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Analysis Of Commercial Vitamin C Tablets

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Experiment 5 Date: 18-10-2005 Analysis Of Commercial Vitamin C Tablets Objective To determine the mass of Vitamin C in 1 pill of Vitamin C tablet. Introduction In this experiment, the concentration of sodium thiosulphate solution is not given. So, we need to standardize it through titration. Sodium thiosulphate reacts with iodine in the following reaction: I2 + 2S2O32- ----------------->2I- + S4O62- After an amount of S2O32- is added, the solution of I2 turns pale yellow. When starch solution is added and more S2O32- is added, the solution reaches its end point, which is colorless. In this way, the molarity of the thiosulphate solution is determined. Vitamin C could be oxidized by iodine in the presence of acid in the following equation: Due to the low solubility of iodine, direct titration of iodine solution and Vitamin C is unsuitable. Then how could the experiment be done? The experiment could be done by adding acidified Vitamin C solution into potassium iodide solution. Then, we add potassium iodate to allow the following reaction to take place: IO3- + 5I- + 6H+ ----------------> 3I2 + 3H2O I2 formed in this reaction could react with the Vitamin C in as mentioned in the above equation. ...read more.


of moles of KIO3 used in each titration= (0.682/214.1)/10 = 3.185 X 10-4 moles. No. of moles of Na2S2O3 used in each titration = 3.185 X 10-4 X 6 =1.91 X10-3 moles. Molarity of Na2S2O3 = 1.91 X 10-3 / 38.067 X 1000 =0.0502M Trial First Second Final burette reading/cm3 16.3 32.0 16.9 Initial burette reading/cm3 0.7 16.3 1.3 Volume of Na2S2O3/cm3 15.6 15.7 15.6 Table showing the data obtained when titrating the Vitamin C solution Mean titre = 15.633cm3 No. of moles of Na2S2O3 used = 15.633/1000 X 0.0502 = 7.849 X 10-4 moles No. of moles of I2 reacted with Na2S2O3 = 3.925 X 10-4 moles No. of moles of I2 formed after potassium iodate was added into the conical flask in part 2 = (5/1000)/5 = 0.001 moles No. of moles of I2 reacted with vitamin C = 0.001 - 3.925 X 10-4 =6.075 X 10 -4 moles So, (6.075 X 10 -4 X 10) =6.075 X 10 -3 moles of Vitamin C was present in 1 Vitamin C pill. Molar mass of Vitamin C= 176 g mol-1 So, mass of vitamin C = 176 X 6.075 X 10 -3 = 1.06g Conclusion The mass of ascorbic acid found in 1 pill of tablet is 1.06g. ...read more.


It should not be added too early because if we add the starch solution too early, black precipitate would be formed. The results of the experiment would thus be affected. 2. I2 vaporizes easily in air. So, to reduce the amount of iodine lost, it is wise to titrate the solution immediately. Otherwise, too much iodine would be lost and the accuracy of the results would be affected to a great extent. 3. To investigate the effect of heating on the vitamin C, Vitamin C is first dissolved in water. Then, it is heated using a Bunsen flame. Afterwards, sulphuic acid, potassium iodate, potassium iodide are added and it is titrated against sodium thiosulphate solution. To investigate the effect of being exposed to air, Vitamin C, already dissolved in acid, is left standing on a bench for 1 night. Then, it is titrated with sodium thiosulphate, with an excess of iodate and iodide added into the flask. 4. The mass of Vitamin C in 1 tablet mentioned in the specifications is 1000mg, which is equal to 1g. My results were higher than that of the specifications. This may be due to experimental errors. This may also be due to the difficulty in getting every tablet to have the exact amount of Vitamin C during the manufacturing process. ...read more.

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This experimental write-up was writtenin a very logical order, with sub-titles marking all the sections to make it easy to access the information. The objective is clearly expressed at the beginning, and then the student, methodically, answers the question by ...

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Response to the question

This experimental write-up was writtenin a very logical order, with sub-titles marking all the sections to make it easy to access the information. The objective is clearly expressed at the beginning, and then the student, methodically, answers the question by showing all the phrases of the experiment, with precise explanations of why things happen. One small gripe would be that the final section with the question answers could have had a copy of the questions so it would be easier for the examiner to know whether they answered the question correctly or not.

Level of analysis

The writer has demonstrated all the necessary skills expected and it is clear that they understand the area well. The discussion section also adds some further depth to the study and shows that they are able to make evaluative comments based on both current knowledge and experience gained from doing the experiment. The calculations are all correct and lead to the correct conclusion of the amount of vitamin C in one pill. The student could have further discussed possible errors, what their implications are and how they worked to reduce them.

Quality of writing

Spelling, grammar and punctuation were all fine, with the correct technical phrases being used when necessary. All the reaction equations have been shown with experimental data and working out set out in a clear and easy to follow manner.

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