What is Sodium Thiosulphate?
Sodium Thiosulphate is an inorganic compound sodium thiosulfate (sodium thiosulphate) has the formula $${{Na}_{2}{S}_{2}{O}_{3}{.}{x}{H}_{2}{O}}$$. It is usually available as a white or colourless pentahydrate, $${{Na}_{2}{S}_{2}{O}_{3}{.}{5}{H}_{2}{O}}$$. The various names for sodium thiosulphate are sodium hyposulfite, hyposulphite of soda. Sodium Thiosulphate is also used to treat cyanide poisoning, pityriasis versicolor, and to reduce cisplatin side effects. It is frequently used after the cyanide poisoning medication sodium nitrite and is usually only recommended in severe cases. It is administered either intravenously or topically.
What is Potassium Iodide?
Potassium iodide is a chemical compound that also serves as a medication and dietary supplement. It is used as a medication to treat hyperthyroidism, in radiation emergencies, and to protect the thyroid gland from certain types of radiopharmaceuticals.It is also used for the treatment of cutaneous sporotrichosis and phycomycosis in developing nations. It is used as a supplement in people with weak iodine intakes.
Common side effects include vomiting, diarrhoea, abdominal pain, rash, and salivary gland swelling. Allergic reactions, headache, goitre, and depression are some of the other side effects. While use during pregnancy may be harmful to the baby, it is still advised in radiation emergencies. KI is the chemical formula for Potassium Iodide. It is commercially produced by combining potassium hydroxide and iodine.
Since at least 1820, potassium iodide has been used in medicine. It is included on the list of essential medicines. Potassium iodide is available as a generic and over-the-counter medication. Potassium iodide is also used in salt iodization.
Aim of the Experiment
The rate of reaction between sodium thiosulphate and potassium iodide is studied in this experiment.
Theory of the Experiment
Indirectly, the reaction between Potassium Iodide and Sodium Thiosulphate includes the formation of iodide ions that are oxidised in an acidic medium. The overall reaction happens in two stages.
$${{Cu}{S}{O}_{4}{+}{4}{K}{I}}$$ $$\to$$ $${{Cu}_{2}{I}_{2}{+}{2}{K}_{2}{S}{O}_{4}{+}{I}_{2}}$$
$${{I}_{2}{+}{2}{Na}_{2}{S}_{2}{O}_{3}}$$ $$\to$$ $${{2}{Na}{I}{+}{Na}_{2}{S}_{4}{O}_{6}}$$
With a starch solution, the evolved iodine produces a blue colour. This is also known as a clock reaction.
Materials Required
The materials required for this experiment are given below:
Six conical flask of volume 250 mL
Stop-watch
Trough
Measuring cylinder of volume 100 mL
2 M Sulphuric acid
5% Starch solution
6% Potassium Iodide solution
0.04M Sodium thiosulphate solution
6% Sodium Thiosulphate solution
Procedure of the Experiment
Mark a 250 mL conical flask with the letter ‘P’.
Fill the container with 25 mL of 6% Potassium Iodide solution, 50 mL of distilled water, and 25 mL of 2.0 M H2SO4.
Shake the contents of the flask vigorously.
Maintain the flask in a temperature bath.
Take five conical flasks, each with a capacity of 250 mL. Label them with the letters Q, R, S, T, and U.
Fill flasks Q, R, S, and T with 6% Na2SO3 solution, starch solution, and distilled water in the proportions listed below, and keep flask U to carry out the reaction.
Pour 20 mL of Na2SO3 solution, 75 mL of distilled water, and 5 mL of starch solution into the conical flask labelled 'Q.' Place the flask in the water bath after thoroughly mixing the contents.
Fill the 'R' conical flask with 15 mL of Na2SO3 solution, 80 mL of distilled water, and 5 mL of starch solution. Place the flask in a water bath and stir the mixture.
Add 10 mL of Na2SO3 solution, 85 mL of distilled water, and 5 mL of starch solution to conical flask 'S.' Place flask S in a water bath after mixing the contents.
5 mL Na2SO3 solution, 90 mL distilled water, and 5 mL starch solution in conical flask 'T' Keep the contents of flask T stirred and in the water bath.
Take the 'U'-shaped conical flask. Pour 25 mL of the solution from conical flask 'P' and 25 mL of the solution from conical flask Q into the conical flask. When half of the solution from flask Q has been added, start the stopwatch. Maintain the flask in the water bath while thoroughly mixing. Time taken for it to appear in blue colour. Time taken for it to appear in blue colour.
Repeat the experiment with the solutions in flasks R, S, and T, using 25 mL of the solution as in the experiment with flask Q, and record the time required to achieve blue colour.
Record your observations as given in a table.
Examine the tabulated data and determine the relationship between the time the blue colour appears and the variation in Na2SO3 concentration.
Important Questions
1.Is sodium thiosulfate an oxidising agent?
