Metal to Aqueous Interface

Physical and Electro-chemistry of metals in salty water and

metals in contact with cells

Reduction Potentials

• What to expect from various metal combinations in aqueous solutions:

• Table of Standard Reduction Potentials • http://hyperphysics.phy-astr.gsu.edu/hbase/tables/electpot.html#c1

(In an electrochemical cell, an electric potential is created between two dissimilar metals. This potential is a measure of the energy per unit charge which is available from the oxidation/reduction reactions to drive the reaction. It is customary to visualize the cell reaction in terms of two half-reactions, an oxidation half-reaction and a reduction half-reaction.)

Ag-Cl coating of electrode…

• Imagine one terminal of a 9v. battery connected to a silver wire in beaker of salty water

• The other terminal to another Ag wire in the same beaker.

At cathode: Ag --> Ag+ + e- (etching)Na+ + e- --> Na 2 Na + 2 H2O --> 2 NaOH + H2 (gas, bubbles) NaOH --> Na+ + OH-The overall cath reaction, powered by the battery, can be described as:2Ag + 2Na+ + 2H2O --> 2Ag+ + 2 H2 + 2 Na+ +2 OH-

Ag electrodes in salty water

At Anode: Ag + Cl- --> AgCl + e-

Safety and cognitive effect of frontal DC brain polarization in healthy individuals

• From the Brain Stimulation Unit and Cognitive Neuroscience (Drs. Iyer, Grafman, and Wassermann, and U. Mattu), Human Motor Control (Dr. Lomarev), and EEG Sections (Dr. Sato), National Institute of Neurological Disorders and Stroke, National Institutes of Health

Conclusions: Limited exposure to direct current polarization of the prefrontal cortex is safe and can enhance verbal fluency selectively in healthy subjects. As such, it deserves consideration as a procedure to improve frontal lobe function in patients.

Lesson?

• Isolate the patient using a capacitor in series with the electrode:

• Blocks DC and “etching” of skin.

Electroplating

• If the solution in the previous example were silver chloride and the cathode was, say, tin, then Ag+ would react with the free electron at the cathode, and would form elemental silver that would plate onto the electrode.

• The cathode would become silver plated tin. • The cathodic reaction can in some cases be described

as a plating electrode, • Plating is an important process in the jewelry industry of

Providence. (Technics) • What happens to the tin (Sn) atom that gave up the

electron? It goes into solution and is replaced by the silver atom.

Electroplating detail • Reading: Philip H. Rieger, late of the Chem Dept at Brown, wrote

Electrochemistry, 2nd Edition, Chapman-Hall (1993), DD553.R3. On p. 398 he says, "...to plate copper on a steel substrate the object to be plated is made the cathode in an electrolysis cell where a piece of copper is used as the anode. There are many subtleties, however, which must be considered in practice. Thus, in plating copper on steel, for example, we immediately recognize that the reaction Cu2+ (aq) + Fe(s) --> Cu(s) + Fe2+(aq)is spontaneous.

• To prevent the dissolution of iron from the substrate, the activity of Cu2+(aq) must be reduced by the addition of a complexing agent which coordinates strongly to Cu(II) but much less so than Fe(II)... In electroplating applications, it is usually desirable to deposit a layer of uniform thickness. This requirement is not difficult to meet if the substrate has a simple geometry. If there are holes or recesses, however, a uniform layer can be quite difficult to achieve."

Etching• If the electrolyte were NaOH (sodium hydroxide), and the anode were a thin

tungsten (W) filament, and the other electrode were a "carbon reference" (like the center pole in a D battery) then the anode reaction would etch W atoms off into the solution.

• Demo: The output of a Variac connected to tungsten and reference electrodes, and set at about 10v RMS, can etch the tungsten wire (125 &mu; diam) to a sharp smooth point, so fine it can't be resolved under a light microscope (< 1 &mu; ). Both phases of the AC cycle can etch:

• W --> W+ e- --> and e- + Na+ --> Na (Na combines with H20 then, and generates H2 gas...)

• W + Cl- --> WCl + e- • such etched electrodes, dipped in epoxy to limit exposed metal to a 20

micron length, are useful for single unit recording from CNS neurons. At 500 Hz such electrodes will have about 500K ohms of impedance.

• Other versions of such etching reactions can be thought of as corrosion, and are responsible for the degradation of buried metal pipes in wet soil.

Ag/AgCl electrodes

• Electrochemical reactions with pure silver are electrically noisy…

• Solution? The Ag-AgCl electrode. • The AgCl coating is a good buffer. The elemental silver at

the surface changes to a AgCl film, which may ionize partially to Ag+ and Cl- in solution.

• With AgCl coating on the cathode the reaction changes to: (B&V vol 3, p 79)e- + AgCl > Ag + Cl-

• Elemental Ag is removed from one electrode and silver chloride powder ends up coating the other electrode.

Practical tips for electrodes attached to humans

• Reading: Geddes' Book (R856/G43/1975) pages 198-215Webster book. page 280

• Grass Instruments: gold-plated electrodes (Lab A16) Au / AuCl

• Avoid solder (lead Pb), and dissimilar metal contacts

• Use good surgical tape and conductive cream

• Do not flex wires at contact; establish strain relief.

• Use AC coupling and an isolation amplifier for patient safety

• Abraid the skin with sandpaper to remove a layer of epidermis and lower impedance.

• Smaller electrode size means better spatial localization. Ground electrode can be a large “plate” (back, leg)

Potentiostat:“The system functions by maintaining the potential of the working electrode at a constant level with respect to the reference electrode by adjusting the current at an auxiliary electrode. It consists of an electric circuit which is usually described in terms of simple op amps.” Wikipedia

Potentiostat: monitor ionic concentrations: 3 op amp ckt:

the UGVF provide feedbackto the Control Amp via the reference electrode

At the working electrode areaction takes place thatcreates charges which drivethe bottom output amp

Counter, Reference and Working are electrodes hosting reactions resultingin charge particle currents

Example: monitoring Au+ conc in plating bath