Beaker for later use and covered with plastic wrap to prevent contamination.
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A vitamin C tablet was dissolved in about 1500.5M sulphuric acid in a beaker.
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The solution was transferred to a volumetric flask and was made up to the mark using deionized water.
of Vitamin C solution was transferred into a clean conical flask and of KI was added
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of potassium iodate solution was pipette into the flask containing the Vitamin C and KI
- The excess iodine was nack titratred against the standardized sodium thiosulphate solution.
- The mass of ascorbic acid in the tablet was calculated
RESULT
Mass of KIO3 weighed by electric balance: 0.65g
Mean volume of Na2S2O3 used = ( 18.4+19.2)/2=18.8cm3
Mean volume of Na2S2O3 used = (7.1+ 7.1)/2= 7.1 cm3
CALCUALTION
Standardization of the given Na2S2O3
No. of moles of KIO3 = = 3.03710-3 mol
6H+ + IO3- + 5I- → 3I2 + 3H2O
No. of moles of I2 formed = 3.03710-3 3 = 9.112 10-3mol
2S2O32- + I2 → S4O62- + 2I-
No. of moles of I2 used in 25 cm3 solution = 9.112 10-4 mol
No. of moles of Na2S2O3 used = 9.112 10-4 2 = 1.8224 10-3 mol
Molarity of the given Na2S2O3 solution = 1.8224 10-3(18.81000)= 0.0969M
Finding the amount of Vitamin C by adding excess I2 and back titrate I2 against standard Na2S2O3.
No. of moles of I2 in 25cm3 solution = = 9.112 10-4
No. of moles of I2 left = (0.0969)(7.11000)2 = 3.440 -4
No. of moles of I2 used = 9.112 x 10-4 – 3.440 x 10-4 = 5.672 x 10-4
No. of moles of vitamin C used in the 25cm3 solution= 5.672 x 10-4
Total no. of moles of vitamin C in the 250cm3 solution = 5.672 x 10-3
Mass of Vitamin C = 5.672 x 10-3 x ( 2+12x6+6+16x6 )= 1.021 g
Percentage error = (1.021 – 1.0000 ) /1 X 100% = 2.1%
DISCUSSION
CHOICE OF BACK TITRATION
Both direct and back titrations are possible methods. The recommended experiment involves an in-situ generation of excess iodine by reacting a fixed amount of iodate(V) in acidic medium in the presence of excess iodide solution. Unconsumed iodine is determined by back titration against a standard solution of sodium thiosulphate. There is an advantage of using Back titration which is consuming all the unstable reactant at the start of the reaction.
In addition, iodine is volatile, Iodine would escape and cause inaccurate concentration of the solution. standard iodine cannot be prepared directly by accurately weighing because iodine is slight soluble in water. Iodine would escape and cause inaccurate concentration of the solution. Also, iodide can be oxidized to iodine in air with the presence of acid, heat and light. As a result, the amount of I2 would increase with time.
4I- + O2 + 4H+ → 2I2 + 2H2O
In this experiment, we dissolve known mass of KIO3 into excess KI and H2SO4 solution to form iodine instead of using iodine solution directly.
IO3- + 5I- + 6H+ → 3I2 + 3H2O
REASON OF STANDARDIZING SODIUM THIOSULPHATE SOLUTION
sodium thiosulphate solution have to be standardized before use as thiosulphate solution in the presence of microorganism, Cu(II), sunlight or in acidic medium. In fact, the sodium thiosulphate solution should be prepared with recently boiled distilled water to ensure no is dissolved. Otherwise, it may be decomposed by acids (e.g ). can be added to keep the solution alkaline and some Hg can be added to suppress bacterial action.
REASON FOR CHOICING IODINE BUT NOT OTHER HALOGEN
Vitamin C is an unsaturated compound which contains a carbon-carbon double bond (C=C) adjacent to two OH groups. The enol group can be easily oxidised to a keto group. Stronger oxidising agent such as bromine is not recommended as the double bond may be ruptured.
CHOICE OF INDICATOR
In this experiment, starch (preserved with salicylic acid) solution is used to determine the end point of titration of iodine solution against sodium thiosulphate solution. It is because iodine solution could turn starch into blue black, it could help indicating whether iodine is completely reacted. It is because starch could form a blue-black complex with iodine. As the colour change of iodine to iodide is insignificant and difficult to observe, so starch is used as the indicator. However, Starch solution is being added in the later stage of titration. It is because starch would combine with iodine irreversibly when iodine is at a very high concentration. At the endpoint, the blue-black colour would completely disappear.
REASON FOR EXACT VOLUME OF KI AND SULPHURIC ACID IS NOT IMPORTANT
KI is in excess and sulphuric acid is used to acidfy the solution only. It is not necessary to know the exact amount of KI and sulphuric acid as both amount are not involved in the calculation of Vitamin C content in the tablets.
DESCRTION OF HOW STANDARD SOLUTION CAN BE TRANSFERRED
Rinsed the beaker with deionized water and then with the solution. Pour the solution carefully with aid of filter funnel from the volumetric flask to the beaker.
THE EXPERIMENT DETAILS OF INVESTIGATING THAT ASCROBIC ACID DETERIORATES ON HEATING AND ON EXPOSURE TO THE AIR
As a first attempt, 25 cm3 of tablets sample was heated to 80 for 10 minutes and the amount of ascorbic was determined by the same method as mentioned.
As for the exposure to air, 25 cm3 of tablets sample was placed In the conical flask and allowed to expose to air. The amount of ascorbic acid was determined by the same method as mentioned.
SOURCE OF ERROR
ERROR IN CONCENTRATION OF SOLUTIONS
After standardizing the sodium thiosulphate solution, it might decompose by air and sunlight. As a result, the concentration of the solution would be lower than we expected and the calculated amount of vitamin C would be smaller.
In addition,iodide can be oxidized by air (promoted by acids, heat, light).As a consequence, Iodine conctentration increase with time.
OTHER CHEMICALS IN THE TABLET MIGHT REACT
In this experiment, we have assumed that only vitamin C in the tablet would react with I2. However, we don’t actually know whether other chemicals and ingredients inside the tablet would react with iodine solution. If some of the ingredients would react with iodine solution, the calculated amount of vitamin C in the tablet would be greater.
CONCLUSION
The mass of vitamin C inside the tablet is 1.021g, the tag on the vitamin C tablet claimed that it contain 1.0000 g vitamin C. The percentage error is 2.1 , which is insignificant. So the claim of the tablet maker is justified.
