V. Baratharaj
September 16, 2016

Determining ozone in process gas (to determine the correct weight of ozone generated by an ozone generator) can be either through a calibrated ozone analyzer or it can also be determined by simple titration method. While using an ozone analyzer - is an expensive method, determining ozone by Titration Method is relatively easy and can be done in a good laboratory.

The method recommended by The International Ozone Association, Quality Assurance Committee, Revised Standardized Procedure 001/96:

Iodometric Method for Determination of Ozone in a Process Gas


The present standard method concerns the determination of ozone in air, oxygen or other process gases.

Range Of Application:

The method is directly applicable in the range of 1 g/m3 to 200 g/m3 of ozone, the volume being expressed at NTP (Normal Temperature Pressure, conditions which are equal to 0°C or 273.15 K and 1.01325 x 105 Pa or 1 ATM).

Reagents (All of Analytical Grade)

  • Quality of the water for make-up of solutions shall comply with ISO No. 3696-1987 (Grade 1).
  • Buffered KI (Potassium Iodide) in water:
    > KI 20 g/L, Na2HPO4, 2H2O (Disodium Hydrogen Phosphate) 7.3 g/L and
    > KH2PO4 (Monopotassium Dihydrogen Phosphate) 3.5g/L
  • Sodium Thiosulfate: Na2S2O3, 0.1 mol/L in water
  • Acidifying solution: H2SO4 (sulfuric acid): 4.5 mol/L
  • Powered KIO3 (Potassium Periodate)
  • Cystalline KI
  • HCI (hydrochloric acid) or H2SO4 0.1 N (certified)
  • Starch indicator: Znl2 (zinc iodide)-starch, prepared by dispersing 4 g starch into an aliquot of water. The dispersion is added to a solution of 20 g ZnCI2 (zinc chloride) in 100 ml water. The solution is boiled until the volume has been reduced to 100 mL and is finally diluted to 1L while adding 2 g of Znl2. The indicator is stable for at least one month when stored in the dark at room temperature.

Standardization of Titrant

5KI + 5H+ = 5HI + 5K+
KIO3 + H+ = HIO3 + K+
HIO3 + 5 HI = 3I2 + 3H2O
3I2 + 6S2O2 = 6I + 3S4O6=


Take 50 mL of water in a 250 conical flask (Erlenmeyer), add 0.05 g of KIO3 and 0.5 g of KI, followed by another volume of about 50 mL water. After mixing it, 10 mL of certified 0.1 N acid should be added. The iodine formed by this, is titrated with the Thiosulfate Solution.


Normality of S2O3= = Normality of acid multiplied by the volume of acid (mL) and divided by the volume of thiosulfate titrant (mL).

Determination of Ozone


  • 200 mL of KL solution is added to a gas washing bottle equipped with an open gas bubbling device (tube or diffuser) under a reagent depth of 15 cm or more (the use of fritted glass diffusers is not recommended)
  • A second identical flask is connected in series as a guard detector for ozone transfer and reaction in the first flask.
  • Process gas containing ozone is bubbled at a flow rate of 1 L/minute or less, until a total (estimated or expected) quantity of approximately 1 mM O3 (it equals 0.048 g) has passed.
  • The iodine formed in the solutions of KI in the flasks, immediately after acidification with 5 mL of the acidifying reagent, is titrated as a freshly standardized sodium Thiosulfate Solution.
  • After titration to a pale yellow color, optionally, 0.5 mL of the starch indicator solution can be added to complete and record the final result (This addition is recommended, but can be optional, depending on the skills and experience of the operators).


O3 + 2I = I2 + O2 +O=
L2+ 2S2O3 = 2I + S4O6=

Concentration of ozone in g/L = 24 x volume of Thiosulfate in L x Normality of Thiosulfate divided by the inlet volume of gas passed in L.


  • All upstream transfer and pressure reducing equipment must be in materials which do not react with ozone, e.g., glass, PTFE
  • The gas contacting systems must have a free exit to ambient pressure.
  • All gas flow must be expressed at NTP, (for high precision or when analyzing high ozone concentrations, the volume must be corrected for local existing atmospheric pressure).
  • Gas flows should be measured with an accuracy of 1% with totalizing volumetric gas meter or with a bubble trap).

Precision and Accuracy

  • Detection limit of the analytical procedure: 0.1 mg/L
  • Repeatability: 2% of the measured ozone concentration


Nitrogen oxides, other oxidants of iodide ion - if present.

V. Baratharaj V. Baratharaj is Head, Ozone Technologies & Systems India Pvt Ltd. He can be reached at, and +91 44 4211 8266.

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