Standardization of Sodium thiosulphate (Na2S2O3) solution by Iodometry

 

Theory

In this experiment the strength of Sodium Thiosulphate (Na₂S₂O₃) solution is determined with the help of a standard Potassium Dichromate solution. This is an oxidation and reduction reaction, as well as “Iodometric reaction”. Titration involving with iodine or dealing with iodine liberated in chemical reaction is called Iodimetric and Iodometric titration respectively. This reaction is iodometric because iodine is got from KI. The objective of this experiment is to determine the strength of this Sodium Thiosulphate. This experiment is based on oxidation and reduction reaction, as well as “Iodometric reaction”. The main purpose of the experiment is to standardize Na₂S₂O₃ solution with standard K₂Cr₂O₇ solution

Chemicals

Sodium bicarbonate, distilled water, KI solution, conc. HCl, Potassium dichromate (K₂Cr₂O₇) solution, Na₂S₂O₃ solution, Starch solution 

Apparatus

Burette, burette stand, pipette, funnel, conical flask, beaker, stirrer, weight balance, watch glass

Principle

Redox titration

Reaction Equation

K₂Cr₂O₇ + 6KI +14HCl → 2CrCl₃ + 3I₂ + 7H₂O + 8KCl

2Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2NaI

Indicator

Starch solution

End point

Disappearance of blue colour

Preparation of 100ml of 0.01M K₂Cr₂O₇ solution

Molecular weight of K₂Cr₂O₇ = 294g

It means 1000ml of 1M K₂Cr₂O₇ solution contains = 294 g of K₂Cr₂O₇

We want to calculate “Given mass” for 0.01M solution

Given mass = Molarity × molar weight

0.01  × 294

= 2.94 g

For 1000ml of 0.01M solution we require 2.94g of K₂Cr₂O₇

Take 0.294g of K₂Cr₂O₇ in 100ml volumetric flask and dissolve in distilled water, ten fill the flask up to the mark

Procedure

1.    Dissolve about 1gm of sodium bicarbonate (NaHCO₃) in about 50ml of distilled water in a conical flask

2.    Add 4ml of 12% solution of iodate-free potassium iodide (KI) and shake well

3.    Now add about 4ml of concentrated HCl acid slowly by while rotating the flask in order to mix the liquids well

4.    Pipette out 10ml of standard dichromate (K₂Cr₂O₇) solution and pour into the same flask. Shake gently for thorough mixing. Cover the flask with a watch glass and allow standing in the dark for 5 minutes for completion of reaction. The solution will be deep brown. In the meantime, fill the burette with the supplied thiosulphate solution in the appropriate manner.

5.    Titrate the liberated iodine (brown colour solution of flask) with the Na₂S₂O₃ solution while shaking the flask until the brown colour fades (light yellow)

6.    Add 2ml of starch solution. At the end point the deep blue colour of the starch-iodine complex disappears leaving the light green colour of the chromate ion 

7.    Calculate the strength of the supplied thiosulphate solution

Table: Standardization of Na₂S₂O₃ by K₂Cr₂O₇ solution

Result

The strength of supplied thiosulphate (Na₂S₂O₃) solution is ………… M.

Discussion

The solution was made acidic enough to make sure the redox reactions get completed. In order to avoid oxidation of HCl by air a reducing environment was provided by adding a small amount of NaHCO₃ into solution. As iodine can be easily lost from solution due to its volatility, flask was kept closed. Iodine in water solutions is usually coloured strong enough so that its presence can be detected visually. However, close to the end point the concentration of iodine is very low, its yellow colour is very pale and can be easily overlooked. If we add starch, iodine gets adsorbed on the surface of starch and the product gives a deep blue colour. In the presence of small amounts of iodine adsorption and desorption are fast and reversible which makes the detection of end point difficult. This is why starch is used. However, when the concentration of iodine is high, its bonding with starch is relatively strong which makes the desorption slow which makes the detection of end point relatively difficult. Thus, at the end point some absorbed iodine may remain untitrated giving erroneous endpoint. This is why starch solution is added at the end point when the concentration of iodine is very small. [Discussion should be written in past-passive form]