Iodometric Method

Iodometric method Principle  The oxidation of potassium iodide by hydrogen hydrogen peroxide peroxide in an acid medium, and the titration, using a sodium thiosulphate solution, of the iodine so released. Reaction : H2O2 + H2SO4 + 2 KI > K 2SO4 + I2 + 2 H2O 2 Na2S2O3 + I2 > Na2S4O6 + 2 NaI Reagents ¥ sulphuric acid solution of approximately 300 g/l, ¥ potassium iodide solution of approximately 100 g/l, ¥ ammonium molybdate solution of approximately 5 g/l, ¥ sodium thiosulphate solution of !." g of #a $%3, 5 &%/l 'normal 0.1 solution(, ¥ starch or thiodene.

Iodometric Titration Principle  The oxidation of potassium iodide by hydrogen hydrogen peroxide peroxide in an acid medium, and the titration, using a sodium thiosulphate solution, of the iodine so released.

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 + K 2SO4 + 2 H2O I2 + 2 Na2S2O3 → Na2S4O6 + 2 NaI Scope of Application 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 leels 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 Precations #oncentrated sulfuric acid is a corrosie, hazardous material and should be handled and disposed of in accordance with the $%&%. 'eoprene gloes and monogoggles are recommended, as is wor(ing under a acuum hood. %ample bottles containing H2O2 should not be stoppered, but rather ented or coered loosely with aluminum foil or paraffin film.

!ea"ents ). *otassium iodide solution !)+ w/". &issole ). grams - into ) mLs demineralized water. %tore capped in cool place away from light. ellow0orange tinted - solution indicates some air oxidation to iodine, which can be remoed by adding a )02 drops of dilute sodium thiosulfate solution. 2. 1mmonium molybdate solution. &issole  grams ammonium molybdate in ) mLs 3' 'H4OH. 1dd 24 grams 'H4'O5 and dilute to ) mLs. 5. %ulfuric acid solution. #arefully add one part H2%O406+ to four parts demineralized water. 4. %tarch indicator. 7. %odium thiosulfate solution !.)'".

Apparats ). 1nalytical balance !8/0 .) mg/L" 2. %mall weighing bottle !9 7 mLs" 5. 27 mL :rlenmeyer flas(  4. 7 mL buret !#lass 1" 7. $edicine dropper 

Procedre ). ;eigh to the nearest .) mg an amount of H2O2 eepeat steps 204 on a blan( sample of water !omitting the H2O2".

#alclation

;eight + H2O2 ? !1 0 @" x !'ormality of 'a 2%2O5" x ).A / %ample weight in grams ;hereB 1 ? mLs 'a2%2O5 for sampleC @ ? mLs 'a 2%2O5 for blan( !eferences #. T. -ingzett, #hem. 'ews, 4)BA3 !)66"C 45B)3) !)66)" . $. -olthoff, #hem ;ee(blad, )AB)A !)2"