Analysis of commercial vitamin C tablets

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Analysis of commercial vitamin C tablets


Objective
           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:
                                       2S
2O32-   +   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.> NaIO
3 is then dissolved in deionized water and is made up to 250cm3 in a volumetric flask.
         3.> Sodium thiosulphate, Na
2S2O3, is standardized by part of standard NaIO3 solution is shown below:
                         (a) Pipette 25cm
3 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, H
2SO4, 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 Na
2S2O3 solution.
         4.> A vitamin C tablet is dissolved in 150cm
3 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 25cm
3 of vitamin C solution (with dil. H2SO4) into a conical flask and about 5cm3 of 1M NaI solution is added.
         7.> Pipette 25cm
3 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+:
                        IO
3-   +   5I-   +   6H+   →   3I2   +   3H2O                                         Equation III
         8.> The mixture is then titrated immediately with standard Na
2S2O3 solution to find out the number of moles of excess I2. Hence, the mass of ascorbic 
               acid reacted can be calculated.

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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
                                                                                              = (9.550761x10
-4 x 2)mol
                                                                                              = 
         1.910152x10-3          mol
                    
                 The table below shows the standardization of
Na2S2O3 solution by titration:-

             ∴ Concentration of standard Na2S2O3 solution = 1.910152x10-3 ÷ (29.8 x 10-3) M =      0.064099     M
                     
                 The table below shows the determination of
I2 which is not reduced by ascorbic acid by titration with standard ...

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