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

Question

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!

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Answer 1 · 2017-10-18T04:14:08

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

...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?