Determination of Vitamin C Concentration by Titration Ascorbic acid (Vitamin C) How much vitamin C is in fresh squeezed orange juice? Compared to orange juice, are vitamin C tablets a good source of vitamin C?

Introduction

Vitamin C, more properly called ascorbic acid, is an essential antioxidant needed by the human body. oxidized vitamin C We will determine the amount of vitamin C using a redox titration method. The chemical name for vitamin C is ascorbic acid. I2 + C6H8O6 → C6H6O6 + 2H+ + 2I– oxidized vitamin C vitamin C We need to have a standardize diodine solution

I2 + starch → Iodine-starch complex I2 + C6H8O6 → C6H6O6 + 2H+ + 2I– vitamin C oxidized vitamin C None of these substances have a distinctive color. How will we know how much I2 to add? After all the vitamin C has reacted, excess I2 begins to react with starch indicator solution to form a colored complex with. I2 + starch → Iodine-starch complex

Acorbic acid + I2 → 2 I - + dehydroascorbic acid Experiment This method determines the vitamin C concentration in a solution by a redox titration using iodine. As the iodine is added during the titration, the ascorbic acid is oxidized to dehydroascorbic acid, while the iodine is reduced to iodide ions. Acorbic acid + I2 → 2 I - + dehydroascorbic acid

Experiment Due to this reaction, the iodine formed is immediately reduced to iodide as long as there is any ascorbic acid present. Once all the ascorbic acid has been oxidized, the excess iodine is free to react with the starch indicator, forming the blue-black starch-iodine complex. This is the endpoint of the titration. The method is suitable for use with vitamin C tablets, fresh or packaged fruit juices and solid fruits and vegetables.

Solutions Needed Iodine solution: (0.005 mol L−1). Weigh 2 g of potassium iodide into a 100 mL beaker. Weigh 1.3 g of iodine and add it into the same beaker. Add a few mL of distilled water and swirl for a few minutes until iodine is dissolved. Transfer iodine solution to a 1 L volumetric flask, making sure to rinse all traces of solution into the volumetric flask using distilled water. Make the solution up to the 1 L mark with distilled water. Starch indicator solution: (0.5%). Weigh 0.5 g of soluble starch and add it to 50 mL of near boiling water in a conical flask. Stir to dissolve then complete to100 mL and cool before using.

Titration 1.Pipette a 20 mL aliquot of the sample solution into a 250 mL conical flask and add about 1 mL of starch indicator solution. 2. Titrate the sample with 0.005 mol L−1 iodine solution. The endpoint of the titration is identified as the first permanent trace of a dark blue-black colour due to the starch-iodine complex. 3. Repeat the titration with further aliquots of sample solution until you obtain concordant results (titres agreeing within 0.1 mL)

Burette containing iodine solution Titration Initially you will see a deep blue color appear in the area that the drop is added. Once you begin to see a deep blue color that persists throughout the solution, add the titrant more slowly and swirl after every drop. Continue adding titrant until you obtain a blue color that lasts 60 seconds. Record the final number of ml Iodine added. Repeat the experiment once or twice Conical flask Add I2 I2 Vitamin C or sample solution + Starch indicator Swirl

Calculations: Assume that the sample solution of vit.C was prepared by extraction of 10 g sample and completed to the total volume of 100ml Calculate the concentration of vit. C in mg/100mL or mg/100g of ascorbic acid, in the sample where: 1ml of Iodine 0.005 mol/L equivalent to 0.88 mg ascorbic acid or vitamin C

Explanation 1ml of Iodine 0.005 mol/L equivalent to 0.88 mg ascorbic acid or vitamin C At the end point of titration C1 V1 of iodine =C2V2 of ascorbic acid Weight of ascorbic acid (mg)= Volume(ml) X Concentration X Molecular weight of ascorbic acid Weight of ascorbic acid (mg )=1mlX0.005X176 = 0.88 mg

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