Analysis of Commercial Vitamin C Tablets

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EXPERIMENT ( 1 )        

Topic :        Analysis of Commercial Vitamin C Tablets

Introduction :        In this experiment the vitamin C content of commercial tablets is determined titrimetrically and compared

with the manufacturers’ specifications.

Vitamin C is ascorbic acid which is rapidly and quantitatively oxidized by iodine in acid solution according

to the following equation :                C6H8O6(aq)        +  I2(aq)                C6H6O6(aq)        +  2 H+(aq)        +  2 I-(aq)

The standard method for determination of ascorbic acid involves the direct titration of the acidified sample with a standard iodine solution. But the low solubility of iodine makes this procedure less than ideal.

The proposed experiment avoids these difficulties by generating in situ a known excess quantity of iodine

by the reaction between iodate(V) & iodide :        IO3-(aq)  +  5 I-(aq)  +  6 H+(aq)    3 I2(aq)  +  3 H2O(l)

After reacting with ascorbic acid, the remaining iodine may then be titrated against standard sodium

thiosulphate solution.

Chemicals :        Vitamin C tablets , 0.05 M Na2S2O3 , 0.5 M H2SO4 , 1 M KI , KIO3 , Starch solution (freshly prepared)

Procedures :        1.        Accurately weigh out 0.6 to 0.7g of potassium iodate(V). Record the mass..

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  1. Dissolve this in deionized water and make up to 250 cm3 in a volumetric flask.
  2. Use such iodate(V) solution to standardize the given thiosulphate solution as follows :
  1. Pipette 25.0 cm3 of KIO3(aq) into a conical flask, add to it about 5 cm3 of 1 M KI(aq) followed

by about 10 cm3 of 0.5 M H2SO4(aq) , then immediately titrate with 0.05 M Na2S2O3(aq) .

  1. Add a few drops of freshly prepared starch solution when the reaction mixture turns pale

yellow and continue to titrate to the end point. Record your results in Table 1.

  1. ...

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