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

Procedures:

Part A. Preparation of Standard Potassium Iodate(V) Solution

. A weighing bottle was weighed. 0.6 to 0.7g of potassium iodate(V) to the weighing bottle carefully with a spatula. The mass of the weighing bottle was weighed with potassium iodate(V). The masses were recorded.

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

3. The weighing bottle was weighed again after the discharge of the content and the weight was recorded.

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

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

6. The solution was made up to 250.00 using distilled water and it was shaken well.

Part B. Standardization of Sodium Thiosulphate Solution

. A stand, a burette clamp and a white tile were collected to construct a titration setup.

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 solution was poured into the burette 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.

4. A 25.00pipette 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 100 beaker for rinsing the pipette.

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

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

8. 10of 0.5M sulphuric acid was measured with a 10clean measuring cylinder and then it was poured into the conical flask.

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

0. The reaction mixture was titrated immediately in the conical flask with the sodium thiosulphate solution until pale yellow was just turned.

1. A few drops of freshly prepared starch solution were added to the conical flask.

2. The reaction mixture was continued to titrate until the colour from dark blue to colourless was just changed. This was a trial titration to estimate the volume of the sodium thiosulphate solution required. The volume of the sodium thiosulphate solution added in titration was calculated.

3. The given sodium thiosulphate solution was added to the burette through a filter funnel if the volume remained was not enough to carry out another titration.

4. Steps 6-13 were repeated to obtain two sets of consistent results. In runs 2 & 3, sodium thiosulphate solution was stop draining at about 3less than the estimated value. Then the sodium thiosulphate solution was added drop by drop until the reaction mixture in conical flask just changed from dark blue to colourless.

Part C. Using Standardized Sodium Thiosulphate to determine the Vitamin C content of the tablet

. The mass of a vitamin C tablet was weighed and recorded. Then it was placed in a dry and clean 250 beaker.

2. 150of 0.5M sulphuric acid was measured using a 100 measuring cylinder. The sulphuric acid was poured into the beaker containing vitamin C tablet.

3. The solution was stirred with a glass rod until the vitamin C tablet was dissolved completely.

4. The resulting solution was poured into a 250volumetric flask. The beaker was rinsed with distilled water and the rinse was transferred into the flask. The rinsing was repeated twice.

5. The solution in the volumetric flask was made up to 250.00 using distilled water and it was shaken well.

6. A 25.00was rinsed with distilled water and the with vitamin C solution.

7. 25.00 of the vitamin C solution was pipetted from the 100 beaker into a clean conical flask.

8. 5 of 1.0M potassium iodide was measured using a 10 measuring cylinder. Then it was poured into the conical flask.

9. 25.00 of the potassium iodate(V) solution was pipetted into the conical flask containing vitamin C, sulphuric acid and potassium iodide.

0. The mixture was stirred thoroughly.

1. The solution was titrated by repeating steps 9 to 14 of Part B. And the results were recorded in Table 2.

Results:

Mass of weighing bottle and potassium iodate(V): 4.629g

Mass of weighing bottle: 3.983g

Mass of potassium iodate(V): 0.646g

Table 1

No. of Titrations

st (Trial)

2nd

3rd

Final Burette reading ()

42.50

25.00

43.10

Initial burette reading ()

24.50

7.10

25.00

Volume of sodium thiosulphate reacted ()

8.00

7.90

8.10

Average titre of sodium thiosulphate = = 18.00

Mass of the vitamin C tablet: 4.604g

Brand name of the vitamin C tablet: Redoxan

Manufacturer's specification of vitamin C tablet: 1000mg

Table 2

No. of Titrations

st

2nd

Final Burette reading ()

7.70

4.00

Initial burette reading ()

.50

7.70

Volume of sodium thiosulphate reacted ()

6.20

6.30

?The trial is not listed in the table 2.

Average titre of sodium thiosulphate = = 6.25

Calculation:

Mass of = 0.646g

Molarity of 250.00standard solution

=

= 0.012075

Number of mole of formed = Number of mole of

=

=0.00090561 mol

Number of mole of used to react with

=

=0.00181122 mol

Concentration of standard solution

=

=0.10062

Number of mole of added to react with vitamin C

= 0.00090561 mol

Number of mole of reacted by excess

= 0.10062 x 0.00625

= 0.000628875 mol

Number of mole of excess

=

=0.000314437 mol

Number of mole of vitamin C reacted

= number of mole of reacted by vitamin C

= number of mole of added - number of mole of excess

= 0.00090561 - 0.000314437

= 0.000591173 mol

Number of mole of vitamin C present in the given tablet

=

= 0.00591173 mol

Mass of vitamin C per tablet

The percentage of vitamin C present in the tablet

Discussion:

. Vitamin C, also called ascorbic acid, is one of the most abundant and easily obtained vitamins in nature. It is a colourless and water soluble acid, and it is a powerful biochemical reducing agent, meaning it readily undergoes oxidation, even from the oxygen in the air.

