Lab G Experimental Procedure (Vitamin C Analysis)

Experimental Method

This experiment will quantitatively analyze a consumer product for vitamin C content. A traditional way to analyze for vitamin C relies on iodometry in the presence of a starch indicator.  Iodine is reduced by ascorbic acid in the addition process as indicated by the reaction equation below:

     [Vit. C] = Unknown          [I₂] = known excess                     Product                           Final [I₂]

This titration technique measures the final concentration of I₂ in the titration sample and then the original concentration of Vitamin C is determined by difference. 

One mole of iodine reacts with one mole of vitamin C. This is a stoichiometric ratio of 1:1.  In this experiment a solution of vitamin C is added to a solution containing a known excess of iodine. When starch is added, the solution will turn blueish-grey in colour and this excess amount of iodine can then be titrated against standardized sodium thiosulphate from Lab F. This is a specific type of titration called a back-titration.  We then determine the moles of vitamin C by difference as follows: 

(Initial moles I₂) – (moles I₂ left over) = (moles vitamin C)