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Friday, August 9, 2013

Preparation and Standardization of 0.1M Sodium Thiosulfate - Lu Le Laboratory


Numerous methods are based on the reducing properties of iodine ion:

2I- I2 + 2e-

Iodine, the reaction product, is ordinarily titrated with a standard sodium thiosulfate solution, with stretch serving as the indicator:

I2 + 2S2O32- 2I- + S4O62-

Solution of sodium thiosulfate are conveniently standardized by titration of the iodine produced when an unmeasured excess of potassium iodide is added to a known volume of an acidified standard potassium iodate solution. The reaction is

IO3- + 5I- + 6H+ 3I2 + 3H2O

Note that each mole of iodate results in the production of three moles of iodine. The procedure that follows is based on this reaction.

Preparation of Solutions

1.     Potassium iodate, 0.0100M. Dry about 1.2g primary-standard KIO3 at 110℃ for at least 1 hr and cool in a desiccator. Weight (to the nearest 0.1mg) about 1.1g into a 500 mL volumetric flask; use a powder funnel to ensure quantitative transfer of the solid. Rinse the funnel well, dissolve the KIO3 in about 200 mL of distilled water, dilute to the mark, and mix thoroughly.

 Potassium iodate

2.     Starch indicator. Add 1g of starch into 100 mL boiling water. Stir it. Leave to cool down.


1.     Pipet 50.00 mL aliquots of standard iodate solution into 250 mL conical flasks.

2.     Treat each sample individually from this point to minimize error resulting from the air oxidation of iodide ion.

3.     Introduce 2 g of iodate-free KI, and swirl the flask to hasten solution.

 Potassium iodide with some iodine from it's desomposition

Potassium iodide solution with some iodine

4.     Add 2 mL of 6M HCl, and immediately titrate with thiosulfate until the solution becomes pale yellow. 

After adding HCl, the solution turned into dark brown immediately


5.     Introduce 5 mL of starch indicator, and titrate with constant stirring to the disappearance of the blue color.


6.     Calculate the molarity of the iodine solution.

Experimental Record

Weight of potassium iodate

=1.028 mmol
Molarity of Potassium Iodate
0.0103 M
Weight of Na2S2O3 sample (hexahydrate)
25.060 g
Theoretical Molarity of Na2S2O3 (aq)
0.1010 M
Consumption of Na2S2O3 (aq)
27.00 Ml

26.80 mL
26.90 mL
Molarity of Na2S2O3 (aq)
0.0890 M
Assay of Na2S2O3
0.0890/0.1010 x 100%

= 88.12%

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