A.2 Preparation of 0.1 M Potassium Bromate Solution
Potassium Bromate that had been dried for 1 – 2 hours at 120*C and then cooled in the dessicator was accurately weighed out to 4.1 g on a analytical balance, transferred quantitatively to a 250 mL volumetric flask and diluted to the mark with distilled water. The falsk was then labeled.
A.3 Standardization of Sodium Triosulphate Solution
25 mL of the standard potassium bromate solution was transferred using a 25 mL pipette into a 250 mL Erlenmeyer flask. 2.5 g of potassium iodide followed by 4 – 5 mL of 4 N hydrochloric acid. This solution was then titrated with sodium thiosulphate until a pale orange-yellow colour appeared. Then the solution was diluted with 100 mL of distilled water and approximately 2 mL of starch solution was added. The solution was then titrated again until colour disappears.
A.4 Assay
An unknown Vitamin C tablet was assigned and the weight of 20 tablets was recorded (B). The mean weight of one tablet was then calculated. Two of the unknown tablets were then grinded using a mortar and pestle unto a coarse powder. The weight of half of a tablet was weighed on an analytical balance three times and transferred into 3 different 250 mL Erlenmeyer flasks and dissolved in 100 mL of distilled water. Into each Erlenmeyer flask 25 mL of potassium bromate was pipetted into the solution, along with 5 mL of 4N HCl and 2.0 g KBr, and topped with a stopper. Solution was swirled until colour change and then 2.5 g of KI was also added. After recording V initial, the solution was then titrated with sodium thiosulphate until most of the liberated iodine has reacted, which is when the dark colour becomes transparent. Then 2 mL of the starch solution was added as indicator. The solution was titrated again until solution goes clear and all the black beads disappear. V final was then recorded.
Results:
2.1 Data
Weighed of Potassium Bromate: 4.1245 g
Table 2.11: Standardization if Potassium Bromate
Table 2.12: Vitamin C Samples for Determination
2.2 Results
Table 2.21: Results of standardization of Potassium Bromate (Molarity)
Table 2.22: Results of standardization of Potassium Bromate (Normality)
Table 2.23: Results of Vitamin C determination (mg)
Table 2.22: Results of Vitamin C determination (mg/tablet)
2.3 Sample Calculations
Standardization of Potassium Bromate
BrO3- + 6S2O32- + 6H+ → Br- + 3S2O32- + 3H2O
For Sample #1:
mol KBrO3 = weight KBrO3 = 4.1245 g = 0.0247 mol KBrO3
MW KBrO3 166.9922 g/ mol
M KBrO3 = mol KBrO3 = _0.0247_mol KBrO3 = 0.098 M KBrO3
V(L) KBrO3 0.250 L
M Na2S2O3 = M KBrO3 * (Vpipette (L))
(Vtitrant (L))
M Na2S2O3 = 0.098 M KBrO3 * 0.02495 L
0.034299 L
= 0.0713 M Na2S2O3
N Na2S2O3 = M KBrO3 * 6
= 0.0713 M KBrO3 * 6
= 0.4278 N thio
Vitamin C Determination
6S2O3- + 2BrO3- + 10 H+ + 2Br- + C6H8O6 → 3S4O62- + 6H2O + 2Br2 + C6H6O6
For Sample #1
mol BrO3- = M KBrO3 * Vpipette(L)
= 0.098 M KBrO3 * 0.02495 L
= 0.0244 mol BrO3-
mol Na2S2O3 = N Na2S2O3 * Vtitrated (L)
= 0.4238 N Na2S2O3 * 0.03378 L
= 0.0014 mol Na2S2O3
6 mol Na2S2O3 = 1 mol Vitamin C
mol Vitamin C = (1/6) * mol Na2S2O3
= (1/6) * 0.0014 mol Na2S2O3
= 0.00024 mol Vitamin C
mol BrO3- (left) = mol BrO3- - mol Vitamin C
= 0.0244 mol BrO3- - 0.00024 mol Vitamin C
= 0.00221 mol BrO3-
1 mol BrO3- = 2 mol Vitamin C
mol Vitamin C = 2 * mol BrO3-
= 2 * 0.00221 mol BrO3-
= 0.004413 mol Vitamin C
mg Vitamin C = mol Vitamin C * MW Vitamin C
= 0.004413 mol Vitamin C * (176.13 g / mol) Vitamin C
= 0.777 g Vitamin C = 777 mg Vitamin C
mg/table Vitamin C = 777 mg Vitamin C * {Mtablet (mg) / Msample(mg)}
= 777 mg Vitamin C * (0.30533 g / 0.1577 g)
= 1504.9 mg/tablet Vitamin C
Discussion
Conclusion
In this experiment we have determined how much Vitamin C (ascorbic acid) is actually present in a Vitamin C tablet.
First we standardized our titrant which was sodium thiosulphate; the molarity concentration was 0.0713 ± 0.0011 M with a 95% confidence interval of 0.0013 and a relative standard deviation of 15.77. The normality concentration was 0.4238 ± 0.0067 N with a 95 % confidence interval of 0.0076 and a relative standard deviation of 15.77.
We then determined the amount present in our Vitamin C sample, which was 776.89 ± 0.364 mg with a 95 % confidence interval of 0.412 and a relative standard deviation of 0.468.
Also the amount of Vitamin C present per tablet B which is 1502.64 ± 11.42 with a 95 % confidence interval of 12.92 and a relative standard deviation of 7.600.
References:
- Chemistry – CHEM 2080 4.0, York University, p. 27-29