Lab 4 - Ascorbic Acid (Vitamin C) Determination by Iodometric Titration

Lab 4 - Ascorbic Acid (Vitamin C) Determination by Iodometric Titration ABSTRACT The objective of this experiment is to determine the concentration of Ascorbic Acid (Vitamin C) through the standardization of iodine solutions. This experiment involves iodometric titration in which iodine reacts with Ascorbic Acid oxidizing it to form dehydroascorbic acid. The redox titration endpoint is determined by the first iodine excess that is complexed with starch, giving a deep blue-violet color. The results of the titration of iodine solution using ascorbic acid as a calibration standard determined the amount of Ascorbic Acid to be -2.225g. The standardization of the iodine solution using ascorbic acid was accurate and precise however there was a calculation error that is exhibited in this report. INTRODUCTION This experiment involves an iodometric titration in which iodine reacts with ascorbic acid (Vitamin C), oxidizing it to dehydroascorbic acid. The redox titration endpoint is determined by the first iodine excess that is complexed with starch, giving a deep blue-violet color. Iodine is relatively insoluble but by complexing Iodine with Iodide (I2), Triiodide (I 3) will result. Ascorbic acid (vitamin C) mild reducing agent that reacts rapidly with triiodide a mild oxidizing agent. As displayed in the following stoichiometry. I 2 + I - ↔ I 3IO3- + 8I- + 6H+ ↔ 3I3 - + 3H2O I 3 + 2S2O 23 - ↔ 3I + S4O6 2- (reaction 1) (reaction 2) Ascorbic + I3- + H2O ↔Dehydroascorbic acid + 3I- + 2H+ (reaction 3) The preparation of triiodide, the concentration of Triiodide ( I3 - ) is determined by the excess Potassium Iodide in the form of KIO3 in the solution. Then leaving Triiodide to oxidize Ascorbic Acid to form dehydroascorbate and three Iodide ions. The purpose of this experiment is to determine the amount of Ascorbic acid in a vitamin using back titration. By extracting the Ascorbic acid from a Vitamin C supplement with Potassium Iodate (KIO3) the conversion to dehydroascorbate by excess Triiodide. The remaining Triiodide is determined by the titration of triodide with a standardized thiosulfate solution. The back titration was performed by adding the titrated reagent after excess exemplified in reaction 2. MATERIALS · Starch indicator: Make a paste of 5 g of soluble starch and 5 mg of Hg2I2 (used to preserve the solution) in 50 mL of distilled water. Pour the paste into 500 mL of boiling distilled water and boil until it is clear. · Sodium thiosulfate: 9 g Na2S2O3 • 5H2O/group. · Sodium carbonate: 50 mg Na2CO3/group. · Potassium iodate: 1 g KIO3/group. · Potassium iodide: 12 g KI/group. · 0.5 M H2SO4: 30 mL/group. · Unknown: Dietary supplement containing X mg of Vitamin C per tablet is suitable/group. · Amber bottles · 0.3 M H2SO4: 180 mL/group. METHODS Preparation and Standardization of Thiosulfate Solution 1% STARCH INDICATOR To make a paste of a starch indicator 5 g of soluble starch and 5 mg of HgI2 is dissolved in 500 mL of boiling water till clear. IODINE SOLUTION To prepare an Iodine solution of 0.07 M Na2S2O3 , approximately 8.7 g of Na2S2O3 • 5H2O is to be dissolved in 500 mL of freshly boiled water containing 0.05 g of Na2CO3. A 0.01 M KIO3 solution is prepared by accurately weighing approximately 1g of solid reagent and dissolving it in the 500-mL volumetric flask. STANDARDIZING SOLUTIONS The thiosulfate solution was standardized by pipetting 50.00 mL of the Iodine Solution ( KIO3 )solution into a flask. Then 2 g of solid KI and 10 mL of 0.5 M H2SO4 is added. Titration of thiosulfate occurs until the solution has lost almost all its color . Then 2 mL of starch indicator is added to complete the titration. This titration step was repeated two additional times in 50.00 mL volumes of KIO 3 solution. Analysis of Vitamin C The following analysis was performed three times. 