# 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!

Oct 18, 2017

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

#### Explanation:

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

$\frac{1}{2} {I}_{2} + {e}^{-} \rightarrow {I}^{-}$ $\left(i\right)$

And nitrite is an reductant, that is oxidized to nitrate ion, $N {O}_{3}^{-}$:

$N {O}_{2}^{-} + {H}_{2} O \rightarrow N {O}_{3}^{-} + 2 {H}^{+} + 2 {e}^{-}$ $\left(i i\right)$

WE adds $\left(i\right)$ and $\left(i i\right)$ together such that the electrons are eliminated...$2 \times \left(i\right) + \left(i i\right)$

$N {O}_{2}^{-} + {I}_{2} + 2 {e}^{-} + {H}_{2} O \rightarrow N {O}_{3}^{-} + 2 {H}^{+} + 2 {e}^{-} + 2 {I}^{-}$

And cancel appropriately.....

$N {O}_{2}^{-} + {I}_{2} + {H}_{2} O \rightarrow N {O}_{3}^{-} + 2 {H}^{+} + 2 {I}^{-}$

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. $H N {O}_{2}$, the source of the nitrite ions. Capisce?