• Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

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

Extracts from this document...


Title: Determination the amount of ascorbic acid(vitamin c). Objective: To determine the vitamin C content in a given commercial effervescent Vitamin C tablets by the reaction between ascorbic acid and bromine, then follow by the addition of potassium iodate to the solution and the back titration of iodine formed from the previous reaction between iodate and bromine. Hence, compare this data with the manufacturer's specification. Principle and Introduction: In this analysis, certain amounts of known molarity potassium bomate solid and dilute sulphuric acid are required to standardize first in a volumetric flask. By the addition of certain amount of ascorbic acid into the mixture, potassium bromide, which is known to excess is added to the mixed solution. By the reaction of potassium bromate, potassium bromide and sulphuric acid, bromine is formed. KBrO3 +5KBr+3H2SO4 3K2SO4+3Br2+3H2O After the formation of bromine, it then further react with ascorbic acid to form bromide ion. +Br2 +2H + +2Br - For the back titration, potassium iodide is added to the remaining bromine solution formed just now to allow the formation of iodine. Br2+2I - 2Br - + I2 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- 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. ...read more.


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. Trail 1 2 3 Initial reading of sodium thiosulphate/cm3 15.90 40.60 0.00 2.00 Final reading of sodium thiosulphate/cm3 40.60 50.00 13.50 28.70 Volume of sodium thiosulphate used /cm3 24.70 24.10 26.70 Average volume of Na2S2O3 added /cm3 25.17 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. ...read more.


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. ...read more.

The above preview is unformatted text

This student written piece of work is one of many that can be found in our GCSE Aqueous Chemistry section.

Found what you're looking for?

  • Start learning 29% faster today
  • 150,000+ documents available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

See related essaysSee related essays

Related GCSE Aqueous Chemistry essays

  1. To employ iodometric titration to determine the content of vitamin C in commercial tablets ...

    The potassium iodate(V) solution was poured into a 250.00 cm3 volumetric flask. And the beaker was rinsed with distilled water for twice, so that any remains on the beaker were drained into the volumetric flask 6. The solution in the volumetric flask was made up to the 250.00 cm3 as indicated by the mark on the flask.

  2. Determination of the vitamin C content (ascorbic acid) of a commercial vitamin Ctablet and ...

    About 5 cm3 of 1M potassium iodide solution was added, then followed by adding about 8 cm3 of 0.5M sulphuric acid into the conical flask. 6. The initial burette reading is recorded in table1. The resulting solution is immediately titrated with sodium thiosulphate solution.

  1. The Determination of an Equilibrium Constant.

    reacted equals the amount of CH3COOH (C2H5OH) produced ?Number of moles of CH3COOC2H5 at equilibrium = 0.784 - 0.35 = 0.434 mole Number of moles of H2O at equilibrium = 5.556 - 0.35 = 5.206 mole 4 [CH3COOC2H5] = 0.434 / 0.25 mole dm-3 [H2O] = 5.206 / 0.25 mole

  2. Indigestion - find out the composition of tablets which are for indigestion and compare ...

    Tums Antacid Calcium carbonate Ph Eur 500mg. 36 tablets - 3 rolls Price: �1.69Inc VAT at 17.5% No dosage given The total weight of each pack Setlers antacid = 250.235g Andrews antacid = 747g Bisodol indigestion relief tablets = 654g Rennie Peppermint = 1010g Tums Antacid = 500mg The cost of all the tablets Setlers antacid =

  1. Investigation into the efficiency of various indigestion tablets.

    Mass of MgCO3 in powder (0.25g) = n x mm = 0.000148 x 84 = 0.012432 g Mass of MgCO3 in powder (1.0g) = 0.012432 x 4 = 0.049728 g = 49.7 mg Mass of MgCO3 in tablet (1.315g) = 0.012432 x 5.26 = 0.0653923 g (times more)

  2. To determine the content of vitamin C in commercial tablets using volumetric analysis

    Standardization of Sodium Thiosulphate Solution 1. A stand, a burette clamp and a white tile was collected to construct a titration set-up 2. A burette was rinsed with distilled water and then with the given sodium thiosulphate solution. 3.

  1. Studies On Vitamin C Degradation in Fruit.

    The former Nobel prize winner Linus Pauling claimed that huge intakes i.e. greater than 500mg per day would in humans prevent colds, however little evidence thus far backs up this hypothesis. Other workers on the internet are promoting the value of Vitamin C in controlling cancer and viruses.

  2. To employ titration technique to determine the content of vitamin C in commercial tablets ...

    The end point of titration was not obvious. Instead, back titration would be employed. In this method, two equations were involved. The reaction between potassium iodate(V) (), potassium iodide () and dilute sulphuric acid () generated in-situ a known excess quantity of iodine.

  • Over 160,000 pieces
    of student written work
  • Annotated by
    experienced teachers
  • Ideas and feedback to
    improve your own work