In this experiment, the vitamin C content of a commercial tablet is determined and compared with the manufacturers’ 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). As the iodine is added during the titration, the ascorbic acid is oxidized to dehydroascorbic acid, while the iodine is reduced to iodide ions as shown in the following equation: (studymode.js, 2010).
Ascorbic acid + I2 (aq) ———–> 2I- (aq) + dehydroascorbic acid
Due to this reaction, the iodine formed is immediately reduced to iodide as long as there is any ascorbic acid present. The amount of ascorbic acid is determined by the stoichiometry of the equations and the difference between the total amount of iodine present and the amount that reacts with the thiosulphate. This method is suitable for use with vitamin C tablets, fresh or packaged fruit juices and solid fruits and vegetables.
Vitamins are a group of small molecular compounds that are essential nutrients in many multi-cellular organisms, and humans in particular. L-Ascorbic acid was first isolated as a pure substance by Albert Azent-Gyorgi and Charles Kingin 1928 (Pauling, 1970), an anti-oxidant and free radical scavenger, is found ubiquitously in fruit and vegetables such as citrus fruits (oranges, lemons, limes, tangerines etc.), melons, tomatoes, peppers, broccoli, green leafy vegetables such as spinach, potatoes and turnips, its quantitative determination is especially important in the production of wine, beer, milk, soft drinks and fruit juices, where it can be a quality indicator (Gerrior & Zizza, 1994)] .Given the essential role played in the human diet and necessary to growth and repair of tissues in all parts of human body .It is necessary to form collagen an important skin proteins ,scar tissue ,tendons, ligaments, and blood vessels. Vitamin C is essential for the healing of wounds, and for the repair and maintenance of cartilage, bones and teeth (Mcevoy, 1993) one of the several ways of determining the ascorbic acid content of a solution is by titration.
Figure structure of vitamin c (ascorbic acid)
Titration is a volumetric analysis and is one of the basic analytical techniques in chemistry which enables one to quantitatively determine how much of a specific substance is contained in a given sample. This technique is involved in almost every aspects of our daily living such as life sciences, clinical chemistry, water pollution, industrial analyses etc. In order to make meaningful titration analysis, one has to understand and learn the principles involved. Several types of titration techniques are now available but, two general types which are still in much use are acid-base and reduction-oxidation (Redox). Acid-base titration involves the neutralization reaction and the products are generally salt and water. Redox titrations involve the titration of an oxidizing agent (or oxidant) with a reducing agent (or reductant) or vice versa. Oxidation process involves loss of electrons while reduction process involves gain of electrons. Thus an oxidizing agent is one which accepts electrons while a reducing agent is one which loses the electrons. There must be a sufficiently large difference between the oxidizing and reducing capabilities of these agents for the reaction to undergo completion with a sharp end point.
MATERIALS AND METHOD
250-mL conical flask
Vitamin C tablets
50-mL, 25ml and 10ml measuring cylinder
10% KI solution
50-mL burette with stand
2.0 M H2SO4 solution
Glass stirring rod
A. STANDARDIZATION OF SODIUM THIOSULPHATE
1. A standard titration setup was framed up using a stand, a burette clamp and a white tile
Figure 2 setup for titration
(Courtesy of http://www.chemteach.ac.nz)
2. A burette was rinsed with distilled water and then with the given sodium thiosulphate solution. 3. With the stopcock closed, the rinsed burette was fully filled up with the sodium thiosulphate solution and then the stopcock was opened so that the tip of the burette was also allowed to be filled up. The initial burette volume was recorded to an accuracy of 2 decimal places 4. A 25.00cm3 pipette was rinsed with distilled water, and then with KBrO3 solution. 5. 25.00cm3 of standard solution of KBrO3 was transferred to a clean conical flask using the rinsed pipette. 6. Using a 25cm3 measuring cylinder, 25ml of 2M H2SO4 was transferred into the conical flask containing the KBrO3 solution. 7. Using a 10ml measuring cylinder, 5ml of 10% KI solution was also added to the conical flask containing the KBrO3 solution. 8. Immediately, the reaction mixture in the conical flask was titrated with the sodium thiosulphate solution until a pale yellow solution was observed. 9. A few drops of starch solution were added to the pale yellow solution in the conical flask. 10. Titration of the mixture was continued until the solution changed from dark blue to colorless. The final burette reading, accurate to 2 decimal places, was recorded. the volume of the sodium thiosulphate solution added was calculated 11. Apart from the first trial, 2 more successive titrations were carried out. The burette was refilled in between runs when the volume remaining was not enough for a complete titration.