1. Dissolved two tablets in 60 mL of 0.3 M H2SO4, using a glass rod to help break the solid. 2. 2 g of solid KI and 50.00 mL of standard KIO3.were added. Then standardized thiosulfate was titrated, adding 2 mL of starch indicator just before the end point. CALCULATIONS DATA & RESULTS End Point Indicator Color: Blue-Violet TRIALS VOLUME OF POTASSIUM IODATE (mL) VOLUME OF SODIUM THIOSULFATE (mL) CONCENTRATION OF SODIUM THIOSULFATE (M) 1 50 24.6 1.138 2 50 24.4 1.148 3 50 24.35 1.150 Average total volume: 24.6+24.4+24.35 /3 =24.45 Average Concentration of Sodium Thiosulfate: 1.145 M TRIAL VOLUME OF SODIUM THIOSULF ATE (mL) ADDED MOLES OF SODIUM THIOSULF ATE MOLES OF MOLES OF IODINE IN IODINE SOLUTION TITRATED MOLES OF RESIDUAL IODINE MASS OF ASCORBIC ACID VIT. C (g) 1 24.6 0.0141 0.00140 0.0140 -0.0127 -2.237 2 24.4 0.0140 0.00140 0.0140 -0.0126 -2.219 3 24.35 0.0140 0.00140 0.0140 -0.0126 -2.219 Average Amount of Ascorbic Acid: -2.225g DISCUSSION The first reaction takes place between iodate (IO3- ) with excess iodide (I- ) in acid environment with the following formation of iodine (I2 ). Solution turns dark-brown. In the presence of Vitamin C, part of the iodine I 2 is reduced to iodide I - which is colorless compared to iodine. On the other hand, vitamin C is oxidized to dehydroascorbic acid (reagents ratio is 1:1): The rest of the iodine is titrated with sodium thiosulfate. By doing some common stoichiometric calculations, it is possible to determine the iodine titrated and therefore the amount of Vitamin C present in the sample you are analysing A suitable method for the determination of vitamin C (C6H8O6) is a titration with potassium iodate (KIO3). Potassium iodate is used as a titrant and is added to an ascorbic acid solution that contains strong acid and potassium iodide (KI). Potassium iodate reacts with potassium iodide, liberating molecular iodine (I2): [1] KIO3 + 5KI + 6H+ → 3I2 + 6K+ + 3H2O As long as the solution contains ascorbic acid, the I2 produced in equation 1 is used up in a rapid reaction with ascorbic acid (equation 2), during which dehydroascorbic acid (C6H6O6) and iodide ion (I-) are formed: [2] C6H8O6 + I2 → C6H6O6 + 2I- + 2H+ Potassium iodide must be added in excess to keep iodine dissolved. Once all the ascorbic acid has been consumed, any excess iodine remains in solution. Since aqueous iodine solutions are brown in colour, iodine can act as its own indicator. However, it is difficult to detect endpoints using iodine colouration alone therefore a starch is added, which formed an intensely blue coloured complex with iodine. CONCLUSION Iodine is readily reduced by Vitamin C and by knowing the initial amount of iodine in the solution and that one reduced by sodium thiosulfate, it is possible to determine the content of Vitamin C in a specific product. In this experiment it was determined that the average amount of Ascorbic Acid (Vitamin C) is found to be -2.225g. Due to the fact that the amount of moles of Iodine reacted (averaged to 0.00140 mol) during the titration was more than the amount of moles of Iodine initially in the titration ( 0.00140 mol) proved that there is a calculation error within this experiment. Because it is not possible for there to be more of a product within a solution than that was originally incorporated. Another conclusion could be that within the reaction taking place more Iodine was yielded from a contaminate within the solution. Overall this experiment objective was observed and the expected results were not determined.