2 Determination of Dichromate by a Coulometric Redox

Method 11.2 Determination of Dichromate by a Coulometric Redox

Titration 13

Description of the Method. The concentration of Cr 2 O 7 2– in a sample is

determined by a coulometric redox titration using Fe 3+ as a mediator and electrogenerated Fe 2+ as the “titrant.” The end point of the coulometric redox titration is determined potentiometrically.

Procedure. The electrochemical cell consists of a Pt working electrode and a Pt counterelectrode that is maintained in a separate compartment connected to the analytical solution by a porous glass disk. The counter electrode’s compartment is

filled with 0.2 M Na 2 SO 4 , the level of which must always be maintained above that

of the solution in the electrochemical cell. Platinum and tungsten electrodes connected to a potentiometer are used to follow the change in potential during

the analysis. A solution of approximately 0.3 M NH 4 Fe(SO 4 ) 2 Representative Methods is prepared for use as

the mediator. The coulometric redox titration is carried out by adding 5.00 mL of

the sample solution, 2 mL of 9 M H 2 SO 4 and 10–25 mL of the NH 4 Fe(SO 4 ) 2

mediator solution. Distilled water is added as needed until the electrodes are covered. Pure N 2 is bubbled through the solution for 15 min to remove traces of O 2 . A flow of N 2 is maintained during the electrolysis, but is turned off momentarily when measuring the potential. A magnetic stir bar is used to stir the solution. Adjust the current to 15–50 mA, and begin the titration. Periodically stop the titration, and measure the potential of the solution. In this manner a titration

—Continued

506 Modern Analytical Chemistry

Continued from page 505 curve of potential versus time is recorded. The time needed to reach the

equivalence point is read from the titration curve.

Questions

1. Does the platinum working electrode serve as the cathode or the anode in this analysis?

Reduction of Fe 3+ to Fe 2+ occurs at the working electrode, making it the cathode in the electrochemical cell.

2. Pure N 2 is bubbled through the solution to remove any trace of dissolved O 2 .

Why is this necessary?

If O 2 is present, some of the electrogenerated Fe 2+ may be oxidized back to Fe 3+ by the reaction

4Fe 2+ (aq) + O 2 (g) + 4H 3 O + (aq) t 4Fe 3+ (aq) + 6H 2 O(l)

Any Fe 2+ lost in this fashion must be replaced by the additional reduction of Fe 3+ , reducing the current efficiency and increasing the time needed to reach the titration’s end point. The net result is that the reported concentration of

Cr 2 O 7 2– is too large.

3. What is the effect on the analysis if the NH 4 Fe(SO 4 ) 2 used to prepare the mediator solution is contaminated with trace amounts of Fe 2+ ?

The Fe 2+ introduced when the mediator solution is first added will react with the Cr 2 O 7 2– before the electrolysis begins. As a result, the amount of Fe 2+ that must be generated is less than expected, decreasing the time needed to reach the end point of the titration. The current efficiency, therefore, is greater than

100%, and the reported concentration of Cr 2 O 7 2– is too small. Trace amount of Fe 2+ can be removed from the mediator solution by adding H 2 O 2 and heating at 50–70 °C until the evolution of O 2 ceases. Alternatively, a blank titration can be used to correct for any Fe 2+ initially present in the mediator.