Extracts from this document...

Introduction

ANALYSIS OF THE AMOUNT OF VITAMIN C IN A COMMERCIAL TABLETS AIM To investigate the content of vitamin C (ascorbic acid) content in a commercial tablets and compare this with the manufacturer's specification. INTRODUCTION Vitamin C is ascorbic acid which is rapidly and quantitatively oxidized by iodine in acidic medium according to the following equation: The standard method for determining ascorbic acid present in a sample is to titrate a standard iodine solution. But the low solubility of iodine makes this procedure less than ideal. In this experiment, a know excess of iodine is generated in situ by the reaction between an iodine and an iodate and that not reacting with the ascorbic acid is then titrated against standard sodium thiosulphate solution. A standard solution of iodine is unstable and has to be frequently standardised by a standard solution of thiosulphate. Direct titration of tablets against a standard solution of iodine is not recommended. ??? Stoichiometrically in-situ generation of iodine according to the equation (aq) + 5(aq) + 6(aq) ??? 3(aq) + 3O(l) ...........(2) provides a known amount of iodine (which is excess) to oxidize the vitamin C content of tablets . ...read more.

Middle

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. ...read more.

Conclusion

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. ?? ?? ?? ?? CHAN TING HIN 6B11 ? 1 ...read more.

This student written piece of work is one of many that can be found in our AS and A Level Physical Chemistry section.