Standardization of Sodium
Thiosulfate against Potassium Bromate
Discussion
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.
Procedure
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
Discussion
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.
Procedure
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.
Notes
[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
|
|
Titration
|
|
|
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
|
98.00%
|
