Experiment 11
Standardization of sodium thiosulphate solution
(24/11, 27/11)
Introduction :
Although sodium thiosulphate are resistant to air-oxidation, they do tend to decompose to give sulphur and sulphur
dioxide in acidic medium (酸性介質) :
S2O32-(aq) + 2 H+(aq)  SO2(g) + S(s) + H2O(l)
Another cause for the instability of thiosulphate solutions can be traced to bacteria that metabolize thiosuphate ion
to sulphate(IV) ions as well as elemental sulphur. Bacterial activity (細菌活性) appears to be at a minimum at a pH
between 9 and 10, or in the presence of a bactericide, such as trichloromethane (三氯甲烷) or sodium benzoate
(苯甲酸鈉).
Potassium iodate(V) 碘(V)酸鉀 is an excellent primary standard for thiosulphate solutions. In this application,
known mass of KIO3 are dissolved in water containing an excess of potassium iodide (碘化鉀). When this mixture
is acidified with a strong acid, liberated iodine (釋出的碘) is then titrated with the thiosulphate solution.
Chemicals and apparatus
Pipette, burette, sodium thiosulphate-5-water, potassium iodate(V), sodium carbonate, potassium iodide solution,
1% starch solution.
Procedure :
A. Preparation of sodium thiosulphate solution
Prepare a sodium thiosulphate solution boiled deionized water.
B. Preparation of a standard potassium iodate(V) solution
Prepare a 250.0 cm3 of 0.02 M potassium iodate(V) solution.
C. Standardization of the sodium thiosulphate solution
Pipette 25.00 cm3 of potassium iodate(V) solution into a conical flask(錐形瓶). Add 12 cm3 of 1 M potassium
iodide solution and 5 cm3 of 1 M sulphuric(VI) acid into the flask respectively. Titrate the liberated iodine against
the sodium thiosulphate solution. When the solution turns to pale yellow, add 2 cm3 of 1 % starch solution.
Continue the titration until colour changes.
Results :
1. Calculate the molarity of the standard potassium iodate(V) solution.
2. Calculate the molarity of the sodium thiosulphate solution.
Questions :
1. Give the procedure for the preparation of
(a) sodium thiosulphate solution, and
(b) potassium iodate(V) solution respectively.
2. In the preparation of sodium thiosulphate solution, explain why
(a) boiled deionized water should be used ;
(b) sodium carbonate should be added.
3. Though sodium thiosulphate is soluble in water, explain why the solution should be boiled during the
preparation.
4. Write a balanced equation of each reaction involved.
5. Describe the colour change of the solution during the titration.
6. Why should the starch solution be freshly prepared ?
7. Explain why starch solution should be added just before the end point.
Preparation of sodium thiosulphate solution
1
1. Weigh out 27 g of sodium thiosulphate solid into a 1-litre beaker.
2. 0.1 g of sodium carbonate is added.
3. Adding 100 cm3 of deionized water to dissolve the solid and filter
off any undissolved particles through cotton.
4. The filtrate is diluted to 1.1 dm3 with boiling deionized water.
5. Store the resulted solution in a plastic bottle.
Preparation of standard potassium iodate(V) solution
1. Weigh out accurately 1.0 g of potassium iodate(V).
2. Dissolve the solid with minimum deionized water and the make up
the solution to 250 cm3.
2