Aim: To employ iodometric titration to determine the content of vitamin C in commercial tablets using volumetric analysis and compares it with the manufacturersÂ’ specifications.
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 oneÂ’s health since the last century, vitamin C tablets become the most popular supplyment to normal diets.
[IMAGE]In this experiment, the vitamin C content of a commercial tablet is determinded and compared with the maunfacturersÂ’ 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. Instead, back titration will be employed.
The titration of a reducing agent with iodine to produce iodide ion is referred to as an iodometric titration. Iodine is generate by adding a weighed amount of standard potassium iodate (KIO3) to an excess of potassium iodide (KI) and then add adding strong dilute sulphuric acid (H2SO4) to produce iodine as shown below:
(1) KIO3(aq) + 5KI(aq) + 3H2SO4(aq) â†’ 3I2(aq) + 3H2O(l) + 3K2SO4(aq)
The known excess of I2 generated by the reaction is immediately reacted with the ascorbic acid sample; finally, the excess unreacted iodine is Â“back-titratedÂ” with standardized sodium thiosulphate(Na2S2O3) as shown in the following equation:
(2) 2 Na2S2O3(aq) + I2(aq) â†’ Na2S4O6(aq) + 2NaI(aq)
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.
A) Preparation of Standard Potassium Iodate(V) Solution
1. A weighing bottle with potassium iodate(V) was weight. And the mass was recorded on the data sheet.
2. The potassium iodate(V) solid was discharged from the weighing bottle to a clean and dry 100 cm3 beaker.
3. The empty weighing bottle was weighed again And the exact mass of potassium iodate(V) used was obtained by weighing by difference.
4. Distilled water was added into the 100 cm3 beaker which the solid potassium iodate(V) was held. The mixture was stirred gently with a glass rod until all powder was dissolved into solution.
5. The potassium iodate(V) solution was poured into a 250.00 cm3 volumetric flask. And the beaker was rinsed with distilled water for twice, so that any remains on the beaker were drained into the volumetric flask
6. The solution in the volumetric flask was made up to the 250.00 cm3 as indicated by the mark on the flask. The flask was stoppered and shaken well to ensure a homologous potassium iodate(V) solution.
B) Standardization of Sodium Thiosulphate Solution
1. A standard titration set-up was framed up using a stand, a burette clamp and a white tile.
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 up to an accuracy of 2 decimal places.
4. A portion of the prepared potassium iodate(V)...