ELECTRODE POTENTIALS

A) Theoretical
	Electrode potential is basically a difference in electric potentials of an electrolyte solution and an electrically conductive solid (called electrode) being in contact with each other.
	Clearly, electric charges (ions and/or electrons) are transferred between the two phases, and the work of such a transference (the electric potential  multiplied by the transferred charge ziFni; where ni is the amount of the i'th substance) has to be be included in the fundamental equation for closed multicomponent two phase systems

						1)

One may introduce the concept of electrochemical potential :

										2)

and rewrite Eq. 1):

								3)

Now, in the usual isotherm-isobar equilibrium the following condition must be fulfilled:

										4)

In words: the chemical potential difference between the phases is balanced by their oppositely signed electric potential difference. This is the potential condition for an electrochemical equilibrium.
	There is a kinetic condition for equilibrium, too, the law of detailed balance:

										5)

In words: electric current carried by component i when it is transferred from phase k to phase l is balanced by the transference of the same component backward. The absolute value of equilibrium currents in Eq. 5) is called exchange current:

									6)

	The larger the exchange current is the faster the equilibrium potenial settles. The magnitudes of currents i's, consequently, that of exchange current depend on the activities of component i in the phases k and l.
	If phases k and l have more components transferred between each other the situation becomes complicated as each component tries to settle its own equilibrium electrode potential while the electrode can possess only one. Two cases may happen:
i)	The exchange currents of the components are of the same order of magnitude. The resulting electrode potential is a combination of their eqilibrium potentials and is called mixed potential. Not any of the processes is in equilibrium in this case, so the measured electrode potential can not be treated in the frame of equilibrium thermodynamics.
ii)	One of the components has an exchange current larger by many orders of magnitude than the others. This component enforces its own equilibrium electrode potential and the others have practically no contribution; as if only one component transferred (so-called "electrode with a main process"). Its potential can be treated thermostatically.

	For a metal, composed of its cations and electrons (phase k), immersed into an electrolyte solution containing the same cations (phase l), after the equilibrium of transfer of the cations (component i) has been settled, Eq. 4) writes:

								7)

	Of course, voltage U (the absolute electrode potential) can not be directly measured. In order to close the electric circuit one must immerse another electrode (so-called reference electrode) into the solution where a second absolute electrode potential (U2) would build up. We could measure only the difference U-U2 that is the electromotive force (EMF) of the galvanic cell composed of the two electrode-electrolyte systems.
	In addition, the second electrode may not find any component in the electrolyte that could be transferred between them, so it may need a second electrolyte. The second electrolyte must be connected to the first one by a salt bridge which contributes to the EMF of the galvanic cell with its "junction potential".
Despite of its name, junction potential is a voltage. We try to keep it on a negligible value. One has to fill up the salt bridge with a concentrated solution of a salt whose cation and anion have mobilities as close to each other as possible.
	The practical solution to the problem of electrode potential measurment was the use of the standard hydrogen electrode as source of U2. The EMF of the galvanic cell

	Pt  H2(p=1bar)  HCl (activity=1)  electrolyte solution  electrode

(where  represents a phase boundary and  a salt bridge) has been defined as electrode potenial .
	Its concentration dependence can be given by using the well-known

									8)

and Eq. 7) to get the Nernst equation:

									9)

(Superscript  denotes the standard value of the potential;  is the mean ionic activity coefficient; see text Electrolytic conductivity.)

	The most important types of electrodes are:
i)	Metal electrodes, transferring cations as outlined above.
ii)	Redoxi electrodes are metal or metal-like , too, but are not able to transfer their cations (Pt, Au, graphite). They could transfer electrons if free electrons existed in electrolyte solutions. As they do not, the solution must have a redoxi system whose components, adsorbed to the electrode surface, can take up and give down electrons. Following the same way as with metal electrodes, however, replacing chemical potential of cation by that of electron and expressing the hypothetical activity of electron in the solution from the eqilibrium constant of the redoxi reaction:

								10)

(Subscript Ox denotes the oxidized, subscript Red the reduced components of the redoxi reaction.)
iii)	Membrane electrodes, where the act of transference takes place between the soaked surface of the membrane and the electrolyte. Similarly to the case ii), the activity of the transferred component in the soaked region of the membrane can be calculated from a related equilibrium constant. Ion-selective electrodes and glass electrode belong to this important group.

B) Equipment
	We assemble a galvanic cell as can be seen on the Figure. It is composed of a I2/I- electrode 1 whose Nernstian operationfulness will be investigated and a quinhydrone electrode 2 which serves as reference electrode. Both electrodes belong to the group of redoxi electrodes.
	Electrolytes are galvanically connected by a salt bridge 3.
	The EMF of the galvanic cell is measured by a voltmeter of high input impedance 4.
	The I2/I- electrode is a flat platinum electrode immersed into an aqueous solution containing I2 and KI. Its redoxi reaction in the solution and on the surface of the platinum is as follows:

				2I-   I2 + 2e-

, consequently, its Nernst equation (replace the related components in Eq. 10)) is
:
									11)

Eq. 11) can be converted to a more practical form (unit for  is mV, T=25C, decimal logarithm instead of natural):

							12)

The standard electrode potential at room temperature, according to the literature, is =628 mV.
	The activities of the components in the solution is influenced by an other reaction:

			 I2 + I-    I3-	,					13)

whose equilibrium constant is K=641dm3/mol.
	The quinhydrone electrode consist of a flat platinum electrode immersed into a buffered solution that is saturated by both quinone and hydroquinone. The redoxi reaction taking place is:

			hydroquinone  quinone + 2H+ + 2e-,

and the related Nernst equation, in its more practical form:

						14)

Due to the saturation, activities of the organic components are constant at a given temperature, so they can be included in the standard electrode potential. Doing this, the Nerst equation becomes:

			/ mV = 699 mV - 59 pH					15)

	The salt bridge is a U-shaped glass tube closed by glass filters on its both ends. It contains 2.5 mol/dm3  KNO3  solution. The diffusion of ions K+ and NO3-  creates the galvanic contact between the electrochemically active solutions. Leakage through the filters is the most serious error-source of this measurement.

C) Experimental
	1. Make a series of solutions by diluting the stock solution with KI solution. Gross concentration of iodine should reach the value 10-9 mol/dm3.
	2. With a Weston cell, decide which input of the voltmeter is V and which one is COMMON.
	3. Assemble the galvanic cell depicted on the Figure. Connect the quinhydrone electrode to input COMMON and the I2/I- electrode to input V. Doing it in this way, the relation between the measured voltage U and the electrode potentials reads:
								16)
	4. Starting from the most dilute solution, measure the EMF of the galvanic cell with each solution of the series, and at last, with the stock solution.
	5. Replacing activities by concentrations, calculate the  content for each member in the series using Eq. 13).
Denoting  content by x, the gross concentration of iodide by c1, and the gross concentration of iodine by c2 , Eq. 13) takes the form:
						17)
Solve the equation above for x and choose the realistic one from the two solutions.
	6. Plot the graph   .
With notation in Eq. 17), (c2-x)/(c1-x)2 has to be written in the argument of the logarithm function.
 	7. Mark the region on the graph where the electrode obeys the Nernst equation. Read intersection and calculate the standard electrode potential using Eq. 15) and 16). Check the slope; if eqilibrium was settled it should be  29-30 mV/decade. If equilibrium was not settled the slope is less. A larger slope signes a measuring error.