Question

How does 1% KI solution detect the presence of excess nitrite ions?

I recently prepared an Azo Dye in lab and one of the things we had to do during the procedure was check for excess nitrite ions using 1% KI solution... Our lab manual, textbook, and even my various internet searches haven't turned up anything other than a color change takes place (which I already knew)! I'm not even sure why we tested for excess nitrite ions so if you could explain that too I would appreciate it! Thanks!

Answer 1

Well, iodine is an oxidant.........

Explanation:

...that is reduced to iodide ion....

`1/2I_2 +e^(-) rarr I^(-)` `(i)`

And nitrite is an reductant, that is oxidized to nitrate ion, `NO_3^(-)`:

`NO_2^(-) +H_2O rarr NO_3^(-) + 2H^+ + 2e^(-)` `(ii)`

WE adds `(i)` and `(ii)` together such that the electrons are eliminated...`2xx(i) +(ii)`

`NO_2^(-) +I_2 +2e^(-)+H_2O rarr NO_3^(-) + 2H^+ + 2e^(-)+2I^(-)`

And cancel appropriately.....

`NO_2^(-) +I_2 +H_2O rarr NO_3^(-) + 2H^+ +2I^(-)`

And so deep purple iodine dissipates in colour to give colourless `I^-` ions....

When we perform diazotization we use in effect nitrous acid, i.e. `HNO_2`, the source of the nitrite ions. Capisce?