2. The experiment used a redox titration to determine the weight percent of vitamin C (ascorbic acid) in a commercial tablet. The titration of a reducing agent with iodine to produce iodide ion was referred to as an iodometric titration. Iodine was slightly soluble in water but was soluble in solutions containing iodide ion. Iodine formed the triiodine complex with iodide.

Excess potassium iodine was added to the reaction mixture to increase the solubility of iodine and to decrease its volatility. The triiodide ion was pale yellow in dilute and red-brown colour in more concentrated solutions.

3. Before the solution was drained into the burette, we should make sure the filter funnel was cleaned and placed stable on the top of the burette. When draining the solution, the mouth of the burette must be below your eye-level and all the people around were wear the safety googles and lab coats.

4. The reaction of iodine with thiosulphate was fast and goes to completion. Iodine oxidizes thiosulphate to the tetrathionate ion

The red-brown colour of the triiodide ion became pale yellow as was reacted and eventually the solution would become colourless when all of had been reacted to form.

5. (Q3) Since it was very difficult to see when the pale yellow colour became colourless, a sharper end-point detection system was needed. Starch formed a dark bluish purple complex with triiodide. Starch MUST not be added until most of had been reached since at high concentrations, the starch might form an insoluble complex that was irreversible. The starch indicator was added just before the end point of the titration when the solution still had a pale yellow colour. The end point of the titration occurred when the bluish coloured "" complex was destroyed and the solution became clear.

If there were unsure of the colour change, the burette reading was recorded and an additional drop of titrant was added. If there was no significant change in colour, the end point had been reached at the previous reading.

6. If there was no colour change when the starch indicator was added, it was because the end point had already passed in the titration and it should be titrated again.

7. (Q4) Iodine would be produced after the addition of sulphuric acid. And iodine decreases with time as iodine is volatile and it could oxidize most organic substances.

So the titration should be carried out immediately while sulphuric acid was added to reduce the loss of due to vapourization.

8. When analyzing the Vitamin C tablet, a glass rod was needed to break up and dissolve the solid. Be aware that some solid binding material in the tablet would not dissolve.

9. (Q5) To investigate vitamin C decomposes upon to air or heat. Ascorbic acid change DCPIP solution from blue to colourless. Three beakers of ascorbic acid were prepared. Use the bunser burner to heat beaker 1. Then, beaker 2 was placed beside the windows and beaker 3 was placed in vacuum atmosphere as control setup.

After a period of time, titrate the exactly same amount of those three sets of solution; say 10.00 with vitamin C solution.

Comparing the titration result between three setups, the volume of standard DCPIP solution used in setup 1 and 2 were less than that in the control setup 3. It proved vitamin C would decompose upon the exposure to air or heating.

0. (Q6) Cooking is likely to reduce the amount of vitamin C of vegetables because it leaks out into the cooking water because it is a water soluble substance.

1. Finally, in the procedure of disposal of the chemicals, all reactants and products should be disposed into sink with water and we had to wash all the apparatus with water to ensure all impurities were removed.

Conclusion:

In the experiment, the vitamin C tablet weighted 4.604 grams, but it had only about 1 gram of vitamin C, the remaining substances might be carbonates and bicarbonates, colourings, flavourings and binder.

From the result obtained, the amount of vitamin C was 0.04 gram more than manufacturer's specification, the difference might due to the poor technique of titration and some solid binding material in the tablet would not be dissolved.

The mass of vitamin C tablet obtained by the experiment was quite close to manufacturer's specification and it was acceptable.

Hazards:

Handle chemicals with care, KIO3 was an oxidant, and sulphuric acid was corrosive to equipment and skin. The thiosulphate solution should be stored in a cool, dry atmosphere away from heat and incompatible substances. Sodium thiosulphate was moderately toxic when ingested; we should wash hands thoroughly after handling.

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