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Analysis of commercial vitamin C tablets

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Analysis of commercial vitamin C tablets Date: 11-10-2010 Objective To determine the vitamin C content in commercial vitamin C tablets by titration between vitamin C (ascorbic acid) & iodine solution. Hence, compare this data with the manufacturer's specification. Procedures 1. 0.6808g potassium iodate(V) , KIO3,was accurately weighed. 2. KIO3 (s) was then dissolved in de-ionized water in a beaker. 3. The solution was made up to 250cm3 in a volumetric flask. 4. Sodium thiosulphate, Na2S2O3, was standardized by part of the standard KIO3 solution in the following steps: (a) 25cm3 of the standard KIO3 solution was pipetted into a conical flask (b) 5cm3 of 1M potassium iodide, KI, together with 10 cm3 of 0.5M sulphuric acid, H2SO4, were also added into the conical flask. (c) The mixture was immediately titrated with approximately 0.1M Na2S2O3 solution. (d) These steps were repeated for 2 times to obtain a more accurate titration data. (e)When the reaction mixture became pale yellow, 5 drops of freshly prepared starch solution were added. (f) The titration was continued until the end-point was reached. (g) The molarity of Na2S2O3 solution was calculated. 5. A vitamin C tablet was dissolved in 150cm3 of 0.5M H2SO4 6. The mixture was then transferred into a clean 250cm3 volumetric flask 7. De-ionized water was added to reach the graduated mark. 8. ...read more.


= No. of mole of I2 produced- No. of mole of I2 reacted with Na2S2O3(aq) =0.0009543925234-0.0007182797836 =0.0002361127398mol ?No. of mole of 10cm3 vitamin C(Ascorbic acid) = No. of mole of I2 reacted with 10cm3 vitamin C(Ascorbic acid) =0.0002361127398mol ?No. of mole of 250cm3 vitamin C(Ascorbic acid) =0.0002361127398x25 =0.005902818495mol ?Mass of Vitamin C(Ascorbic acid) per tablet =0.005902818495x(12x6+8+16x6) =1.038896055 ~1.04g 3. State the function of the starch solution. Explain why it should be added only when the reaction mixture becomes pale yellow. Starch is often used in chemistry as an indicator for redox titrations where iodine is present as starch forms a very dark blue-black complex with iodine. Moreover, the indicator is not affected by the presence of iodide (I-). In order for the starch solution to change colour, both starch and iodide must be present. Starch forms an unstable complex which is blue coloured in low concentrations of iodine. However, this complex is in high concentration of iodine .When the mixture turns pale yellow ,it indicates that iodine is partly reduced to iodide ions. That means the colour change to yellow indicates that the concentration of iodine in the mixture is no longer too high .If the mixture has not become pale yellow and the starch indicator is added too early, the conical flask still contains high concentration of iodine .As ...read more.


The volume of standard Na2S2O3 used for titration should be larger. It implies that the number of mole of vitamin c reacted with I2 decreases. In short, the Vitamin C content per tablet decreases due to the effect of exposure to air. To determine the effect of heating ,a vitamin C tablet should be dissolved in 150cm3 H2SO4(aq).Then, heat the conical flask containing the mixture in a bath of boiling water for about 10 minutes. After that, carry out the procedure above to determine the vitamin C (Ascorbic acid)content through titration .Finally, compare the result with the value obtained in the experiment this time(which serves as control). The volume of standard Na2S2O3 used for titration should be also larger. It implies that the number of mole of vitamin c reacted with I2 decreases. In short, the Vitamin C content per tablet decreases due to the effect of heating * Conclusion Through the standardization of Na2S2O3 solution ,it was found that The molarity of Na2S2O3 solution is 0.0994M.Later on, the mass of vitamin C per tablet obtained by the titration above is1.04g which is quite close to the manufacturer's specification(about 1 g) and acceptable. Reference http://hk.search.yahoo.com/search/kp?p=excess+ki+sulphuric+acid&rs=1&fr2=rs_bottom http://hk.knowledge.yahoo.com/question/question?qid=7006100204912 http://en.wikipedia.org/wiki/Iodine http://www.newworldencyclopedia.org/entry/Iodine http://www.outreach.canterbury.ac.nz/chemistry/documents/vitaminc_iodine.pdf http://hk.search.yahoo.com/search/kp;_ylt=A8tU3w7p26ZMI3gADEDaygt.?p=word%E6%89%93%E5%88%86%E7%B7%9A&fr2=sb-top http://hk.knowledge.yahoo.com/question/question?qid=7008110601656 F.6 Chemistry Notes Section I by Ms Sin W L End ?? ?? ?? ?? F.6 Chemistry TAS/Experiment 4/P.1(6) 1 ...read more.

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