- The empty weighing bottle was weighed again And the exact mass of potassium iodate(V) used was obtained by weighing by difference.
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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.
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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
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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
- A standard titration set-up was framed up using a stand, a burette clamp and a white tile.
- A burette was rinsed with distilled water and then with the given sodium thiosulphate solution.
- 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.
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A portion of the prepared potassium iodate(V) solution was poured from the volumetric flask to a clean and dry 100 cm3 beaker.
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A 25.00 cm3 pipette was rinsed with distilled water, and then with the potassium iodate(V) solution.
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25.00 cm3 of potassium iodate(V) solution was transferred from the beaker into a clean conical flask using the rinsed pipette.
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Using a 10 cm3 measuring cylinder, 5 cm3 of 1.0M potassium iodate(V) solution was added to the same conical flask.
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Also, with another clean measuring cylinder, 10 cm3 of 0.5M sulphuric acid was added to acidify the mixture in the conical flask.
- Immediately, the reaction mixture in the conical flask was titrated with the sodium thiosulphate solution until a pale yellow solution was observed.
- A few drops of starch solution were added to the pale yellow solution in the conical flask.
- Titration of the reaction mixture was continued until the solution changed from dark blue to colourless. The final burette reading, accurate to 2 decimal places, was recorded. The volume of the sodium thiosulphate solution added was calculated.
- Apart from the very first trial run, 3 more titrations were carried out. The burette was refilled in between runs when the volume remained was not enough for one complete titration.
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Steps 6-11 were repeated for 3 times. Sodium thiosulphate solution was added carefully drop by drop starting from the point when it was 3 cm3 less than the estimated value. The solution was turned colourless definitely by the one last drop.
C) Determination of the Vitamin C content in the Tablet
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The mass of a vitamin C table was weighed and recorded. Then the tablet was placed in a dry and clean 250 cm3 beaker.
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Using a 100 cm3 measuring cylinder, around 150 cm3 of 0.5M sulphuric acid was poured into the beaker which contained the vitamin C tablet.
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The resulting solution was drained into a 250 cm3 volumetric flask. The beaker was rinsed with distilled water twice and all the rinsing water was discharged into the flask.
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The solution in the volumetric flask was made up to 250 cm3 and the flask was shaken gently. A portion of the vitamin C solution was poured out from the flask into a dry and clean 100 cm3 beaker.
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A 25.00 cm3 pipette was first rinsed with distilled water and then with the Vitamin C solution.
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25.00 cm3 of the vitamin C solution was pipetted from the 100 cm3 beaker into a clean conical flask.
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5 cm3 of 1.0M potassium iodate(V) solution was added into the vitamin C solution in the conical flask using a 10 cm3 measuring cylinder.
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Lastly, 25.00 cm3 of the previously prepared standard potassium iodate(V) solution was transferred to the same conical flask.
- The solution was immediately titrated with sodium thiosulphate solution in the burette, just as in part (B), step 9-13.
- The volume of sodium thiosulphate used in each titration was recorded and the average volume was calculated.
Results and Calculations
Mass of weighing bottle and potassium iodate(V): 4.647g
Mass of weighing bottle: 4.000g
Mass of potassium iodate(V) weighed: 0.674g
0.674g potassium iodate(V) = 0.674g ÷ (39.1+127+16x3)g mol-1
= 3.148 x 10-3 mol
Concentration of the prepared standard potassium iodate(V) solution:
3.148 x 10-3 mol ÷ 0.25dm3 = 0.0126 mol dm-3
Table 1 (for part B)
Average titre of sodium thiosulphate = 18.70 cm3
Mass of the vitamin C tablet: 5.802g
Brand name: Redoxon
Manufacturer’s specification of vitamin C tablet: 1000mg of Vitamin C per tablet
Table 2 (for part C)
Average titre of sodium thiosulphate = 7.67 cm3
Questions and Discussions
1) From the stoichiometry of equations (1) & (2) stated in the introduction part:
IO3-(aq) + 5I-(aq) + 6H+(aq) → 3I2(aq) + 3H2O(l)
6S2O32-(aq) + 3I2(g) → 3S4O62-(aq) + 6I-(aq)
Mole ratio of KIO3 : Na2S2O3 = 1 : 6
Concentration of the prepared standard KIO3(aq) : 0.0126 mol dm-3.
No. of moles of KIO3(aq) in 25.00cm3 : 0.0126mol dm-3 x 0.025dm3
= 0.000315 mol
No. of moles of Na2S2O3 required for one titration: 0.000315mol x 6
= 0.00189mol.
