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Peroxide Quantification via Iodometric Titration

Technical Overview

This procedure describes a method to measure the presence of hydrogen peroxide (H2O2) through two major steps.

In the first step, an iodide solution is added to the analyte under acidic conditions in the presence of molybdate catalyst. H2O2 oxidizes iodide to iodine per the following reaction:

H2O2 + 2 KI + H2SO4 → I2 + K2SO4 + 2 H2O                                                                        (1)


The iodine formed (related to the initial amount of Peroxide by 1:1 stochiometry) is then titrated with a thiosulfate solution of known concentration.

I2 + 2 Na2S2O3 → Na2S4O6 + 2 NaI                                                                                      (2)


Early in the titration, the presence of iodine is indicated by a characteristic yellow-brown color of the solution.

In the final stages of the titration (after the solution has reached a pale, yellow color), starch is added as an indicator to determine the final end point.

This is because starch forms a deep, dark blue complex with minute amounts of triiodide ions that are formed only in the presence of both Iodine and Iodide in solution. Hence, the blue color will disappear once all the Iodine is reduced to Iodide.

Note: Starch should not be introduced within the early stages of the thiosulfate titration as it is believed that starch can bind permanently with some iodine/iodide species affecting the accuracy of the titration.

Materials

Potassium Iodide, KI
CAS #:
Manufacturer: Spectrum Chemical
Product #: P1335-125G
Molar Weight: 166.0028 g/mol

Ammonium Molybdate, (NH4)6Mo7O24.4H2O
CAS #:

Manufacturer: EMD Chemical
Product #: AX1310-3
Molar Weight: 1163.9 g/mol, 1235.86 g/mol (tetrahydrate)

Sulfuric Acid, H2SO4
CAS #:
Manufacturer:
Product #: 
Molar Weight: 98.079 g/mol

Sodium Thiosulfate, Na2S2O3
CAS #:
Manufacturer: Acros Organics
Product #: AC447942500
Molar Weight: 158.11 g/mol (anhydrous) 248.18 g/mol (pentahydrate)

Starch Indicator, (C6H10O5)n
CAS #:
Manufacturer: LabChem
Product #: LC25310-51
Molar Weight: variable
Comments:1% (w/v) Aqueous solution, stabilized

Procedure

Reagents:

1. Potassium iodide solution (2 wt%).

Dissolve 2.0 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. (3.5M)

Carefully add one part H2SO4-98% to ____ parts demineralized water.

4. Starch indicator.

5. Sodium thiosulfate solution (0.01M).

Method:

1. For ___mLs of analyte, add ___mLs of sulfuric acid, __mLs of ammonium molybdate solution, and ___mLs of Potassium Iodide solution. (You need enough)

2. Titrate 0.01M Sodium Thiosuflate Solution against the analyte until the analyte is pale yellow.

3. Once the brown colour of Iodine becomes hard to discern from transparency, add starch indicator to determine the final endpoint of the Sodium Thiosulfate titration.

(This method yields good resolution for peroxide amounts above ~10 µmol)

Safety Notes

Sulfuric Acid may corrode the skin at high concentrations or with improper use.

Ammonium paramolybdate is harmful if swallowed or inhaled. It causes irritation to the eyes, skin, and respiratory tract. (As with a lot of chemicals)

Calculation Notes

From equation (1) and (2), stochiometric analysis gives:

n(H2O2) = ½ * n(Na2S2O3) (due to 1:2 Iodine/Thiosulfate Ratio)

n(H2O2) = ½ * V(Na2S2O3) * c(Na2S2O3) (as n = c*V)

Primary References

1. Iodometric Titration, http://www.h2o2.com/technical-library/analytical-methods/default.aspx?pid=70&name=Iodometric-Titration, Accessed Sept 2014

2. McCloskey et al, Combining Accurate O2 and Li2O2 Assays to Separate Discharge and Charge Stability Limitations in Nonaqueous Li-O2 Batteries


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