Vitamin C

Topics: Iodine, Titration, Potassium Pages: 4 (1193 words) Published: October 29, 2012


Ascorbic acid, C6H8O6, MM =  176.13, can be oxidized to form dehydroascorbic acid by bromine via the following reaction:

The bromine or iodine used in the titration is generated by adding an excess amount of KBr or KI to an acidified solution of the sample according to the following reaction:   BrO3-  +   5 Br-   +   6 H+    3 Br2  +   3 H2O IO3-  +   5 I-   +   6 H+    3 I2  +   3 H2O

Sodium thiosulfate solutions can be standardized by direct titration of the I2 generated in in the KIO3  reaction using the starch-iodine complex as the indicator (remember that the iodine is actually in the form of the triiodide ions in the presence of excess KI due to the  I2  +  I-  I3-, equilibrium.)   In the determination of ascorbic acid, both the KBrO3 and KIO3 methods employ a back titration with a standard thiosulfate solution of the excess Br2 or I2 generated by adding a precisely known amount of a primary standard KBrO3 or KIO3 solution to an ascorbic acid solution.  In the case of the KBrO3 procedure, potassium iodide is added to react with Br2 to form I2. Br2  +   2 I-     2 Br-  +   I2

In both methods the excess iodine formed is back-titrated with standard sodium thiosulfate.  I2  +  2  S2O3-2    2 I-  +  S4O6-2
Ascorbic acid can undergo air oxidation requiring that the procedure be performed with minimal delay. Reagents:|
| starch solution| 1.5 M H2SO4|
| potassium bromide| potassium bromate|
| potassium iodide| potassium iodate|
POTASSIUM BROMATE EXPERIMENTAL PROCEDURE (this is an alternative procedure) 

Preparation of Standard 0.015 M Potassium Bromate
Weigh approximately 2.6 g of dried and cooled reagent grade KBrO3 to the nearest 0.1 mg. Transfer quantitatively to a 1000 mL volumetric flask. Initially, dissolve the KBrO3 in about 200 mL of distilled water. Dilute to the mark and mix thoroughly.  Calculate the...
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