Preparation and standardization of Sodium thiosulfate standard solution
Citation: Chaudhari, M. (2024). Standardization of sodium thiosulfate. Pharmacy Infoline. https://doi.org/10.5281/zenodo.10815788
Pharmaceutical Analysis I Practical: Limit test for (1) Chloride (2) Sulphate (3) Iron (4) Arsenic Preparation and standardisation of (1) Sodium hydroxide (2) Sulphuric acid (3) Sodium thiosulfate (4) Potassium permanganate (5) Ceric ammonium sulphate Assay (1) Ammonium chloride (2) Ferrous sulphate(3) Copper sulphate (4) Calcium gluconate (5) Hydrogen peroxide (6) Sodium benzoate (7) Sodium Chloride Determination of Normality (1) Conductometric titration of strong acid against a strong base (2) Conductometric titration of strong acid and weak acid against a strong base (3) Potentiometric titration
Aim
To Prepare and standardize 0.01 M sodium thiosulphate standard solution
Principle
The principle of standardization of sodium thiosulphate is based on redox iodometric titration with potassium iodate (or potassium bromate) as a primary standard. Potassium iodate a strong oxidizing agent is treated with excess potassium iodide in acidic media which liberates iodine which is back titrated with sodium thiosulphate
Procedure
Preparation of 0.1 M Sodium thiosulfate standard solution
Dissolve____ g of sodium thiosulphate and 0.2 g of sodium carbonate in carbon dioxide-free water and dilute to 1000 ml with the same solvent.
Standardization of 0.1 M Sodium thiosulfate solution
Dissolve 0.200 g of potassium bromate, weighed accurately, in sufficient water to produce 250.0 ml. To 50.0 ml of this solution add 2 g of potassium iodide and 3 ml of 2 M hydrochloric acid and titrate with the sodium thiosulphate solution using the starch solution, added towards the end of the titration, as an indicator until the blue colour is discharged.
Reference
Indian Pharmacopoeia 2014; Vol. 1; Pg. No. 316
Sodium Thiosulfate Standard Solution
Sodium thiosulfate (Na2S2O3) is a versatile compound used in various analytical techniques, particularly for iodometric titrations. These titrations involve the reaction of iodine with the analyte. However, due to the instability of standard iodine solutions, sodium thiosulfate is employed as a titrant. A standard sodium thiosulfate solution is one with a precisely known concentration, and proper preparation and standardization are crucial for accurate analysis.
Materials
- Sodium thiosulfate pentahydrate (Na2S2O3.5H2O)
- Distilled water
- Volumetric flask (1000 mL)
- Analytical balance
- Magnetic stirrer or stirring rod
- Potassium dichromate (K2Cr2O7) (primary standard)
- Potassium iodide (KI)
- Hydrochloric acid (HCl)
- Sodium bicarbonate (NaHCO3)
- Starch indicator solution
- Conical flask (500 mL)
- Burette
Preparation
- Weighing: Accurately weigh approximately 24.8 grams of sodium thiosulfate pentahydrate using an analytical balance.
- Dissolution: Transfer the weighed sodium thiosulfate into a clean 1000 mL volumetric flask. Add about 100 mL of distilled water and swirl to dissolve the crystals.
- Addition of stabilizer: Add a small amount of sodium carbonate (around 0.2 grams) to the solution. This helps to stabilize the sodium thiosulfate and prevent its decomposition.
- Dilution and mixing: Fill the flask roughly ¾ full with distilled water. Swirl the flask to ensure complete mixing. Make up the volume to the mark with distilled water using a wash bottle to remove any residual sodium thiosulfate on the neck of the flask. Stopper the flask securely and mix thoroughly by swirling or using a magnetic stirrer for at least an hour.
Note: It is recommended to prepare the sodium thiosulfate solution a few hours before standardization to allow complete equilibration.
Standardization
- Preparation of potassium dichromate solution: Accurately weigh around 210 mg of previously oven-dried potassium dichromate (primary standard). Dissolve the K2Cr2O7 in 100 mL of distilled water in a clean 500 mL conical flask.
- Acidification and addition of KI: Swirl the flask to dissolve the potassium dichromate. While swirling, carefully add 3 grams of potassium iodide (KI) and 2 grams of sodium bicarbonate (NaHCO3). Then, add 5 mL of concentrated hydrochloric acid (HCl).
- Reaction and standing time: Stopper the flask gently and swirl to mix the contents. Let the solution stand in the dark for exactly 10 minutes. This allows for the complete reaction between KI and K2Cr2O7, liberating iodine.
- Titration: Add a few drops of starch indicator solution to the flask. The solution will turn a deep blue color. Using a burette, titrate the solution with the prepared sodium thiosulfate solution until the blue color just disappears. This endpoint signifies the complete consumption of free iodine by the sodium thiosulfate. Record the volume of sodium thiosulfate solution used (V).
Calculation
The concentration of the sodium thiosulfate solution is usually expressed in terms of normality (N). The normality can be calculated using the following equation:
Normality (N) of Na2S2O3 = (weight of K2Cr2O7 x purity of K2Cr2O7) / (10 x V x equivalent weight of Na2S2O3)
Where:
- Weight of K2Cr2O7 (g) – Mass of potassium dichromate used
- Purity of K2Cr2O7 – Decimal value representing the purity of K2Cr2O7 (obtained from the certificate of analysis)
- V (mL) – Volume of sodium thiosulfate solution used in the titration
- Equivalent weight of Na2S2O3 (g/eq) – The equivalent weight of sodium thiosulfate, which depends on the specific reaction involved in the titration. In this case, the equivalent weight is equal to the molecular weight of Na2S2O3 divided by 2 (assuming the reaction consumes 2 moles of thiosulfate ions per mole of dichromate).
Important Considerations
- Use analytical grade reagents throughout the preparation and standardization process.
- Distilled water is crucial to avoid contamination that might affect the accuracy of the standardization.
Citation: Chaudhari, M. (2024). Standardization of sodium thiosulfate. Pharmacy Infoline. https://doi.org/10.5281/zenodo.10815788
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