Iodometric Titration

Principle

H2O2 oxidizes iodide to iodine in the presence of acid and molybdate catalyst. The iodine formed is titrated with thiosulfate solution, incorporating a starch indicator.

H₂O₂ + 2 KI + H₂SO₄ → I₂ + K₂SO₄ + 2 H₂O

I₂ + 2 Na₂S₂O₃ → Na₂S₂O₆ + 2 NaI

Scope of Application

This method is somewhat less accurate than the permanganate titration, but is less susceptible to interferences by organics, and is more suitable for measuring mg/L levels of H2O2.

Interferences

Other oxidizing agents will also produce iodine, whereas reducing agents (and unsaturated organics) will react with the liberated iodine. The contribution from other oxidizing agents can be determined by omitting the acid and molybdate catalyst.

Safety Precautions

Concentrated sulfuric acid is a corrosive, hazardous material and should be handled and disposed of in accordance with the MSDS. Neoprene gloves and monogoggles are recommended, as is working under a vacuum hood.

Sample bottles containing H2O2 should not be stoppered, but rather vented or covered loosely with aluminum foil or paraffin film.

Reagents

1. Potassium iodide solution (1% w/v). Dissolve 1.0 grams KI into 100 mLs demineralized water. Store capped in cool place away from light. Yellow-orange tinted KI solution indicates some air oxidation to iodine, which can be removed by adding a 1-2 drops of dilute sodium thiosulfate solution.
2. Ammonium molybdate solution. Dissolve 9 grams ammonium molybdate in 10 mLs 6N NH4OH. Add 24 grams NH4NO3 and dilute to 100 mLs.
3. Sulfuric acid solution. Carefully add one part H2SO4-98% to four parts demineralized water.
4. Starch indicator.
5. Sodium thiosulfate solution (0.1N).

Apparatus

1. Analytical balance (+/- 0.1 mg/L)
2. Small weighing bottle (< 5 mLs)
3. 250 mL Erlenmeyer flask
4. 50 mL buret (Class A)
5. Medicine dropper

Procedure

1. Weigh to the nearest 0.1 mg an amount of H2O2 equivalent to a titer of 30 mLs (0.06 grms of H2O2) using a 5 mL beaker and medicine dropper. Transfer sample to Erlenmeyer flask.
2. Add to Erlenmeyer flask 50 mL of demineralized water, 10 mL of sulfuric acid solution, 10-15 mLs of potassium iodide solution, and two drops ammonium molybdate solution.
3. Titrate with 0.1 N sodium thiosulfate to faint yellow or straw color. Swirl or stir gently during titration to minimize iodine loss.
4. Add about 2 mL starch indicator, and continue titration until the blue color just disappears.
5. Repeat steps 2-4 on a blank sample of water (omitting the H2O2).

Calculation

Weight % H₂O₂ = (A – B) x (Normality of Na₂S₂O₃) x 1.7 / Sample weight in grams

Where: A = mLs Na₂S₂O₃ for sample; B = mLs Na₂S₂O₃ for blank

References

C. T. Kingzett, Chem. News, 41:76 (1880); 43:161 (1881)

I. M. Kolthoff, Chem Weekblad, 17:197 (1920)