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Monday, August 26, 2013

The Determination of Ascorbic Acid in Vitamin C Talbets by Titration with Potassium Bromate - Lu Le Laboratory

Standardization of Sodium Thiosulfate against Potassium Bromate

Iodine is generated by the reaction between a known volume of standard potassium bromate and an unmeasured excess of potassium iodide:

BrO3- + 6 I- + 6 H+ Br- + 3 I2 + 3 H2O

The iodine produced is titrated with the sodium thiosulfate solution.

1.     Pipet 25.00-mL aliquots of the KBrO3 solution into 250-mL a conical flask and rinse the interior walls with distilled water.
2.     Treat each sample individually beyond this point. Introduce 2 to 3 g of KI and about 5-mL of 3 M H2SO4
3.     Immediately titrate with Na2S2O3 until the solution is pale yellow.

4.     Add 5-mL of starch indicator, and titrate with Na2S2O3 to the disappearance of the blue color.

5.     Calculate the concentration of the thiosulfate solution.

The Determination of Ascorbic Acid in Vitamin C Tablets by Titration with Potassium Bromate

Ascorbic Acid, is cleanly oxidized to dehydroascorbic acid by bromine:

An unmeasured excess of potassium bromide is added to an acidified solution of the sample. The solution is titrated with standard potassium bromate to the first permanent appearance of excess bromine; this excess is then determined iodometrically with standard sodium thiosulfate. The entire titration must be performed without delay to prevent air oxidation of the ascorbic acid.

1.     Weight (to nearest milligram) 3 to 5 vitamin C tablets [1]. Pulveize them thoroughly in a mortar, and transfer the powder to a dry weighting bottle.
2.     Weight individual 0.40-g to 0.50-g samples (to the nearest 0.1mg) into dry 250-mL conical flasks.
3.     Treat each sample individually beyond this point. Dissolve the sample [2] in 50-mL of 1.5 M H2SO4; then add 5g of KBr.

4.     Titrate immediately with standard KBrO3 to the first faint yellow due to excess Br2. Record the volume of KBrO3 used.

5.      Add 3 g of KI and 5-mL of starch indicator; back-titrate [3] with standard 0.05 M Na2S2O3.

6.     Calculate the percentage of ascorbic acid in the sample.

[1] This method is not applicable to chewable vitamin C tablet.
[2] The binder in may vitamin C tablets remains in suspension throughout the analysis. If the binder is starch, the characteristic color of the complex with iodine appears on addition of KI.
[3] The volume of the thiosulfate needed for the back-titration seldom exceeds a few milliters.

Report Sheet
Concentration of Standard KBrO3 (aq)
0.0150 M
Concentration of Standard Na2S2O3 (aq)
0.0500 M
Weight of Vitamin C Samples

Sample 1
0.4000 g
Sample 2
0.4000 g

Test 1

Volume of KBrO3 (aq)
51.25 mL
Volume of Na2S2O3 (aq)
3.50 mL
Test 2

Volume of KBrO3 (aq)
51.20 mL
Volume of Na2S2O3 (aq)
3.10 mL
Weight of Vitamin C
(3x0.0150 x V(KBrO3) – (0.0500 x V(Na2S2O3) /2 ) x 176.12 x 10-3
Test 1
0.3910 g
Test 2
0.3920 g
Average Weight of Vitamin C
0.3920 g
Percentage Error
2.00 %
Assay of Vitamin C Sample

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