# Potentiometry

## In-Class Exercises

Suppose that your analyte is an electroactive species, which means that it can donate or accept electrons at an electrode.  Ex:  Trying to detect the redox state of iron (relative amounts of Fe2+ and Fe3+) in water.

Let's construct an electrochemical cell to detect the redox state of a solution that contains both iron species.  Start your sketch below:  Two beakers containing solutions.  The one on the right contains the two analytes.

1. We are doing electrochem, so we know we likely will need to know what redox reaction could happen here. Look at your table of standard potentials and find the relevant half reaction and its standard potential.

1. Will this reaction progress as an oxidation or a reduction in the electrochemical cell that we are about to build?

1. To take advantage of the analyte's electroactivity for analytical purposes, we need to be able to conduct electrons in and out of the solution. What could you add to your drawing?  Notes:

1. This is only a half-cell. What else do you need in your system to measure the potential of the complete cell?  Notes:

1. What do you want the potential of the reference electrode to be, changing or constant?

1. How could you ensure that the reference potential remains constant?

1. Write a general equation for the voltage of the complete cell.

1. Sketch a Ag/AgCl reference electrode and annotate its parts (notes from lecture)

### Part II.  Quantification

1. Write an equation for the electrochemical potential of a saturated Ag/AgCl electrode.

For saturated KCl/AgCl, E =

1. Suppose you want to measure the relative amounts of Fe2+ and Fe3+ in solution. You build an electrochemical cell by inserting Pt wire into the iron solution, and connect it to a saturated Ag/AgCl reference electrode at 298 K. How does the cell potential vary with the ratio of [Fe2+]/[Fe3+]?  Write an equation.

1. You record the potential, then remove your Pt electrode then add an enzyme that catalyzes a redox reaction with iron. After stopping the reaction and removing the enzyme, you put the Pt electrode back in.  The measured potential is 59.1 mV higher than before.  How much did the ratio of [Fe2+]/[Fe3+] change?