Balanced Chemical Equation and Role in Iodometric Titration
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 Explained
1. What happens when sodium thiosulphate reacts with potassium iodide?
There is no direct reaction between sodium thiosulphate and potassium iodide unless an oxidizing agent is present to form iodine. In typical redox chemistry:
- KI provides iodide ions (I-).
- An oxidizing agent (like Cl2 or Fe3+) converts I- to I2.
- Sodium thiosulphate (Na2S2O3) then reacts with iodine to reduce it back to iodide.
2. What is the balanced equation for the reaction between sodium thiosulphate and iodine formed from potassium iodide?
The balanced equation is 2S2O32-(aq) + I2(aq) → S4O62-(aq) + 2I-(aq). In full molecular form:
- 2Na2S2O3(aq) + I2(aq) → Na2S4O6(aq) + 2NaI(aq)
3. Why is sodium thiosulphate used in iodometric titration with potassium iodide?
Sodium thiosulphate is used because it is a standard reducing agent that reacts quantitatively with iodine. In iodometric titration:
- An oxidizing agent liberates I2 from KI.
- The liberated iodine is titrated with Na2S2O3.
- The reaction is stoichiometric and sharp at the end point.
4. What type of reaction occurs between iodine and sodium thiosulphate?
The reaction between iodine and sodium thiosulphate is a redox reaction. Specifically:
- Iodine (I2) is reduced to iodide (I-).
- Thiosulphate (S2O32-) is oxidized to tetrathionate (S4O62-).
5. What is the role of potassium iodide in iodometric reactions?
Potassium iodide acts as a source of iodide ions (I-) that are oxidized to iodine. In iodometric analysis:
- An oxidizing agent converts 2I- → I2 + 2e-.
- The liberated iodine is then titrated with sodium thiosulphate.
6. How do you calculate the amount of iodine using sodium thiosulphate titration?
The amount of iodine is calculated using the stoichiometric ratio 1 mole I2 : 2 moles S2O32-. Steps:
- Use M × V to find moles of Na2S2O3.
- Divide by 2 to get moles of I2.
- Apply required mass or concentration formula.
7. What is the colour change observed when sodium thiosulphate reacts with iodine?
The brown colour of iodine disappears and the solution becomes colourless at the end point. During titration:
- Iodine (I2) gives a brown solution.
- With starch indicator, a blue-black complex forms.
- Addition of Na2S2O3 reduces I2, causing the blue colour to vanish.
8. What is iodometry and how is sodium thiosulphate involved?
Iodometry is a redox titration method in which iodine is liberated and then titrated with sodium thiosulphate. The process involves:
- Oxidizing agent + KI → I2
- I2 + 2S2O32- → S4O62- + 2I-
9. Is the reaction between sodium thiosulphate and iodine reversible?
The reaction is practically irreversible under titration conditions. Once iodine is reduced to iodide and tetrathionate forms, the reaction proceeds to completion because:
- Tetrathionate (S4O62-) is relatively stable.
- The equilibrium strongly favors product formation.
10. What are common mistakes when studying the reaction between sodium thiosulphate and potassium iodide?
A common mistake is assuming sodium thiosulphate reacts directly with potassium iodide without iodine formation. Important points to remember:
- No reaction occurs between Na2S2O3 and KI alone.
- An oxidizing agent is required to produce I2.
- The correct stoichiometric ratio is 2:1 (thiosulphate to iodine).