B. ANALYSIS OF VITAMIN C TABLETS
Figure 3 vitamin c tablets
(Courtesy of lule.blogspot.com)
1. A tablet of vitamin c was transferred into a 250ml conical flask and approximately 40cm3 of 2M H2SO4 was added. The tablet was then crushed to dissolve until a solution formed.
2. Using a measuring cylinder, 10cm3 of 10% KI solution was added to the tablet solution and 25cm3 of standard KIO3 solution was pipetted in to the flask.
3. The solution was then immediately titrated with standard thiosulphate solution in the burette until the dark color changed to the original color of the tablet.
4. This procedure was repeated with 2 more samples of the tablet.
5. The volume of sodium thiosulphate used in each titration was recorded and the average volume was calculated.
RESULTS AND CALCULATIONS
Table results from standardization of sodium thiosulphate (A) Number of moles of KBrO3 in 25cm3 solution = [this formula is used because the molarity of the solution is given and the volume is known] 0.00964×0.025=0.000241mol
Mass of KBrO3 solution used= [this formula is used in deriving the mass of a substance when the no. of moles is given (or derived)]=0.000241mol×167g/mol=0.040247g= 40.25mg
Concentration of KBrO3 =
No. of moles of Na2S2O3 required for one titration = 0.00241mol×6= 0.01446mol
Moles of I2 liberated = 2.41×10-4 moles
Concentration of Na2S2O3= 0.298mol/dm3
Table 2 results from vitamin C analysis (B)
Brand of vitamin C: Cenovis
Manufacturer’s specification of vitamin c tablet: 1000mg of vitamin c per tablet
Molar mass of vitamin C (C6H8O6) = 12×6+1×8+16×6=176g/mol
Balanced Equation for reaction = KIO3+5KI+3H2SO4→ 3I2+3H2O+3K2SO4
Number of moles of KIO3 in 25cm3 solution = 0.00100 mol
Moles of I2 produced by 25cm3 of KIO3= 0.00100×3= 0.003 moles
No of moles of Na2S2O3 needed to react with excess I2
Moles of I2 that reacted with sodium thiosulphate
No. of mole of Na2S2O3 =
No. of mole of original I2 – no. of mole of excess I2 =0.000723- 0.0005=0.000223mol
No. of mole of I2 that reacted with ascorbic acid = 0.006-0.0005 = 0.0055
Mass of ascorbic acid = 176×0.0055= 0.92g =920mg
The experiment was successfully carried out and the results were encouraging. The analysis of the vitamin c content of the Cenovis tablet was carried out using titration. The vitamin c tablet was crushed into a conical flask and specific amounts of KI and KIO3 were added. The solution was then titrated with standard thiosulphate solution until the required color was derived. The results were recorded and calculations to determine the amount of ascorbic acid in each tablet was also successfully done. However the results from the experiment showed different values from the manufacturer’s specifications. The manufacturer (Cenovis) claims that each vitamin c tablet contains 1000mg of ascorbic acid while this experiment shows otherwise. After calculations were done, the experiment found each vitamin c tablet to be containing about 920mg of ascorbic acid which is below the manufacturer’s specification. However this error could be as a result of different factors; 1. The manufacturer of the tablet made wrong analysis of the tablet. 2. This experiment contained errors which arose from wrong reading of instruments, wrong calibration of instruments, or errors in uncertainty.
Gerrior, S., & Zizza, C. (1994). Nutrient content of the U.S food supply, 1909-1990. Washington D.C: U.S department of Agriculture. Mcevoy, G. (1993). Drug information the american hospital formulary service. American society of health-system pharmacist. Pauling, l. (1970). vitamin c and the common cold.
studymode.js. (2010). commercial vitamin c tablet analysis. Retrieved from studymode: http://www.studymode.com/essays/Commercial-Vitamin-c-Tablets-Analysis-470526.html