How do you test for the presence of iodine?

Answer 1

Testing for Iodide, I-

Sodium iodide, NaI, is the source of iodide anion for this experiment.

Reaction with bleach involves three steps. The brown color shows the presence of I3- ions.

Hypochlorite ion yields chlorine:

OCl- (aq) + Cl- (aq) + H2O

Cl2 (aq) + 2 OH-

Chlorine reacts with iodide anion:

Cl2 (aq) + 2 I- (aq)

I2 (aq) + 2 Cl- (aq)

Triiodide ion is formed:

I2 (aq) + I- (aq) I3- (aq)

Starch reacts with iodine and iodide to form a characteristic blue/black complex. A corn starch packing peanut is shown here.

Silver ion reacts with iodide to form silver iodide, AgI.

Ag+ (aq) + I- (aq) AgI (s)

Reaction of iodide with sulfuric acid produces hydrogen sulfide gas and brown triiodide solution in a series of reactions:

I- (aq) + H2SO4 (aq) HI (aq) + HSO4- (aq)

8 HI + H2SO4 (aq) H2S (g) + 4 I2 (aq) + 4 H2O

I2 (aq) + I- (aq) I3- (aq)

Again, starch is used to confirm the presence of iodine and iodide.