Title: Determination the amount of ascorbic acid(vitamin c)

-1 X watch glass

-1 X burette

-2 X conical flask

-2 X glass rod

-1 X white tile

-Potassium bormate solid, KBrO3(s)

-Potassium bromide solution, KBr (0.3M,aq)

- Potassium iodide solution, KI (0.1M,aq)

-Diute sulphuric acid, H2SO4 (2M, aq)

-Standardize sodium thiosulphate solution, Na2S2O3 (0.0249M,aq)

-starch indicator

-Effervescent Vitamin C tablet

Procedure:

Preparation of Vitamin C solution

1. An effervescent Vitamin C tablet was added into 150 cm3 of water in a beaker.
2. A watch glass was put above the beaker.
3. Time was required to wait until the tablet was completely dissolved.
4. When effervescence stopped, content in the beaker was transferred to a 250 cm3 volumetric flask.
5. Content on the watch glass was rinsed back to the 250cm3 volumetric flask.
6. Deionizied water was added to the volumetric flask up to the graduation mark.
7. The solution was mixed well.

Preparation of mixture of Potassium bromate solution and diluted sulphuric acid

1. 1.25g of potassium bromate was weighed and added into a beaker.
2. 30cm3of diluted sulphuric acid was pipetted into the beaker.
3. 150 cm3 of water was pipetted into the beaker.
4. The substances in the beaker were mixed well by a glass rod.
5. The content in the beaker was transferred into a volumetric flask.
6. Deionozed water was added to the volumetric flask up to the graduation mark.
7. The solution was mixed well.

Formation of Bromine and dehydroascorbic acid

1. 10 cm3 of the content mixed(H2SO4+KBrO3) was pippetted from the volumetric flask to a conical flask.
2. 25cm3 of ascorbic acid solution was pipetted from volumetric flask to the conical flask.
3. 20cm3 of Potassium bromide was pippeted into the flask.
4. The flask is stoppered.
5. The solution inside the flask is mixed fully for 20 minutes until a dark-brown color is formed.

Formation of iodine

1. 10cm3 of potassium iodide was added to the conical flask.
2. The flask is stoppered.
3. The solution inside the flask is mixed fully.

Back titration with sodium thiosulphate solution

1. The mixture in the conical flask was then titrated immediately with standard sodium thiosulphate solution.
2. The sodium thiosulphate solution on top is added to the solution mixture
3. The liquid is swirled during the titration.
4. When a pale yellow color is observed, starch indicator is added to the mixture.
5. The sodium thiosulphate solution is added to the solution mixture until the end point is reached, that is the disappearance of the dark blue color.
6. The volume of sodium thiosulphate solution used for reaching the end-point was recorded.
7. Steps in Formation of iodine and Back titration with sodium thiosulpahte soultino were repeated twice to calculate the mean volume of sodium thiosulphate used.
8. The mass of ascorbic acid reacted was then calculated.

 Precation

-Ascorbic acid (vitamin C) is unstable and can be easily oxidized. Oxidation can be highly speeded up when it's heated or dissolved in water. To obtain the best results of the experiment, the tablet is dissolved in water only when titration is about to begin. Besides, the tablet (or the vitamin C solution) should avoid storage in direct sunlight, keep in a cool place and in an air-tight bottle. Moreover, only pour out enough amount of vitaminC from the volumetric flask just about to use and then stopper it immediately.

-Starch solution should be added only when the solution becomes pale yellow.

-The mixture of reacted vitamin C and excess I2 should be titrated with Na2S2O3 immediately to reduce I2 lost due to vapourization.

-Handle chemicals with care, H2SO4 is an irritant which may irritate the skin especially if there is a wound.

-Avoid exposure to I2 solution since the I2 vestige on skin is quite hard to remove, it will remain for a long time.

Result:

The table below shows the standardization of Na2S2O3 solution by titration.

Calculations:

Involved equations:

KBrO3 +5KBr+3H2SO4                     3K2SO4+3Br2+3H2O

                       

+Br2                                             +2H + +2Br -                                                                                                

Br2+2I -                                2Br - + I2

          2S2O32-   +   I2                       S4O62-   +   2I-       

Given: Molarity of diute sulphuric acid (H2SO4) =2M

      Molarity of Poatssium iodide  (KI)    =0.1M

      Molarity of potassium bromide (KBr)   =0.3M

      Molarity of sodium thiosulpahte (Na2S203)=0.0249M

Calculations on the number of mole of iodine reacted.

By  2S2O32-   +   I2                       S4O62-   +   2I-       

Mole of S2O32- : mole of I2   =2:1

∴No of mole of iodine reacted= (volume of S2O32 used X Molarity of S2O32 ) ÷ 2

                         = (25.17÷1000 X 0.0249 ) ÷ 2

                         =3.134 X 10-4mol.

