electrodes.pptx

7/28/2019 ELECTRODES.pptx

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Electrodes

Electrodes play an important role in makingsatisfactory records of bioelectric signals and theirchoice requires careful consideration.

They should be comfortable for the patients to wearover long periods of time and should not produce anyartefacts.

Another desirable factor is the convenience ofapplication of the electrodes.


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The mechanism of electric conductivity in the bodyinvolves ions as charge carriers.

Thus, picking up bioelectric signals involvesinteracting with these ionic charge carriers andtransducing ionic currents into electric currentsrequired by wires and electronic instrumentation.


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electrode-electrolyte interface

This transducing function is carried out byelectrodesthat consist of electrical conductors in contact with theaqueous ionic solutions of the body.

This introduces the electrode-electrolyte interfaceprocess, electrode characteristics and different types of

electrodes to be used for measuring ECG,EMG,EEGand other bio potentials.


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The Electrode-Electrolyte Interface

When electrodes in their simplest form made of piecesof metal, are placed on or inside the body, they comein contact with body fluids which may be consideredas ELECTROLYTES.

Due to this contact between a metal and electrolytesolution, an electrochemical reaction produces adifference ofpotential between the metal andsolution.

The chemical reactions that occur between metals andelectrolytes influence the performance of Biopotentialelectrodes.


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The electrolyte represents the body fluidcontaining ions.

A net current that crosses the interface, passing fromthe electrode to the electrolyte, consists of

(1) electrons moving in a direction opposite to that ofthe current in the electrode,

(2) cations (denoted by C+ ) moving in the samedirection as the current, and

(3) anions (denoted by A-) moving in a directionopposite to that of the current in the electrolyte.


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Electrodeelectrolyte interface

The electrode consists of metallic atoms C.The electrolyte is an aqueous solution containing cations of the electrodemetal and anions.


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For charge to cross the interfacethere are no freeelectrons in the electrolyte and no free cations oranions in the electrodesomething must occur at theinterface that transfers the charge between these

carriers.What actually occur are chemical reactions at the

interface, which can be represented in general by thefollowing reactions:

where n is the valence of C and m is the valence of A


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The figure shows a silver electrode in an electrolyte(salt) solution.

Since it is a good conductor of electricity, has anabundance loosely held free valance electrons, some ofthese enter the solution, making the neutral

electrode positively charged with respect to theelectrolyte.


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Half-cell Potential

This results in a potential difference called a half-cellpotential.

The value of this half-cell voltage for a silver electrodeis approximately0.8 Vand for copper electrode it isapproximately0.3 V.This relationship is known as the Nernst equation

Where a 1 and a 2 are the activities of the ions on either side of the membrane, R isthe universal gas constant, Tis the absolute temperature, n is the valence of the ions,and Fis the Faraday constant.


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The following figure shows both the electrodesplaced in the electrolyte and, since the half-cellpotentials for the silver and copper electrodes are0.8&0.3V respectively, both the electrodes beingpositive with respect to the electrolyte, the PDbetween the electrodes equals 0.5V.


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Silver-copper electrodes Half-cell Potentials

electrolyte

0.5 v

CopperSilver

+ ve+ ve


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Electrical Double Layer

Some sort of separation of charges exist at the metal-electrolyte interface which results in an ElectricalDouble Layer, wherein one type of charge is dominanton the surface of the metal and the opposite chargeis distributed in excess in the electrolyte lyingadjacent to the electrode.


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Oxidation or reduction reactions at the electrode-electrolyte interface lead

to a double-charge layer


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Half-cell Potential

The knowledge of the half-cell potential, which isdetermined by the

1) Metals involved, 2) The concentration of its ions in solution and

3) The temperature

Half cell potential cannot be measured without asecond electrode.

The half cell potential of the standard hydrogen

electrode has been arbitrarily set to zero.


