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Analysis of commercial vitamin C tablets

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Analysis of commercial vitamin C tablets 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. Principle In this analysis, certain amount of iodine solution, which is known in excess, is added to acidified ascorbic acid solution. Brown iodine can be easily reduced by acidified ascorbic acid to form colourless iodide ion: � Equation I 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- � Equation II 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. Hence, the mass of ascorbic acid reacted by iodine can be calculated. Chemicals vitamin C tablet(s), NaIO3, 1M NaI solution, 0.5M H2SO4, approximately 0.06M Na2S2O3, freshly prepared starch solution Apparatus electronic balance, beaker, volumetric apparatus, pipette, glass rod, white tile, dropper, measuring cylinder Procedure 1.> Accurately weigh 0.6 to 0.7g sodium iodate, NaIO3 and record its mass. 2.> NaIO3 is then dissolved in deionized water and is made up to 250cm3 in a volumetric flask. 3.> Sodium thiosulphate, Na2S2O3, is standardized by part of standard NaIO3 solution is shown below: (a) ...read more.


Precaution 1.> Ascorbic acid (vitamin C) is unstable and can be easily oxidized. Oxidation can be highly speeded up when it's heated or dissolved in water. 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 vitamin C from the volumetric flask just about to use and then stopper it immediately. 2.> Starch solution should be added only when the solution becomes pale yellow. 3.> The mixture of reacted vitamin C and excess I2 should be titrated with Na2S2O3 immediately to reduce I2 lost due to vapourization. 4.> Handle chemicals with care, NaIO3 is an oxidant, H2SO4 is an irritant which may irritate the skin especially if there is a wound. 5.> Avoid exposure to I2 solution since the I2 vestige on skin is quite hard to remove, it will remain for a long time. Discussion ~The function of vitamin C~ Vitamin C (ascorbic acid) is a white, odourless powder, and a water-soluble vitamin. When dry, it's reasonably stable in air but in solution, it oxidizes rapidly in the presence of air. In fact, it's known that vitamin C is the most unstable vitamin. The oxidation can be speeded up by heating or presence of moisture. ...read more.


+ I-(aq) � I3-(aq) This can also explain that why iodine solution appears reddish (colour of I3-) but not brown. 8.> To obtain the results more accurately, it's recommended that we shall use the vitamin C tablets which are tasteless and contain no colourings. However, orange-flavoured tablets is also accepted but not grape-flavoured, since it's colour may affect determination of the end point (blue to colourless!). 9.> Theoretically, vitamin C tablets which has orange-flavoured always has a higher vitamin C content than that of grape-flavoured, since people think that the former is more sour than latter and manufacturers will add more vitamin C to the former one as flavourings. Enriching the tablets with vitamin C can attract more people to buy. 10.> In the experiment, each tablet used weighs about 4.5g, but has only 1g of vitamin C, the remains may be bicarbonates/carbonates (to have a cooling effect), colourings, flavourings, binder (stick the powder together), etc. 11.> From the result obtained, it is about 0.58% less than manufacturer's specification, the difference may due to the poor technique of titration, self- oxidation of vitamin C, etc. Conclusion Vitamin C is an essential vitamin to prevent scurvy, and it can be found in citrus fruit and green vegetables. Although it's so useful, now it is still too soon to say that whether Pauling is right or not. It may need several years until there is enough evidence. The mass of vitamin C tablet obtained by the experiment is quite close to manufacturer's specification which is acceptable. ?? ?? ?? ?? ...read more.

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