Title: Determination of the vitamin C content ( ascorbic acid ) of a commercial vitamin C tablet and compare the consequence with the value specified by the maker.
In this experiment. we have to find the vitamin C content of a commercial vitamin C tablet which is the mass of ascorbic acid in the tablet. As I is a weak oxidizing agent used chiefly for the finding of strong cut downing agents. ascorbic acerb C6H8O6 ( aq ) can be quickly and quantitatively oxidized by I in acidic status as shown in the undermentioned equation:
I2 ( aq ) + C6H8O6 ( aq ) > C6H6O6 ( aq ) + 2H+ ( aq ) + 2I- ( aq )
This method involves the direct titration of the ascorbic acid with a standard I solution in an acidic medium. However. I is non really soluble in H2O ( 0. 001M ) . this makes the method less than ideal.
Alternatively of fade outing solid I in H2O. we can obtain extra measure of iodine solution by adding an immeasurable surplus of K iodide solution to a known volume of an acidified criterion K iodate solution harmonizing to the undermentioned equation:
IO3- ( aq ) + 5 I- ( aq ) + 6 H+ ( aq ) 3I2 ( aq ) + 3H2O ( cubic decimeter )
Then the experiment is instantly followed by a back titration of the extra liberated I with standard Na thiosulphate solution as shown below:
I2 ( aq ) + 2S2O32- ( aq ) > 2I- ( aq ) + S4O62- ( aq )
With iodine solution. the thiosulphate ion is oxidized quantitatively to tetrathionate ion. The sum of iodine solution produced utilizing this method can so be calculated.
The above method have to be repeated for two times. The first clip without the ascorbic acid. but the 2nd clip with. By happening out the sum of iodine solution produced by this method and the existent sum of iodine solution reacted with the ascorbic acid. and with the aid of the first chemical equation above. the mass of the ascorbic acid can so be calculated.
A. Preparation of a standard solution of K iodate KIO3
1. About 0. 65g of K iodate KIO3 is weighed out accurately. so the exact reading is recorded in the consequence sheet.
2. The solids were dissolved in deionized H2O and were made up to 250 cm3 in a volumetric flask.
B. Standardization of thiosulphate solution with iodate solution.
3. The burette is rinsed. and so it is filled with Na thiosulphate solution.
4. 25. 00 cm3 of the K iodate solution is pipetted into a 250 cm3 conelike flask by utilizing a pipette and a pipette filler.
5. About 5 cm3 of 1M K iodide solution was added. so followed by adding about 8 cm3 of 0. 5M sulfuric acid into the conelike flask.
6. The initial burette reading is recorded in table1. The resulting solution is instantly titrated with Na thiosulphate solution.
7. After the reaction mixture had turned pale yellow. about 10 beads of newly prepared starch solution were added to it as an index. The titration is continued until the dark bluish coloring material of the reaction mixture changed to colourless. The concluding burette reading is recorded in table 1.
8. The titration is repeated for two times. Stairss 3 to 6 are repeated.
9. The staying standard solution of K iodate solution is transferred from the volumetric flask to a clean beaker.
C. Preparation of a standard solution of vitamin C tablet
10. One commercial vitamin C tablet is dissolved in approximately 150 cm3 of 0. 5M sulfuric acid in a beaker.
11. The resulting solution is transferred to a 250 cm3 volumetric flask. Deionized H2O was added to do up to 250 cm3.
D. Back titration of extra I with standard Na thiosulphate solution
12. 25. 00 cm3 of the vitamin C solution is pipetted into a conelike flask by utilizing a pipette and a pipette filler.
13. About 5 cm3 of 1M K iodide solution was added. so followed by adding 25. 00 cm3 of the standard K iodate solution into the conelike flask utilizing a pipette and a pipette filler.
14. The initial burette reading is recorded in table 2. The ensuing solution incorporating extra I is instantly back titrated with the standard Na thiosulphate solution.
15. After the reaction mixture had turned pale brown. about 10 beads of newly prepared starch solution were added as an index. The titration is continued until the reaction mixture turns pale orange. The concluding burette reading is recorded in table 2.
