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To employ iodometric titration to determine the content of vitamin C in commercial tablets using volumetric analysis and compares it with the manufacturers' specifications

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Date of Experiment: 20th November, 2004 Analysis of Commercial Vitamin C Tablets Aim: To employ iodometric titration to determine the content of vitamin C in commercial tablets using volumetric analysis and compares it with the manufacturers' specifications. Introduction Vitamin C is an essential substance for maintaining good health and it is proved to be the agent which prevents scurvy. Most animals can synthesize their own vitamin C, but some, such as human cannot. Owing to the increasing concern for one's health since the last century, vitamin C tablets become the most popular supplyment to normal diets. In this experiment, the vitamin C content of a commercial tablet is determinded and compared with the maunfacturers' specification. Vitamin C is water-soluble and is an enantiomer of ascorbic acid. (Commercial vitamin C is often a mixture of ascorbic acid and other ascorbates.) Ascorbic acid, C6H8O6, is a reducing agent that reacts rapidly with iodine (I2) in acidic medium to produce iodide ion (I-) and dehydroascorbic acid, as shown in the following equation: + I2(aq) -----------> + 2H+(aq) + 2I-(aq) Ascorbic acid (Vit. C) Dehydroascorbic acid However, since iodine is only slightly soluble in water, ascorgic acid should not be titrated directly by a standard iodine solution, since the end point of titration is not o obvious. ...read more.


4. The solution in the volumetric flask was made up to 250 cm3 and the flask was shaken gently. A portion of the vitamin C solution was poured out from the flask into a dry and clean 100 cm3 beaker. 5. A 25.00 cm3 pipette was first rinsed with distilled water and then with the Vitamin C solution. 6. 25.00 cm3 of the vitamin C solution was pipetted from the 100 cm3 beaker into a clean conical flask. 7. 5 cm3 of 1.0M potassium iodate(V) solution was added into the vitamin C solution in the conical flask using a 10 cm3 measuring cylinder. 8. Lastly, 25.00 cm3 of the previously prepared standard potassium iodate(V) solution was transferred to the same conical flask. 9. The solution was immediately titrated with sodium thiosulphate solution in the burette, just as in part (B), step 9-13. 10. The volume of sodium thiosulphate used in each titration was recorded and the average volume was calculated. Results and Calculations Mass of weighing bottle and potassium iodate(V): 4.647g Mass of weighing bottle: 4.000g Mass of potassium iodate(V) weighed: 0.674g 0.674g potassium iodate(V) = 0.674g � (39.1+127+16x3)g mol-1 = 3.148 x 10-3 mol Concentration of the prepared standard potassium iodate(V) solution: 3.148 x 10-3 mol � 0.25dm3 = 0.0126 mol dm-3 Table 1 (for part B) ...read more.


It is believe that the sodium thiosulphate used for each titration in part C will be greater, since the vitamin C content decreases upon exposure to air. 6) Cooking means heating or boiling the food. When vegetables are cooked, the vitamin C they contain is heated vigourously. Knowing that boiling temperatures will destroy vitamin C, the amount of vitamin C in the vegetables will be definitely reduced upon cooking. Further Discussion (i) Acidification of the vitamin C sample also serves to stabilize the ascorbic acid, which will other wise decompose and be undetectable. (ii) As stated in the introduction part, iodine has a limited solubility in water. It dissolves well in the solution of potassium iodide only because it will react with I- to form the very soluble red-brown complex, triiodide ion, I3-. So it is reminded that the iodine generated from the redox reaction of iodide and iodate is actually in the form of the triiodide ions in the presence of excess KI due to the I2 + I- I3- equilibrium. (iii) Ascorbic acid can undergo air oxidation requiring that the procedure be performed with minimal delay. (iv) The structure of ascorbic acid (centered around a five-membered ring of four carbons and one oxygen atom) includes two adjacent alcohol(OH) functional groups. Conclusion (1) Neutralization is an exothermic reaction. (2) Stronger the acid or alkali, greater in magnitude will be the enthalpy change ?H obtained. ...read more.

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