Ans. Thiosulfate ion $${{(}{S}_{2}{O}_{3}^{2-}{)}}$$is a moderately strong reducing agent used in an indirect procedure to determine oxidising agents in which iodine is an intermediate agent. In solutions containing high levels of iodine, starch decomposes irreversibly. As a result, it is clear that sodium thiosulfate is a critical chemical compound in human life. Register with Vedantu and download the mobile application on your smartphone to learn more about this compound and its reaction with hydrochloric acid.
2.What is the use of sodium thiosulphate in laboratories?
Ans. Sodium thiosulphate is a critical compound in both chemical and pharmaceutical laboratories. Because of its medicinal properties, it is a common reagent in pharmaceutical labs. Because it reacts in equimolar amounts with elemental iodine, it is also used in chemistry labs for iodine titration. In the titration, it actually acts as a reducing agent. It is used as a reducing agent in many other chemistry lab reactions. It's also used to dechlorinate water by lowering its toxicity. When it reacts with chlorine, it produces harmless chlorides that are removed from the water.
Multiple Choice Questions
1. In titration of iodine with sodium thiosulfate, the equivalent weight of sodium thiosulphate is ( Mol Wt 248)
(a) 248
(b) 124
(c) 62
(d) 24.8
Answer: (b)
2. How many grams of sodium thiosulphate should be taken to prepare 100 ml of 0.1 N solution
(a) 8 gm
(b) 1.24 gm
(c) 2.48 gm
(d) 12.4 gm
Answer: (b)
Conclusion
Sodium Thiosulphate, also known as thiosulfuric acid or disodium salt, is an inorganic salt that comes in pentahydrate form.
The various names for sodium thiosulphate are sodium hyposulfite, hyposulphite of soda.
Sodium Thiosulphate is also used to treat cyanide poisoning, pityriasis versicolor, and to reduce cisplatin side effects.
Potassium iodide is a chemical compound that also serves as a medication and dietary supplement. It is used as a medication to treat hyperthyroidism, in radiation emergencies, and to protect the thyroid gland from certain types of radiopharmaceuticals.
FAQs on Reaction Between Sodium ThioSulphate and Potassium Iodide
1. What is the fundamental reaction that occurs between iodine and sodium thiosulphate?
The fundamental reaction is a redox reaction where iodine (I₂) is reduced to iodide ions (I⁻), and the thiosulphate ion (S₂O₃²⁻) is oxidised to the tetrathionate ion (S₄O₆²⁻). This reaction forms the basis of iodometric titrations and is crucial for quantitative analysis.
2. What is the primary role of potassium iodide (KI) in reactions involving sodium thiosulphate?
In the context of titrations, potassium iodide (KI) itself does not directly react with sodium thiosulphate. Its primary role is to react with a separate oxidizing agent (like copper sulphate or potassium dichromate) to liberate a precise amount of iodine (I₂). This liberated iodine is then immediately titrated using a standard sodium thiosulphate solution.
3. How is the reaction between sodium thiosulphate and iodine applied in iodometric titrations?
This reaction is the cornerstone of iodometric titrations, a method used to determine the concentration of oxidizing agents. The process involves two key steps:
- An oxidizing analyte is added to an excess of potassium iodide (KI), which liberates a stoichiometric amount of iodine.
- This liberated iodine is then titrated against a standard solution of sodium thiosulphate until all the iodine has reacted.
4. What is the balanced chemical equation for the reaction between sodium thiosulphate and iodine, and what are the oxidation states of sulphur before and after the reaction?
The balanced ionic equation for the reaction is: 2S₂O₃²⁻(aq) + I₂(aq) → S₄O₆²⁻(aq) + 2I⁻(aq). In this reaction, the oxidation states of sulphur change as follows:
- In the thiosulphate ion (S₂O₃²⁻), the average oxidation state of sulphur is +2.
- In the product, the tetrathionate ion (S₄O₆²⁻), the average oxidation state of sulphur is +2.5.
5. Why is starch solution added as an indicator near the end of an iodine-thiosulphate titration and not at the beginning?
Starch forms an intense blue-black complex with iodine, making it an excellent indicator. However, it must be added only when the solution's colour has faded to a pale yellow. If added at the start when the iodine concentration is high, a very stable starch-iodine complex can form that does not easily break down. This would lead to an inaccurate endpoint, as the indicator would not release the iodine to react with the final drops of thiosulphate.
6. What is the difference between iodometric and iodimetric titrations, and where does the sodium thiosulphate-iodine reaction fit?
The key difference is what is being titrated:
- Iodimetry is the direct titration of a reducing substance using a standard iodine solution as the titrant.
- Iodometry is an indirect method where an oxidizing agent is reacted with excess iodide ions to liberate iodine. This liberated iodine is then titrated with a standard reducing agent.
7. Why is sodium thiosulphate not considered a primary standard, and how is its solution prepared for titrations?
Sodium thiosulphate (Na₂S₂O₃·5H₂O) is not a primary standard because it is an efflorescent salt, meaning it can lose its water of crystallisation, which changes its molar mass. Its solutions can also be decomposed by atmospheric carbon dioxide or bacteria. Consequently, a solution of sodium thiosulphate must be standardized by titrating it against a known concentration of a primary standard like potassium dichromate (K₂Cr₂O₇) to determine its exact concentration before use.