16. The titration is repeated for two times. Stairss 12 to 15 are repeated.
Data Collection and presentation:
Mass of K iodate KIO3 used: 0. 65g
Molarity of K iodide solution used: 1M
Molarity of sulfuric acid used: 0. 5M
Index used: approximately 10 beads of Starch solution
Table 1 ( vol of Na2S2O3 solution used in the first titration )
Trial 1 2
Final burette reading ( cm3 ) 25. 95 44. 65 27. 50
Initial burette reading ( cm3 ) 7. 10 25. 95 8. 85
Volume of Na2S2O3 solution added ( cm3 ) 18. 85 18. 70 18. 65
Average volume of Na2S2O3 solution added ( cm3 ) ( 18. 70 + 18. 65 ) ? 2 = 18. 675 cm3
Table 2 ( vol of Na2S2O3 solution used in the 2nd titration )
Trial 1 2
Final burette reading ( cm3 ) 12. 60 19. 65 26. 70
Initial burette reading ( cm3 ) 5. 40 12. 65 19. 70
Volume of Na2S2O3 solution added ( cm3 ) 7. 20 7. 00 7. 00
Average volume of Na2S2O3 solution added ( cm3 ) ( 7. 00 + 7. 00 ) ? 2 = 7. 00 cm3
Observation ( color alteration )
1st titration: brown a picket xanthous a dark blue a colourless
2nd titration: brown a picket brown a dark blue a orange
( after adding iodide solution ) ( during titration ) ( with amylum solution ) ( after the terminal point is reached )
1. IO3- ( aq ) + 5 I- ( aq ) + 6 H+ ( aq ) 3I2 ( aq ) + 3H2O ( cubic decimeter )
2. I2 ( aq ) + 2S2O32- ( aq ) > 2I- ( aq ) + S4O62- ( aq )
3. I2 ( aq ) + C6H8O6 ( aq ) > C6H6O6 ( aq ) + 2H+ ( aq ) + 2I- ( aq )
From the consequences obtained. and the three equations of the reactions. we have:
For the first titration:
No. of moles of IO3- ( aq ) in 25 cm3 of the standard solution
= 0. 65g / ( 39. 1 + 127 + 16?3 ) g/ 10 = 0. 000304 mol
No. of moles of I2 ( aq ) produced = 0. 000304 ? 3 = 0. 000911 mol
From equation ( 2 ) . no. of moles of S2O32- ( aq ) = 2 no. of moles of I2 ( aq )
= 0. 000911 ? 2 = 0. 00182 mol
Molarity of Na2S2O3 = 0. 00182 / 0. 0187 = 0. 0974 M
For the 2nd titration:
No. of moles of Na2S2O3 reacted = 0. 0974 ? 0. 007 = 0. 000682 mol
No. of moles of I2 ( aq ) = 1/2 No. of moles of Na2S2O3 reacted = 0. 000341 mol
no. of moles of I2 ( aq ) reacted with ascorbic acid = 0. 000911 – 0. 000341 = 0. 000569 mol
no. of moles of C6H8O6 ( aq ) = no. of moles of I2 ( aq ) = 0. 000569 mol
mass of C6H8O6 ( aq ) in 25 cm3 of standard solution = 0. 000569 ? ( 12 ?6 + 8 ?1 + 16 ?6 ) = 0. 1003 g
mass of vitamin C in 1 tablet = 0. 1003 ? 10 = 1. 003 g = 1003 milligram
Answers to the Questions to be discussed:
Q. 1 The mass of ascorbic acid per tablet is 1003mg as shown on the above.
Q. 2 ( a ) . The starch solution is used as an index.
It gives a dark bluish coloring material to the solution when surplus oxidising agent is present. in which the concentration ratio of I to iodide is high. And the dark bluish coloring material disappears when there is extra cut downing agent. that is when there is a higher concentration of iodide ions.
During the titrations of I with Na thiosulphate. the sum of I is continuously diminishing. comparatively. the sum of iodide ions is increasing. as a consequence. the dark bluish coloring material is bit by bit diappeared. At the end-point. all the I has reacted with Na thiosulphate. the coloring material alterations aggressively from dark blue to colourless. Thus. starch solution can function as an index.
( B ) . The amylum solution have to be added merely when the reaction mixture becomes pale xanthous since irreversible decomposition of amylum will happen when amylum is added to solutions with really high concentrations of I. When the coloring material of the reaction mixture becomes pale yellow. it indicates that the titration is about complete. the concentrations of I will non be every bit high as that at the beginning of the titration. As a consequence. decomposition can be avoided and the color alteration can be observed more clearly.
Q. 3 The exact volumes of potassim iodide solution and sulfuric acid used in the titration are non of import. Although they are the reactants. both of them are non involved in the formation of the merchandise I2 ( aq ) we want. Therefore. in the computation of the no. of moles of I2 ( aq ) . they are non to be included in the computation. However. they are needed for the reaction to happen in the undermentioned ways.
For the sulfuric acid. it is used to sour the mixture to allow the reaction IO3- ( aq ) + 5 I- ( aq ) + 6 H+ ( aq ) 3I2 ( aq ) + 3H2O ( cubic decimeter ) to happen outright.
For the K iodide solution. it is used as a dissolver as I is dissolved in a little volume of reasonably concentrated K iodide soluiton. Iodine is moderately soluble in this medium as the undermentioned reaction: I2 ( s ) + I- I3- . As a consequence. their exact volumes are non of import.
Q. 4 The titration has to be carried out instantly after the add-on of sulfuric acid as the sulfuric acid will sour the mixture and casue the reaction:
IO3- ( aq ) + 5 I- ( aq ) + 6 H+ ( aq ) 3I2 ( aq ) + 3H2O ( cubic decimeter ) to happen outright.
