Iodide + hydrogen peroxide + nitric acid?

Question

wad happens in 'to a portion of aqueous iodide solution, add a few drops of hydrogen peroxide followed by a few drops of dilute nitric acid' ? it results in a brown solution and black solids but why?

Answer 1

Hydrogen peroxide will first oxidize the iodide into triiodide.

3 I(-) + H2O2 + 2 H (+)   -->   2 H2O + I3(-)

Mechanism

H2O2 + I(-)   -->   IOH + OH(-)  (1)

H2O2 + H(+) <--> H3O2(+)

H3O2(+) + I-   -->   IOH + H2O  (2)

I(-) + IOH   -->   I2 + OH(-)

I2 + I-   -->   I3(-)

Then nitric acid will oxidize triiodide anion into iodine (brown solid).

NO3(-) + 3 H(+) + 2 e(-)   -->   HNO2 + H2O (1) (E=0.789)

3 I2 + 2 e(-)  --> 2 I3(-) (2) (E=0.94)

(1)  -  (2)

HNO3 + 2 I3(-) + 2H(+)   -->   HNO2 + 3 I2 + HNO2