Question

What type of change, physical or chemical, occured upon mixed the two solutions? Explain your reasoning from the reaction, potassium iodate and starch indicator(H2SO4 and NaHSO3)

Answer 1

In analytical methods involving iodine, normal redox indicators are not necessary, because starch is available, a sensitive internal indicator that experiences a highly specific interaction with iodine. The free iodine forms a very intense blue color with dispersed colloidal starch and this coloration serves as an indication of the presence of iodine.

An amount as small as 2 x 10-7 M iodine gives a noticeable blue color with starch.

In the presence of iodide in 10-5 M concentration the blue color of starch-iodine can be easily seen with a concentration of 5 x 10-6 M iodine.

As can be seen, the starch-iodine method is no more sensitive than the self-coloring of iodine, but the blue color of the starch-iodine is more distinctive, in particular in colored solutions.