Question

Estimation of ozone can be made quantitatively by:
(A) decomposition into ${{O}_{2}}$ and adsorption of ${{O}_{2}}$ in pyrogallol.
(B) volumetric method using $KI$ and titration of the liberated iodine using hypo solution.
(C) oxidation ozonolysis method
(D) all of the above.

Answer 1

Hint: Ozone or trioxygen, ${{O}_{3}}$ is an inorganic molecule which is present in the stratosphere in the atmosphere. It dissolves in water and is the most stable allotrope of oxygen. Think about the method using which the quantity of dissolved ozone can be estimated.

Complete step by step answer:
- Ozone is the most stable allotrope of oxygen which forms an ozone layer in the stratosphere.
- Ozone readily dissolves in water.
- Dissolved ozone in water can be estimated volumetrically by using an iodometric method.
- In the iodometric method, there is indirect titration of iodine. Iodine is not added directly to the analyte but liberated iodine is titrated against hypo solution that is, sodium thiosulfate.
- In estimation of ozone, ozone dissolved in water is made to react with potassium iodide solution buffered with a borate buffer having pH 9.2, iodine is liberated.
\[2KI+{{O}_{3}}\to KI+KOI+{{O}_{2}}\]
\[KI+KOI+{{H}_{2}}O\to 2KOH+{{I}_{2}}\uparrow \]

- This liberated iodine is titrated with standardized hypo solution using starch as an indicator.
\[{{I}_{2}}+2N{{a}_{2}}{{S}_{2}}{{O}_{3}}\to 2NaI+N{{a}_{2}}{{S}_{4}}{{O}_{6}}\]

- From the reaction, we can see that one mole of ozone liberates one mole of iodine. One mole of iodine consumes two moles of sodium thiosulfate. So, from the volume of sodium thiosulfate consumed, the amount of iodine can be back calculated which is equal to the quantity of ozone dissolved in water.
-Therefore, estimation of ozone can be made quantitatively by volumetric method using $KI$ and titration of the liberated iodine using hypo solution.
So, the correct answer is “Option B”.

Note: Remember iodometric titration is the one in which iodine is titrated indirectly that is, liberated iodine is estimated. In iodometric titration, iodine is titrated directly. Estimation of dissolved oxygen is done using an iodometric method. Another method used in estimation of oxygen is Winkler’s method which is carried out in acidic conditions.