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.
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.
vitamin C tablet(s), NaIO3, 1M NaI solution, 0.5M H2SO4, approximately 0.06M Na2S2O3, freshly prepared starch solution
electronic balance, beaker, volumetric apparatus, pipette, glass rod, white tile, dropper, measuring cylinder
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.