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.
Ag electrodes in salty water
At Anode:
Ag + Cl- --> AgCl + e-
At cathode:
Ag --> Ag+ + e- (etching)
Na+ + e- --> Na
2 Na + 2 H2O --> 2 NaOH + H2 (gas, bubbles)
NaOH --> Na+ + OHThe overall cath reaction, powered by the battery, can be described as:
2Ag + 2Na+ + 2H2O --> 2Ag+ + 2 H2 + 2 Na+ +2 OH-
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
μ 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 + esuch 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 198-215
Webster 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:
Counter, Reference and
Working are electrodes
hosting reactions resulting
in charge particle currents
the UGVF provide feedback
to the Control Amp via the
reference electrode
At the working electrode a
reaction takes place that
creates charges which drive
the bottom output amp
Example: monitoring Au+ conc in plating bath