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Half-cell Potentials for Common Electrode

Materials at 250C

The metal undergoing the reaction shown has the sign and potential E0

when referenced to the hydrogen electrode


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Electrode offset potentialwith silver electrodes

applied on to the skin surface

The cross section of two silver plates used asbiopotential electrodes on the surface of the skin,which act as an electrolyte.

The metals will develop equal half cell potentials; ifchemically identical, the resulting PD betweenelectrodes will equal to Zero.

The difference of potential between the terminals ofbiopotential electrodes in contact with the body iscalled the electrode offset voltage.


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The offset would be zero if the electrodes are

chemically identical. However, in practice this is not so and some value of

the offset voltage is usually present between theterminals of the applied biopotential electrodes.

This offset voltage will thus also be amplified alongwith the physical variable picked up from the body.

If an ECG is being recorded, which is in the range of1mV, the 0.8 V half-cell potential is approximately 1000times greater.


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POLARIZABLE AND NONPOLARIZABLE

ELECTRODES

Theoretically, two types of electrodes are possible:those that are perfectly polarizable and those that areperfectly nonpolarizable.

This classification refers to what happens to an

electrode when a current passes between it and theelectrolyte.

Perfectly polarizable electrodes are those in which noactual charge crosses the electrodeelectrolyteinterface when a current is applied.

Of course, there has to be current across the interface, but this currentis a displacement current, and the electrode behaves as though it were acapacitor.


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Perfectly nonpolarizable electrodes are those in whichcurrent passes freely across the electrodeelectrolyteinterface, requiring no energy to make the transition.

Thus, for perfectly nonpolarizable electrodes there areno overpotentials.

Electrodes made ofnoble metals such as platinum

come closest to behaving as perfectlypolarizableelectrodes.


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Because the materials of these electrodes are relativelyinert, it is difficult for them to oxidize and dissolve.

Thus current passing between the electrode and the

electrolyte changes the concentration primarily of ionsat the interface, so a majority of the overpotential seenfrom this type of electrode is a result of Vc, theconcentration overpotential.

The electrical characteristics of such an electrode showa strong capacitive effect.


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Ag-AgCl electrode: perfectly nonpolarizable electrode

A large offset voltage due to chemically unmatchedelectrodes, may thus interfere with the desiredbiosignals and result in undesired artefacts.

Both the half-cell potential and noise from theelectrodes, due to chemical activity taking place with

in them, may be reduced by a proper choice ofelectrode material and bychloriding the silverelectrode-called the Silver-Silver-Chloride (Ag-AgCl)electrode.


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The coating of chloride is done chemically by placing

two pieces if pure silver (one of them the electrode) ina bromide-free sodium chloride solution (of %5concentration) and connecting the two pieces to a DCvoltage, the positive being connected to the

electrode to be chlorided.

Chloride ions from the salt solution then combine withthe silver ions of the electrode and a thin film ofchloride molecules is deposited on the silver electrodemaking it look grey.

Chloriding is best done in a dark room for a duration of about three minuteswith a 3 volt battery.


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The second process for producing Ag/AgCl electrodesuseful in medical instrumentation is a sinteringprocess that forms pellet electrodes.


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sintering Process

The electrode consists of an Ag lead wire surroundedby a sintered Ag/AgCl cylinder. It is formed by placingthe cleaned lead wire in a die that is then filled with amixture of powdered Ag and AgCl.

The die is compressed in an arbor press to form thepowdered components into a pellet, which is thenremoved from the die and baked at 400 0C for severalhours.

These electrodes tend to have a greater endurancethan the electrolytically deposited AgCl electrodes,and they are best applied when repeated usage isnecessary.


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The electrolytically deposited AgCl has a tendencyto flake off under mechanical stress, leavingportions of metallic Ag in contact with the electrolyte,

which can cause the electrodes half-cellpotentialto be unstable and noisy.


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calomel electrode

A second kind of electrode that has characteristicsapproaching those of the perfectly nonpolarizableelectrode is the calomel electrode.

