AL Chem F.6 titration(Na2S2O3)

This is an experiment employing the technique of back titration to determine the amount of vitamin C in a sample of juice. In the 6 steps listed, the first 4 steps are experimental procedure, and the last 2 are calculations. The objectives of the 4 experiments steps are:

1) Prepare a standard solution of I2.
(The concentration of I2 is then known.)

2) Standardize a Na2S2O3 solution by the standard I2 solution.
(The concentration of Na2S2O3 is then known.)

3) Add an excess and known amount of the I2 solution to a known volume of juice.
(All the vitamin C in the juice reacts with the I2, and I2 is in excess. If the amount of reacted I2 is known, the amount of vitamin C would be determined.)

4) The amount of excess I2 is determined by a titration against the standard Na2S2O3 solution.
(The total amount of I2 added is known in step 3, and the amount of excess I2 is determined in step 4. Therefore, the amount of I2 that reacted with vitamin C in the juice would be determined. Consequently, the amount of vitamin C in the juice would be determined using the data in step 3.)

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Why is it necessary to perform step 2 ?

A standard solution of Na2S2O3 should be used in step 4. Na2S2O3 solution is prepared by dissolving Na2S2O3Ÿ5H2O crystals in water. The commercially available Na2S2O3 should not be used as a primary standard, because commercially available Na2S2O3Ÿ5H2O crystals are not pure. The Na2S2O3Ÿ5H2O crystals usually contain some impurities such as S, Na2SO3, Na2SO4, Na2CO3 and NaCl, and the crystals are also efflorescent (i.e. to release the water of crystallization in dry air) and deliquescent (i.e. to absorb water from moist air and to dissolve in the water absorbed). In other words, it is necessary to standardize the Na2S2O3 solution. If the concentration of Na2S2O3 is calculated based on the mass of impure Na2S2O3Ÿ5H2O crystals and the volume of the solution, a significant error would be caused.
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