To determine the vitamin C content in commercial vitamin C tablets by titration between vitamin C (ascorbic acid) & iodine solution. Hence, compare this data with the manufacturer's specification. Principle
In this analysis, certain amount of iodine solution, which is known in excess, is added to acidified ascorbic acid solution. Brown iodine can be easily reduced by acidified ascorbic acid to form colourless iodide ion: ← Equation I
The excess iodine is then back titrated by standard sodium thiosulphate solution, by using a burette and starch solution (used as end point indicator) Similarly, iodine can also be decolourized by thiosulphate ion: 2S2O32- + I2 → S4O62- + 2I- ← Equation II Any iodine present will react with starch to form a blue-black complex. However, when all available iodine has been reacted, the blue-black complex becomes colourless which signals the end-point. Hence, the mass of ascorbic acid reacted by iodine can be calculated. Chemicals
vitamin C tablet(s), NaIO3, 1M NaI solution, 0.5M H2SO4, approximately 0.06M Na2S2O3, freshly prepared starch solution Apparatus
electronic balance, beaker, volumetric apparatus, pipette, glass rod, white tile, dropper, measuring cylinder Procedure
1.> Accurately weigh 0.6 to 0.7g sodium iodate, NaIO3 and record its mass. 2.> NaIO3 is then dissolved in deionized water and is made up to 250cm3 in a volumetric flask. 3.> Sodium thiosulphate, Na2S2O3, is standardized by part of standard NaIO3 solution is shown below: (a) Pipette 25cm3 of NaIO3 solution into a conical flask and about 5cm3 of 1M sodium iodide, NaI, together with 5-10 cm3 of 0.5M sulphuric acid, H2SO4, are also added. Then, the mixture is immediately titrated with approximately 0.06M Na2S2O3 solution. Repeat this step for 3-4 times to obtain a more accurate titration data. (b) When the reaction mixture becomes pale yellow, a few drops of freshly prepared starch solution is added and continue to titrate until the end-point is reached. (c) Calculate the molarity of Na2S2O3 solution. 4.> A vitamin C tablet is dissolved in 150cm3 of 0.5M H2SO4 5.> The mixture is then transferred into a clean 250cm3 volumetric flask and deionized water is added to reach the graduated mark. 6.> Pipette 25cm3 of vitamin C solution (with dil. H2SO4) into a conical flask and about 5cm3 of 1M NaI solution is added. 7.> Pipette 25cm3 of standard NaIO3 solution into the conical flask which contains a mixture of vitamin C, H2SO4 and NaI solution. ●Note: I2 is generated by IO3-, I- & H+: IO3- + 5I- + 6H+ → 3I2 + 3H2O ← Equation III 8.> The mixture is then titrated immediately with standard Na2S2O3 solution to find out the number of moles of excess I2. Hence, the mass of ascorbic acid reacted can be calculated. Results
Mass of NaIO3 : 0.63 g ∴ Molarity of 250cm3 standard NaIO3 solution : (0.63 ÷197.89 ÷ 0.25)M = 0.012734 M By equation III, no. of moles of I2 formed = no. of moles of NaIO3 x 3 ÷10 = (0.63 ÷197.89 x 0.3)mol = 9.550761x10-4 mol By equation II, no. of moles of S2O32- needed to react all I2 = no. of moles of I2 formed x 2...