Learn about the preparation and standardization of 0.1 N sodium thiosulphate through a laboratory experiment or practical.
Aim:
To prepare and standardize 0.1 N sodium thiosulphate solution using potassium iodate as primary standard.
Reference: Indian Pharmacopoeia (IP) 2014; Volume-1; Page No. 316
Requirements:
Glasswares: Burette, burette stand, iodine flask, volumetric pipette, beaker, volumetric flask, funnel, glass rod, and wash bottle, etc.
Chemicals: LR grade sodium thiosulphate (Na2O3S2), potassium iodate, potassium iodide, dilute sulphuric acid (H2SO4) and starch indicator, etc.
Apparatus: Digital/analytical balance, and Ultrasonicator.
Principle:
Iodine is liberated when a known concentration of potassium iodate is reacted with excess potassium iodide in an acidic environment. Liberated iodine is directly titrated with sodium thiosulphate to determine its strength. Due to the conversion of iodine into sodium iodide, the endpoint of the titration is determined by the disappearance of the permanent blue color. The following is the reaction that is involved in this titration.
The molecular weight of sodium thiosulphate (Na2S2O3) is 248 g/mol.
Preparation of 0.1 N sodium thiosulphate:
Take 24.8 g of sodium thiosulphate (Na2O3S2) and dissolve in 200 ml of distilled water in a volumetric flask, and properly mixing it. Once it has completely dissolved, make up the volume to 1000 ml.
Preparation of 0.1 N potassium iodate:
Take 356 mg of potassium iodate (KIO3) and dissolve in 50 ml of distilled water in a volumetric flask, and properly mixing it. Once it has completely dissolved, make up the volume up to 100 ml.
Preparation of dilute sulphuric acid:
Take 57.00 ml of sulfuric acid (H2SO4) using a pipette, dilute in 1000 ml of distilled water in a volumetric flask, and properly mixing it.
Preparation of starch indicator solution:
Dissolve 01.00 g of soluble starch in 05 mL distilled water and add to 100 ml boiling water containing 10 mg of mercuric iodide, stirring constantly.
Titration procedure:
- All glassware should be cleaned and dried according to standard laboratory procedures.
- Before filling the burette for the titration, rinse it with distilled water and then pre-rinse it with a portion of the titrant solution. Pre-rinsing is required to make sure that all solution in the burette is the desired solution, not a contaminated or diluted solution.
- Take the unknown stock solution of titrant in a clean and dry beaker then fill the burette using the funnel.
- Remove air bubbles from the burette and adjust the reading to zero.
- Take 10.00 ml of prepared solution of potassium iodate and pour into an iodine flask.
- For 10 minutes, place the flask in the dark (Protect from light.
- Add 2 drops of starch indicator solution.
- Titrate the sample solution with sodium thiosulphate until the endpoint is reached.
- The change in color from blue to colorless indicates that the endpoint is approaching.
- To get accurate results, repeat the titration three times.
- Properly record the readings of the burette.
- Take their mean and calculate the molarity of sodium thiosulphate solution.
Observation table:-
|
Sr. No. |
Content in conical flask
|
Burette reading |
Volume of titrant used (ml) |
|
|
Initial |
Final |
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1 |
|
|
|
|
|
2 |
|
|
|
|
|
3 |
|
|
|
|
|
Mean: |
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Calculations:-
N1 V1 = N2 V2
N2 = V1 N1/V2
Where,
N2 is normality of Na2S2O3
V1 is a volume of KIO3 solution used
N1 is a normality of KIO3 solution
N2 is a volume of Na2S2O3 solution used
Result:-
The strength of the prepared sodium thiosulphate solution was found to be_____N.
Commonly asked questions on titration are as follows.
Why do we standardize sodium thiosulphate solution?
Standardization of a prepared sodium thiosulphate solution is performed to determine the exact normality.
Which indicator is used in the standardization of sodium thiosulphate?
The principle of sodium thiosulphate titration is based on redox iodometric titration using potassium iodate as the primary standard in which iodine starch solution is used as the indicator.
What are the different types of titration?
Depending on the chemical reaction that occurs between the titrant and the solute or analyte acid-base, redox, complexometric and precipitation titrations are types of titrations.
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