We are determining the true content of L-ascorbic acid or the antiscorbic vitamin in a typical Vitamin C tablet.

Authors Avatar

Analytical Chemistry 2080

Experiment #5: Determination of Vitamin C

Name: Elizabeth Conway

TA: Anna

Date performed: Wednesday, November 7th, 2007

Date Due: Wednesday, November 14th 2007

Experiment #5: Determination of Vitamin C.

Introduction:

We are determining the true content of L-ascorbic acid or the antiscorbic vitamin in a typical Vitamin C tablet.  Typical Vitamin C tablets claim to have a high Vitamin C content. Structure shown below:

C6H8O6      MW: 176.13

In an unknown sample, a reaction of bromate with an excess of bromide in acid solution is performed, which will oxidize the ascorbic (Vitamin C). The result is a quick reaction which is also stoichiometric.

BrO3-        +        5Br-        6H+        →        3Br2        +        3H2O

C6H8O6        +        Br2        →        +        2H+        +        2Br-

Iodide is then reacted with the excess of bromine to produce iodine.

Br2        +        2I-        →        I2        +        2Br-

The newly produced iodine is titrated with sodium thiosulphate solution

I2        +        2S2O32-        →        S4O62-        +        2I-

The reaction from the standardization titration is:

BrO3-        +        6I-        +        6H+        →        3I2        +        3H2O        +        Br-

Experimental:

A.1 Preparation of 0.4 N Sodium Triosulphate

Approximately 25 grams of Na2S203*5H2O was weighed on a top loader balance and put into a 250 mL Volumetric flask, also 0.1 g of Na3CO3 was added.  It was then dissolved using boiled distilled water, 3 drops of chloroform was then added, and the solution was diluted to the mark. The flask was then labeled and was then put into a cabinet to protect it from the light.

Join now!

A.2 Preparation of 0.1 M Potassium Bromate Solution

Potassium Bromate that had been dried for 1 – 2 hours at 120*C and then cooled in the dessicator was accurately weighed out to 4.1 g on a analytical balance, transferred quantitatively to a 250 mL volumetric flask and diluted to the mark with distilled water. The falsk was then labeled.

A.3 Standardization of Sodium Triosulphate Solution

25 mL of the standard potassium bromate solution was transferred using a 25 mL pipette into a 250 mL Erlenmeyer flask.  2.5 g of potassium iodide followed by 4 – 5 mL of ...

This is a preview of the whole essay