Question

Determination of Ascorbic Acid in Vitamin C Tablets by iodometric titration:

Given:

Preparation of starch indicator: Mix 1.029 gram soluble starch then dilute to 500mL with boiling hot Deionized water. Calculate the molarity.

Preparation of .5M H2SO4: Mix 2.7mL H2SO4 with 100mL Deionized water. Calculate the molarity.

Preparation of .3M H2SO4: Mix 3.33mL H2SO4 with 200mL Deionized water. Calculate the molarity.

Preparation of .01M potassium iodate: Take 1.1273 grams of primary standard KIO3, and dilute to 500mL with Deionized water. Calculate the molarity.

Preparation of .07M Sodium Thiosulfate: Boil 500mL Deionized water. Add .051 grams Na2Co3. Add 8.7898 grams (Na2S2O3 * 5H2O). Calculate the molarity.

Standardization of the sodium thiosulfate solution:

Trial titration: Pipet 50ml of .01M potassium iodate into a 250 Erlenmeyer flask. Add 2.002 grams KI. Then add 10mL of .05M H2SO4 and titrate with sodium thiosulfate solution until your mixture turns yellow. My mixture turned yellow at 44.2mL of thiosulfate used. Then I added 5ml starch solution and the solution turned blue. I then titrated it again with an additional 1.1mL til my mixture turned pale. Calculate the molarity of your thiosulfate solution for the titration.

Final titration: Weight out one vitamin C tablet= .3461g and dissolve in 60mL of .3M H2SO4 solution. Add 2.0204g KI and add 75mL KIO3 solution. Swirl then add 10mL of .5M H2SO4 and titrate with your sodium thiosulfate solution. it took 20.2mL to titrate my mixture to yellow. Then add 5mL starch solution, your solution turns dark blue then titrate with sodium thiosulfate until your solution is clear. It took an extra 1.0mL sodium thiosulfate solution.

Calculate the number of mg for the vitamin C tablet for the titration. Then calculate the percentage, by mass, of vitamin C in each tablet based off titrations.

Answer 1

1. The molarity of the sulphuric acid solution –

2.7 mL of concentrated sulphuric acid dissolved in 100 mL of water.

The molarity of the conc. Sulphuric acid solution is – 18.4 M

M_initialV_initial = M_finalV_final

18.4 X 2.7 = M_final X (100+2.7)

M_final = 18.4 X 2.7 /(100+2.7) = 0.48 M

2. The molarity of the sulphuric acid solution –

3.33 mL of concentrated sulphuric acid dissolved in 200 mL of water.

The molarity of the conc. Sulphuric acid solution is – 18.4 M

M_initialV_initial = M_finalV_final

18.4 X 3.33 = M_final X (200+3.33)

M_final = 18.4 X 3.33 /(200+3.33) = 0.30 M

3. Molarity of KIO₃ = 1.1273g dissolved in 500mL –

Molecular weight of KIO₃ is 214.

Molarity =(1.1273/214) X (1000/500) = 0.0105 M

4. Molarity of sodium thiosulphate = 8.7898g dissolved in 500mL –

Molecular weight of sodium thiosulphate is 158.11

Molarity =(8.7898/158.11) X (1000/500) = 0.11 M

5. The titre reading for trail run (without vitamin c) is 44.2 + 1.1 = 55.3 mL = V_s

Standardization of sodium thiosulphate

Volume of primary standard used KIO₃ V_k= 50 mL

Concentration calculated in step 3. Molarity M_k = 0.0105 M

M_k V_k = M_s V_s

M_s = M_k V_k / V_s = 0.0105 X 50 / 55.3 = 0.0095 M

6. The titre reading for with vitamin c tablet is 20.2 + 1 = 21.2 mL

The weight of the ascorbic acid can be determined by

the factor divide by 1000 is taken to conver the volume to liters from mL.

the next two terms in the equation is convertion factors. 2 mole thiosulphate required for reaction with 1 mol of iodite ion in tern 1 mol of iodate ion oxidizes the 1 mol of ascorbic acid. the last term is to conver the concentration to weight by multipying with molecular weight.

molecular weight of ascorbic acid = 176.13

on substituting all the values the

weight of ascorbic acid = 0.0177g

The weight of tablet taken is 0.3461 g for titration in that 0.0177 g is ascorbic acid.

ascorbic acid % in tablet = 0.0177/0.3461 X 100 = 5.11%