Calculations on the number of mole of bromine remaining in the mixture.

By    Br2+2I -                                2Br - + I2

Mole of Br2 :Mole of I2 = 1:1

∴No of mole of bromine remaining =3.134 X 10-4mol

Calculations on the number of mole of total bromine formed initially.

As 1.25 g of KBrO3 is weighed out,

∴ molarity of KBrO3 solution = (mass of KBrO3 ÷ Molar Mass of KBrO3) ÷(volume of KbrO3)

                          = (1.25÷ (39+3X16+80)) ÷ (250÷1000)

                          = 0.03M

 No. of mole of in 10cm3KBrO3 = Molarity of KBrO3 X Volume of KBrO3

                                       = 0.03 X 10 ÷1000

                          =3 X 10-4 mol.

By KBrO3 +5KBr+3H2SO4                     3K2SO4+3Br2+3H2O

No. of mole of KBrO3 : No of mole of Br2 = 1 : 3

∴ No of mole of total bromine formed initially= 3 X No. of mole of KBrO3

 in 10 cm3

                                       = 3 X 3 X 10-4

                                           = 9 X 10-4 mol.

Calculations on the number of mole of bromine reacted with Vitamin C

Br2 used to react with Vitamin C = no of mole of bromine formed initially – no of mole of bromine remains in the solution after addition of vitamin C

                          = 9 X 10-4 -3.134 X 10-4

                          =5.866 X 10-4 mol.

Calculations on the number of mole of Vitamin C in each tablet

By

                         

+Br2                                                         +2H + +2Br -                                                                                                

∴No. of mole of C6H8O6 : No. of mole of Br2= 1:1

   ∴ No. of mole of C6H8O6 = 5.866 X 10-4 mol.

As 25 cm3 of ascorbic acid solution was pipetted to react

∴No. of mole of C6H8O6 in 250 cm3 = 5.866 X 10-3 mol.

Calculations on the mass of vitamin C in each tablet

By equation: No. of mole = mass ÷ molar mass

          Mass of C6H8O6 = No. of mole of C6H8O6 X molar mass

                       = 5.866 X 10-3 X ( 12X 6 + 8 + 6X 16)

                       =1.04g

Discussions:

Discussion on natures of vitamin C

Vitamin C (ascorbic acid) is a white, odourless powder, and a water-soluble vitamin. When dry, it's reasonably stable in air but in solution, it oxidizes rapidly in the presence of air It is an unsaturated compound containing a carbon-carbon double bond adjacent to two OH group. The enol group can be easily oxidized to a keto group. Stronger oxidizing agent such as bromine is not recommended as the double bond may be ruptured.

Discussion on experiments

Vitamin C is a reducing agent which can reduce I2 into colourless I-. However, the back titration is used instead of direct titration with I2 because  I2 can be easily vapourize thus the molarity of I2 solution is difficult to control, thus using 'standard' I2 solution for titration is impossible. Moreover, back titration enjoys an advantage of consuming all the unstable reactant at the start of the reation.In the experiment, the known amount of I2 is generated, the excess amount of I2 is reduced by Na2S2O3 immediately, to reduce I2 lost by vapourization. This also explains why I2 is placed in conical flask but not burette when standardizing Na2S2O3.

Starch solution can form a blue-black complex with I2, since it can detect a very little amount of I2 present, it's used to determine the end-point,with the colour changes from pale blue to colourless (no I2 present).Since starch irreversibly combines with iodine at a high concetntaion of iodine, the starch is added at the later stage of titration.

In general, iodine solution is brown in color. While in this experiment, the resulting iodine mixture gives a color of dark brown. As iodine is only slightly dissolved in water , when it mixed with iodide ion, reddish color iodate ions are formed and contribute to a deeper brown color.

I2(s)      +      I-(aq)                   I3-(aq)

Discussion on errors

Vitamin C is sensitive to heat and light, and will oxidize readily by reacting with the oxygen in the atmosphere. As a result, the amount of vitamin C is reduced when oxidation results.

  As iodine is lost to the atmosphere by vaporization when it is formed, so that the calculated value may not be so accurate.

  As the original vitamin c solution have already a similar color with the iodine solution, so the end point is more difficult to be exactly reach and hence a greater error results.

Discussion on improvements.

  To obtain the results more accurately, the vitamin C tablets which contain no colorings should be used.

  The mixture containing potassium bromate, potassium iodate and sulphuric acid should be mixed well by even a longer time to make sure all the reactants are fully reacted.

Discussion on the results and calculation

  In this experiment, the calculated mass of vitamin c is 1.04 g and the expected value is 1.00g. The 4 % error may due to the above errors mentioned.

Conclusion:

  The amount of ascorbic acid(vitamin C) in a given tablet of commercial effervescent Vitamin C tablet is 1.04 g.