Analysis of commercial vitamin C tablets

Results

Standardization of Na2S2O3 solution

Weighing :Mass of solid (potassium iodate(V) ) used                = 0.6808 g

Titration :        Burette contained        :        Na2S2O3                        

Conical flask contained        :        KI,KIO3,        H2SO4        

Burette readings:

Average titrant used = (19.20+19.20)/2

                =19.20 cm3

Determination of vitamin C (Ascorbic acid)content in commercial vitamin C tablet

Titration :        Burette contained        :Standard Na2S2O3                        

Conical flask contained        :Vitamin C,KI,KIO3,H2SO4        

Burette readings:

Average titrant used = (14.50+14.40)/2

                =14.45 cm3

Questions

1. Calculate the concentration of the give sodium thiosulphate solution.

No. of mole of KIO3 titrated  = [0.6808/(39.1+126.9+16x3) ]/10
=(0.6808/214)/10
=0.0003181308411mol

∴No. of mole of I2 produced=3x No. of mole of KIO3 titrated

=0.0009543925234mol

∴No. of mole of Na2S2O3(aq) titrated=2x No. of mole of I2 produced
=2x0.0009543925234
=0.001908785047mol

∴Molarity of Na2S2O3(aq) =No. of mole of Na2S2O3(aq)

 titrated/Average volume of Na2S2O3(aq)

                                   titrated

                      =0.001908785047/(19.20/1000)
=0.099415887
~0.0994M

∴Concentration of Na2S2O3(aq) in gdm-3
=0.099415887x(22.99x2+32.06x2+16x3)
=15.71765187
~15.7
                                           

2. Calculate the mass of vitamin C(Ascorbic acid) per tablet.
   No. of mole of KIO3  used= [0.6808/(39.1+126.9+16x3) ]/10
                        =(0.6808/214)/10
                        =0.0003181308411mol
   
   ∴No. of mole of I2 produced=3x No. of mole of KIO3  titrated
                         =0.0009543925234mol
   ∵No. of mole of Na2S2O3(aq) titrated=MV
                                 =0.099415887x(14.45/1000)
                                 =0.001436559567mol
   ∴No. of mole of I2  reacted with Na2S2O3(aq)= No. of mole of Na2S2O3(aq) titrated/2
                                       =0.001436559567/2
                                       =0.0007182797836mol
   ∴No. of mole of I2 reacted with 10cm3 vitamin C(Ascorbic acid)
    = No. of mole of I2 produced- No. of mole of I2  reacted with Na2S2O3(aq)
    =0.0009543925234-0.0007182797836
    =0.0002361127398mol
   ∵No. of mole of 10cm3 vitamin C(Ascorbic acid)
    = No. of mole of I2 reacted with 10cm3 vitamin C(Ascorbic acid)
    =0.0002361127398mol
   ∴No. of mole of 250cm3 vitamin C(Ascorbic acid)
    =0.0002361127398x25

=0.005902818495mol
∴Mass of Vitamin C(Ascorbic acid) per tablet
 =0.005902818495x(12x6+8+16x6)
 =1.038896055
 ~1.04g

3. State the function of the starch solution. Explain why it should be added only when the reaction mixture becomes pale yellow.

    is often used in chemistry as an  for  where  is present as starch forms a very dark blue-black  with iodine. Moreover, the indicator is not affected by the presence of  (I-). In order for the starch solution to change colour, both starch and iodide must be present. Starch forms an unstable complex which is blue coloured in low concentrations of . However, this complex is in high concentration of iodine .When the mixture turns pale yellow ,it indicates that iodine is partly reduced to iodide ions. That means the colour change to yellow indicates that the concentration of iodine in the mixture is no longer too high .If the mixture has not become pale yellow and the starch indicator is added too early, the conical flask still contains high concentration of iodine .As a result, highly stable complex is formed, the starch indicator then cannot be decolourised and the complex will spoil the reaction medium. Consequently, the titration cannot continue. Therefore the iodine must be diluted first with the titrant  ,sodium  to ensure no highly stable complex is formed. After that the starch can be added.

4. After the addition of sulphuric acid to the reaction mixture ,what would happen if titration is not carried out immediately?
   
   After the addition of sulphuric acid to the reaction mixture ,iodine will be readily produced. Iodine is very weakly soluble in the water, and can be easily lost from the solution due to its volatility.
   By the chemical equation,2S2O32-   +   I2   →   S4O62-   +   2I-          
   It is known that the lost of Iodine   will lower its number of mole in the conical flask. Moreover, when iodine is added to water or in the aqueous solution, OI- and I- ions will be formed.
                      I2(l) + H2O(l) →OI-(aq) + 2H+(aq) + I-(aq)
   In both cases, the number of mole of I2 will be lowered. Consequently, as the calculation of number of mole of   S2O32-        depends on the actual   number of mole of I2 ., the number of mole of   S2O32- calculated will be lower than the actual one. Due to the inaccuracy of standardization of sodium thiosulphate solution, the results and calculation later on for the determination of vitamin C (Ascorbic acid) content in commercial vitamin C tablet will also be unreliable.
5. It is well known that vitamin C(ascorbic acid) decomposes if it is exposed to air or heated. Briefly describe how you would investigate these two factors?

To determine the effect of exposure to air, a vitamin C tablet should be dissolved in 150cm3  H2SO4        (aq).Then, Expose the conical flask containing the mixture and leave its mouth open for 2-3 days. After that, carry out the procedure above to determine the vitamin C (Ascorbic acid)content through titration .Finally, compare the result with the value obtained in the experiment this time(which serves as control). The volume of standard Na2S2O3 used for titration should be larger. It implies that the number of mole of vitamin c reacted with I2 decreases. In short, the Vitamin C content per tablet decreases due to the effect of exposure to air.

To determine the effect of heating ,a vitamin C tablet should be dissolved in 150cm3  H2SO4(aq).Then, heat the conical flask containing the mixture in a bath of boiling water for about 10 minutes. After that, carry out the procedure above to determine the vitamin C (Ascorbic acid)content through titration .Finally, compare the result with the value obtained in the experiment this time(which serves as control).  The volume of standard Na2S2O3 used for titration should be also larger. It implies that the number of mole of vitamin c reacted with I2 decreases. In short, the Vitamin C content per tablet decreases due to the effect of heating

• Conclusion

Through the standardization of Na2S2O3 solution ,it was found that The molarity of Na2S2O3 solution is 0.0994M.Later on, the mass of vitamin C per tablet obtained by the titration above is1.04g which is quite close to the manufacturer's specification(about 1 g) and acceptable.    

  Reference

http://www.newworldencyclopedia.org/entry/Iodine

F.6 Chemistry Notes Section I by Ms Sin W L

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