Average titre in the standardization of sodium thiosulphate in part B: 18.70 cm3
Concentration of sodium thiosulphate solution: 0.00189mol ÷ 0.0187dm3
= 0.101mol/dm-3
2) After reacting with vitamin C, average titre for the titrations: 7.67cm3
No. of moles of Na2S2O3(aq) needed to react with the excess I2(aq):
0.101mol/dm-3 x 0.00767dm3 = 0.000752 mol
2S2O32-(aq) + I2(g) → S4O62-(aq) + 2I-(aq)
No. of moles of excess I2(aq): 0.000752 ÷ 2 = 0.000375 mol
Originally, I2(aq) liberated from 25.00 cm3 KIO3 and excess KI in acid medium:
IO3-(aq) + 5I-(aq) + 6H+(aq) → 3I2(aq) + 3H2O(l)
3 x 0.0126mol dm-3 x 0.025dm3 = 0.000945mol
No. of mole of Vitamin C in 25.00cm3
= No. of mole of original I2 - No. of mole of excess I2
= 0.000945 – 0.000375
= 0.00057 mol
No. of mole of Vitamin C in 250.00 cm3 or in the tablet = 0.00057 ÷ (0.025/0.25)
= 0.0057 mol
Molar mass of vitamin C (C6H8O6) = 12x6 + 1x8 + 16x6 = 176g mol-1
Mass of Vitamin C per tablet: 0.0057mol x 176g mol-1 = 1.0032g
= 1003.2mg
Percentage of vitamin C per tablet: 1.0032/5.802 x 100% = 17.29%
3) Starch solution is used as an indicator. Iodine forms a complex with starch which is dark blue. The endpoint of the titration can be detected by the complete disappearance of the blue colour. By knowing the total quantity of iodine formed, and the quantity left after reaction with vitamin C, the amount of iodine reacted with the vitamin C can be calculated, hence the vitamin C concentration.
The starch solution cannot be added earlier but only when the reaction mixture(iodine solution) fades to a straw colour(pale yellow), by the time most of the iodine has been reduced. It is because with iodine is still in high concentration, it would form a blue-black precipitate complex with starch irreversibly which does not dissolve again easily even though there is an excess of thiosulphate, so iodine would be locked up and would not be free to react.
4) After the addition of sulphuric acid to the reaction mixture, titrations should be carried out immediately. It was because iodine can be easily vapourized and escaped from the solution, causing lost of reacting substance. If this is the case, iodine reacted with thiosulphate will be less than usual. The volume used in the titrations will become less, and thus the calculated concentration of vitamin C present in the tablet will give a greater value.
Moreover, besides the forward reduction of iodine to iodide, after all vitamin C is reacted, iodide can also be oxidized as shown:
4I-(aq) + O2 (g) + 4H+ → 2I2(g) + 2H2O(l)
This will be the backward reaction altering the concentration of iodine present. If titration is not carried out immediately, due to the regeneration of iodine in the system, sodium thiosulphate required to reach the end point of titration will be increase, leading to an overestimation of vitamin C content in the tablet.
5) It is known that vitamin C decomposes upon the exposure to air or heating. To clarify, the following experiments could be done, and the results will be compared to that obtained from a control experiment(simply the one done before as outlined above).
In order to investigate the exposure factor, place the vitamin C tablet nakedly in atmospheric air before doing the whole experiment again. It is believe that the sodium thiosulphate used for each titration in part C will be greater, since the vitamin C content decreases upon exposure to air.
6) Cooking means heating or boiling the food. When vegetables are cooked, the vitamin C they contain is heated vigourously. Knowing that boiling temperatures will destroy vitamin C, the amount of vitamin C in the vegetables will be definitely reduced upon cooking.
Further Discussion
(i) Acidification of the vitamin C sample also serves to stabilize the ascorbic acid, which will other wise decompose and be undetectable.
(ii) As stated in the introduction part, iodine has a limited solubility in water. It dissolves well in the solution of potassium iodide only because it will react with I- to form the very soluble red-brown complex, triiodide ion, I3-. So it is reminded that the iodine generated from the redox reaction of iodide and iodate is actually in the form of the triiodide ions in the presence of excess KI due to the I2 + I- I3- equilibrium.
(iii) Ascorbic acid can undergo air oxidation requiring that the procedure be performed with minimal delay.
(iv) The structure of ascorbic acid (centered around a five-membered ring of four carbons and one oxygen atom) includes two adjacent alcohol(OH) functional groups.
Conclusion
(1) Neutralization is an exothermic reaction.
(2) Stronger the acid or alkali, greater in magnitude will be the enthalpy change ΔH obtained.
