Ions in Solutions + Electrodes

 

Ohm's law - the linear relation between potential difference and current flow - applies to aqueous ionic solutions, such as blood, cytoplasm and sea water. Complications are introduced by two factors:

 

The current is carried by at least two types of ions (one anion and one cation) and often by many more. For each ion, current flow in the bulk solution is proportional to the potential difference. For a first approximation, the conductance of the whole solution is simply the sum of the conductances contributed by each ionic species. When the current flows through ion channels, it is carried selectively by only a subset of the ions in the solution.

At the electrodes, current must be transformed smoothly from a flow of electrons in the copper wire to a flow of ions in solution.

 

Many sources of errors (artifacts) are possible.

Several types of electrodes are used in electrophysiological measurements:

The most common is a silver/silver chloride (Ag/AgCl) interface, which is a silver wire coated with silver chloride. If electrons flow from the copper wire through the silver wire to the electrode AgCl pellet, they convert the AgCl to Ag atoms and the Cl - ions become hydrated and enter the solution. If electrons flow in the reverse direction, Ag atoms in the silver wire that is coated with AgCl give up their electrons (one electron per atom) and combine with Cl –ions that are in the solution to make insoluble AgCl. This is, therefore, a reversible electrode,i.e., current can flow in both directions.

 

 

The Silver/Silver Chloride Electrode

 

There are several points to remember about Ag/AgCl electrodes:

1.The Ag/AgCl electrode performs well only in solutions containing chloride ions;

2.Because current must flow in a complete circuit, two electrodes are needed. If the two electrodes face different Cl - concentrations (for instance, 3 M KCl inside a micropipette and 120 mM NaCl in a bathing solution surrounding the cell), there will be a difference in the half-cell potentials (the potential difference between the solution and the electrode) at the two electrodes, resulting in a large steady potential difference in the two wires attached to the electrodes. This steady potential difference, termed liquid junction potential, can be subtracted electronically and poses few problems as long as the electrode is used within its reversible limits;

3.If the AgCl is exhausted by the current flow, bare silver could come in contact with the solution. Silver ions leaking from the wire can poison many proteins. Also, the half-cell potentials now become dominated by unpredictable, poorly reversible surface reactions due to other ions in the solution and trace impurities in the silver, causing electrode polarization. However, used properly, Ag/AgCl electrodes possess the advantages of little polarization and predictable junction potential. A micropipette is a pulled capillary glass into which the Ag/AgCl electrode is inserted.

 

The silver/silver chloride electrode is reversible but exhaustible.

 

 

 

 

The Platinum Electrode

 

Another type of electrode, made of platinum (Pt),is irreversiblebut not exhaustible. At its surface, Pt catalyzes the electrolysis of water. The gaseous H2 or O2 produced, depending on the direction of current flow, leaves the surface of the electrode. If both electrodes are Pt electrodes, the hydroxyl ions and protons are produced in equal numbers;however, localpH changes can still occur.