Question

I'm doing a lab report determination of ascorbic acid in vitamin C tablet.

The equation used are: IO₃^- + 8 I^- + 6 H+ <--> 3 I₃^- + 3 H2O

Back titration with thiosulfate: 2 S₂O₃^-2 + I₃^- --> S₄O₆^2- + 3 I^-

This is my value from the lab:

Mass of Na2S2O3 (g) : 7.8459 g ; Mass of KIO3 (g) : 2.5272 g

Mol of KIO3 dispensed: 0.01181 mol

Mass of tablet: 0.5986 g; mass of pulverized tablet: 0.5708 g

Volume of thiosulfate dispensed (mL): 29.9 mL

So far I calculated:

Mol thiosulfate: 0.002100 mol (not sure if its right)

Now I need to figure out how to find:

Mol of IO3- consumed by thiosulfate?

Mol of IO3- consumed by vitamin C?

Mol of ascorbic acid?

How do I find those?

Answer 1

We can divide the experiment in parts to understand the process:

Step I : Standardization of the provided solution of Na₂S₂O₃ by titrating it with the I₃^– (aq) produced by the reaction of dissolved samples of primary standard KIO₃(s) with excess KI. (Note – The molar mass of KIO3 is 214.00 g/mol)

Step II: Determination of the IO₃^– in the unknown by titrating (with the Na2S2O3) the I₃^– (aq) generated by the reaction of pipetted samples of the unknown with excess I ^– (aq).

Here we use the :

1M potassium iodide solution (KI(aq))

1M hydrochloric acid solution (HCl(aq))

Starch indicator

Part 1 Preparation of the KIO3 (Standard) Samples

weigh accurately 2.14 gm of KIO3(s), carefully tipping each sample into a clean 100ml volumetric flask Add deionized water and swirl gently to dissolve the KIO3 (0.1M)

Part 2: Titrations of Standard and Unknown Samples

To the burette fill a Na2S2O3(aq) To the first standard sample, add, using a graduated cylinder, 10 mL of the 1 M HCl(aq) and 10 mL of the KI(aq). Titrate with the Na2S2O3 solution to a color between pale orange and yellow. Add 1-2 mL of starch indicator and continue the titration until the solution becomes colorless.

To calculate the Concentration of Na2S2O3:

Calculate the moles of KIO₃ used, (Given: 0.01181M) the moles of S₂O₃^2– (aq) required (based on the stoichiometry = 0.0059M = 0.059ml) and hence [Na2S2O3]) from the volume of titrant required.

To Find Concentration of IO3 – (aq) in the Unknown

We use the titration volume and the calculated [Na2S2O3] (0.059 ml), the moles of IO3 – (from the stoichiometry= 0.01181M) and then the concentration of IO3 – in unknown sample.

Determination of Vitamin C in a Tablet

Weigh the tablets together, record the weight, and then grind all tablets to a powder with a mortar and pestle.

Weigh (to ± 0.1 mg) approximately 0.6 g of the powder into a small beaker, dissolve in 10- 20 mL 0.6 M H2SO4 and carefully transfer to a 500-mL volumetric flask. Dilute to the mark with distilled water. This is stock solution.

Pipet 10 mL of the stock solution into an flask and carefully add about 0.1 g of oxalic acid

Into this flask, pipet 25.00 mL of the standard KIO3 solution, and add about 2 g of KI and 10 mL of 0.6 M H2SO4. Titrate with Na2S2O3 solution as before, adding 2 mL of starch solution just prior to the endpoint. Record the volume of titrant used.

Calculations:

Calculate the molar concentration of the standard KIO3 solution.

Calculate the molar concentration of the Na2S2O3 titrant from the concentration of the KIO3 solution and the volume of titrant needed to titrate 25 mL of the standard KIO3. Report your values for the concentration of Na2S2O3

Calculate the molar concentration of ascorbic acid in each of your sample solutions from the volume of Na2S2O3 titrant required to titrate the excess iodine, knowing the total amount of KIO3 which was added, and using the average molarity of the Na2S2O3 titrant determined

From each of your three determinations, calculate the mass (in milligrams) of ascorbic acid

in sample.

Determine the average mass of ascorbic acid per tablet, and the average %wt of ascorbic acid per tablet.