vITAMIN c

Topics: Iodine, Titration, Vitamin C Pages: 7 (1420 words) Published: October 18, 2014
11/10/2014
experiment 1 Analysis of Commercial Vitamin C Tablets

Objective
To employ titration technique to determine the content of vitamin C in commercial tablets using volumetric analysis and compare it with the manufacturers' specifications.

Introduction
Deficiency of essential antioxidant Vitamin C in human will lead to scurvy. In this experiment, the vitamin C content of commercial tablets is determined and then compared with the manufacturers' specifications. Vitamin C, ascorbic acid C6H8O6, reacts with iodine and is oxidised to dehydroascorbic acid rapidly in acidic medium.

first

Considering iodine is only slightly soluble in water, the above titration cannot be performed directly by a standard iodine solution because of the insignificant end point. Thus, alternative method, back titration, will be employed. Two equations are involved.

First, standard potassium iodate(V) (KIO3), potassium iodate(KI) and dilute sulphuric acis(H2SO4) react to find out the excess quantity of iodine. Below shows the equation.

second

Second, the iodine generated reacts with ascorbic acid immediately. While the excess iodine is the titrated by standardized sodium thisulphate(Na2S2O3) as shown the equation below. Then, the iodine reacted with ascorbic acid can be determined followed by the content of ascorbic acid.

third

Starch indicator is added for the determination of the end point of the titration. Starch reacts with excess iodine in solution to form the blue-black starch-iodine complex. Thus, the colorless solution indicates there is no iodine in the solution.

Procedure

A) Preparation of standard potassium iodate(V) solution
1)A weighing dish was weighed. 0.6g of potassium iodate(V) was added to the weighing dish carefully with a spatula. The mass of the weighing dish with potassium iodate(V) was weighed. The mass was recorded.

2)The potassium iodate(V) was poured from the weighing dish to a clean and dry 100cm3 beaker.

3)The weighing dish was weighed again after the discharge of the content. The weight was recorded.(This is a technique known as weighing by difference)

4)40cm3 of distilled water was added into the beaker. The mixture was stirred with a glass rod gently until all potassium iodate(V) had dissolved.

5)The potassium iodate(V) solution was poured into a 250.00cm3 volumetric flask. The beaker was rinsed with distilled water thoroughly and the rinse was transferred into the volumetric flask.

6)It was made up to 250.00cm3 using distilled water and it was shook well.

B) standardization of sodium thiosulphate solution
1)A stand, a burettle clamp and a white tile were collected construct a titration set-up.

2) A burette was rinsed with distilled water and then with the given sodium thiosulphate solution.

3)The stopcock of the burette was closed and the sodium thiosulphate solutionwas poured into it until the liquid level was near the zero mark. The stopcock of the burette was opened to allow the titrant to fill up the tip and then the liquid level was adjusted near zero but not above zero.

4)A 25.00cm3 pipette was rinsed with distilled water.

5)Then a small amount of the potassium iodate(V) solution was poured from the volumetric flask to a clean and dry 100cm3 of the beaker into a clean conical flask.

6)After that, 25.00cm3 of the potassium iodate(V) solution was pipette from the beaker into a clean conical flask.

7)5cm3 of 1.0M potassium iodide solution was measured with a 10cm3 measuring cylinder. It was poured into the conical flask.

8)10cm3 of 0.5M sulphuric acid was measured with a 10cm3 measuring cylinder. It was poured into the conical flask.

9)The initial burette reading was recorded in Table 1. The reading was accurate to 2 decimal places.

10)The reaction mixture was immediately titrate in the conical flask with the sodium thiosulphate solution until it just turned pale yellow.

11)A few drops of freshly prepared starch solution...
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