It is used primarilyas a reference electrode forelectrochemical determinations and is frequently appliedas the reference electrodewhen pH is measured.

The calomel electrode is often constructed as a glass tube

with a porous glass plug at its base filled with a paste ofmercurous chloride or calomel (Hg2Cl2) mixedwith asaturated potassium chloride (KCl) solution.


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Like AgCl, the Hg2Cl2 is only slightly soluble in water,so most of it retains its solid form.

A layer of elemental mercury is placedon top of thepaste layer with an electric lead wire within it.

This entire assembly is then positioned in the center of alarger glass tubewith a porous glass plug at its base.

The tube is filled with a saturated KCl solution so thatthe Hg2Cl2 layer of the inner tube is in contact with thiselectrolyte through the porous plug of the inner tube.


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We have a half-cell made up of Hg in intimatecontact with an Hg2Cl2 layerthat is in contact withthe saturated KCl electrolyte.

The porous plug at the bottom of the electrodeassembly is used to make contact between the internalKCl solution and the solution in which the electrode isimmersed.

This is actually a liquidliquid junction that can resultin a liquidliquid junction potential, which will add tothe electrode half-cell potential.


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ELECTRODE BEHAVIOR AND CIRCUIT MODELS

The electrical characteristics of electrodes have beenthe subject of much study.

Often the currentvoltage characteristics of the

electrodeelectrolyte interface are found to be nonlinear,and, in turn, nonlinear elements are required for modelingelectrode behavior.

Specifically, the characteristics of an electrode are sensitiveto the current passing through the electrode, and theelectrode characteristics at relatively high current densitiescan be considerably different from those at low currentdensities.


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The characteristics of electrodes are alsowaveformdependent.

When sinusoidal currents are used to measure theelectrodes circuit behavior, the characteristics are also

frequency dependent.


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For sinusoidal inputs, the terminal characteristics ofan electrode have both a resistive and a reactivecomponent.

Over all but the lowest frequencies, this situation canbe modeled as a series resistance and capacitance.

The half-cell potential described earlier was the result of thedistribution of ionic charge at the electrodeelectrolyte interface thathad been considered a double layer of charge.

This, of course, should behave as a capacitorhence the capacitivereactance seen for real electrodes.


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CIRCUIT MODELS

The series resistancecapacitance equivalent circuitbreaks down at the lower frequencies, where this modelwould suggest an impedance going to infinity as

the frequency approaches dc.

To avoid this problem, we can convert this series RC

circuit to a parallel RC circuit that has a purelyresistive impedance at very low frequencies.


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biopotential electrode equivalent circuit

If we combine this circuit with a voltage sourcerepresenting the half-cell potential and a seriesresistance representing the interface effects andresistance of the electrolyte, we can arrive at the

biopotential electrode equivalent circuit model shown


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In this circuit, Rd and Cd represent the resistive andreactive components just discussed.

These components are stillfrequency and current-

density dependent.

In this configuration it is also possible to assignphysical meaning to the components.

Cdrepresents the capacitance across the double layerof charge at the electrodeelectrolyte interface.


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The parallel resistance Rdrepresents the leakageresistance across this double layer.

All the components of this equivalent circuit havevalues determined by the electrode material and itsgeometry, andto a lesser extentby the material of

the electrolyte and its concentration.


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The equivalent circuit of Figure demonstrates that the

electrode impedance is frequency dependent.At high frequencies, where 1=wCRd, the impedance

is again constant but its value is larger, being Rs + Rd.At frequencies between these extremes, the

electrode impedance is frequency dependent.

The impedance of Ag/AgCl electrodes variessignificantly from that of a pure silver electrode atfrequencies under 100 Hz.


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Impedance as a function of frequency for Ag electrodes coatedwith an electrolytically deposited AgCl layer.The electrode area is 0.25 cm2.Numbers attached to curves indicate number of mAs for each deposit.

electrodes.pptx

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