a standard sodium thiosulfate solution. ... Therefore, when the blue-black color disappears, the iodine has been completely reduced to the iodide ion.