Lab G Experimental Procedure (Vitamin C Analysis)
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)