I2 ( aq ) is so quickly produced which lacks stableness due to the volatility of the solute.
Iodine can loss from an unfastened vas in a comparatively short clip. For this ground. titration has to be carried out instantly to forestall any side reactions and loss of I from go oning.
Q. 5 First. rinse the beaker with deionized H2O. Second. take the stopper and pour a few cm3 of the standard solution from the volumetric flask to the beaker for rinsing. After rinsing. utilize a stirring rod to direct the flow of liquid from the volumetric flask to the beaker easy. The KIO3 solution is so transferred to a beaker.
Q. 6 To look into that ascorbic acid ( vitamin C ) deteriorates on warming and on exposure to the air. we can put up two experiments as the same as the one done on the above. However. there will be differences on the two commercial vitamin C tablets.
Both of the tablets will be foremost dissolved in the sulfuric acid in a beaker reaspectively. Then one of the ensuing solution will be left on exposure to the air for 24 hours. And the other one will be heated in the H2O bath for 5-10 proceedingss. To make this. we have to put up a Bunsen burner with a tripod and wire gauze to keep the beaker on it. Afterwards. the two beakers of solution will be used to replace the vitamin C solution prepared by fade outing the vitamin C tablet straight in the sulfuric acid. The experiment will so be the same as the above.
And the consequence will demo that the sum of I reacted with the ascorbic acid will be about nothing. the volume of thiosulphate solution recorded for the first tiration and the 2nd titration would be about the same for the two experiments. This indicates that the ascorbic acid deteriorates. so that no ascorbic acid can be reacted with iodine solution.
Overall Remark to the Experiment:
1. Uncertainty / Error Analysis:
1. The Na2S2O3 in the burette turned milky while the last titration was undergoing. This causes mistakes to the experimental consequence. It is due to the undermentioned chemical reaction:
S2O32- ( aq ) + H+ ( aq ) = HSO3- ( aq ) + S ( s )
Although Na thiosulphate solution is immune to air oxidization. they tend to break up to give sulfur and H sulfate ion. The solution turned to milky due to the suspension of the solid sulfur. However. this procedure occurs at a really low rate. The rate of the decomposition reaction additions significantly when the solution becomes acidic and when there are bacteriums.
In the experiment. some sulfuric acid may hold been wrongly assorted with the Na thiosulphate solution in the burette to give rise to this mistake or there may be some bacteriums.
2. When the vitamin C tablet was added to the sulfuric acid. there was effervescence. The tablet floated on the surface and moved about with fizzing sound. Some of the ensuing solution might hold jumped out of the beaker due to the effervescence and this may do an mistake that some of the mass of the ascorbic acid may hold lost off.
3. The color alteration at the terminal point for the 2nd titration with the ascorbic acid nowadays is really hard to detect because the coloring materials changed from brown to really pale brown. and so back to the original coloring material orange which is due to the nowadays of the pigment of vitamin C. Excess Na2S2O3 ( aq ) solution may hold been added to the reaction mixture due to this ground and this causes an mistake.
4. Iodine solution lacks stableness due to the volatility of the solute. Iodine solution can loss from an unfastened vas in a comparatively short clip. Consequently. the sum of iodine solution produced and the sum of iodine solution reacted can non be known accurately adequate with some loss of iodine solution. This mistake may casue divergence to the replies.
5. The iodide ions present in the solution can undergo air oxidization which can do alterations in the molar concentration of an iodine solution:
4I- + O2 ( g ) + 4H+ > 2I2 + 2 H2O
This reaction causes the molar concentration of the I to increase.
2. Discussion and Conclusion ( ways to minimise mistakes ) :
The value of the mass of ascorbic acid calculated from the experiment is somewhat greater than the value specified by the maker. It is likely due to the mistakes stated above.
To better the experiments. mistakes should be minimized as shown below:
1. To minimise the first job stated supra. standard solution of Na thiosulphate should be prepared under moderately unfertile conditions to forestall bacterial activity. Furthermore. cleansing of the setup is critical to forestall any acids from blending up with the thiosulphate soltuion.
2. To forestall leaping out of solution from the beaker during fade outing the tablet in the sulfuric acid. we can utilize a larger beaker or a conelike flask.
3. To forestall air oxidization of iodide ion and the loss of iodine solution. the titrations have to be done really rapidly and instantly after. This can minimise the uncertainness.
Alteration of the experiment:
In the experiment. K iodate solution is used to standardise a thiosulphate solution as iodine solution is produced for titration with Na thiosulphate.
Alternatively of utilizing K iodate solution. we can utilize iodine solution straight for titration. Although the I has low solubility. it can still be prepared by fade outing I in a little volume of concentrated solution of K iodide. To guarantee complete solution. attention should be taken to avoid dilution of the concentrated solution until the last piece of solid I has disappeared. This can be done by filtrating the solution through a sintered glass melting pot before standardisation.