Transcript Electrochemistry

MLAB 2401: CLINICAL
CHEMISTRY
KERI BROPHY-MARTINEZ
Designs in Instrumentation
Electrochemistry
ELECTROCHEMISTRY

Basic principle
 Electrodes are used to selectively measure
particular ions
 Instruments utilizing electrodes measure the
potential difference (charge) that builds up at
an interface when two different concentrations
of the same ion are in contact with each other
 The electrodes are sensitive to this potential
difference
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ELECTROCHEMISTRY: TERMS

Oxidation
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
Anode
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
gain of electrons (electrode becomes negative)
Cathode


the electrode where oxidation occurs (where electrons
are lost)
Reduction


loss of electrons (leaves electrode positive)
the electrode where reduction occurs (where the
electron gain occurs)
Polarography

the measurement of the gain or loss of electrons in a
chemical reaction by detecting a change in the potential
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ELECTROCHEMISTRY

Galvanic cell – two half cells used to demonstrate
the flow of electrons during the processes of
oxidation and reduction
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oxidation
Galvanic c ell
reduction
ELECTROCHEMISTRY

Electrolytic cell differs from the galvanic cell in
that
The cathode and anode are immersed in the same
solution
 There is an electromotive force used to drive the
chemical reaction

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ELECTROCHEMISTRY

Testing Procedures involving electrochemical
principles

Potentiometry
pH meters
 Ion-selective electrodes
 Blood gas analysis


Chloridometers
POTENTIOMETRY
Measurement of potential or voltage between two
electrodes in a solution
 Nerst equation
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
Predicts amount of change in millivolts due to a
temperature change
As the temperature rises, so does hydrogen ion
activity
POTENTIOMETRY

System Components
Reference electrode
 Indicator or measuring electrode
 Liquid junction
 Readout device

POTENTIOMETRY

Reference electrodes


The electrode against which the potential charge
created in the indicator electrode is measured.
Consists of a metal and its salt in contact with a
solution containing the same anion. These are half-cell
potentials extremely stable and easy to reproduce
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POTENTIOMETRY
 Reference


Electrodes
Calomel electrode composed of
mercury/mercurous chloride is dependable but
large, bulky, and affected by temperature
Silver/silver chloride reference electrodes
are more compact and handle temperature
fluctuations better - overall better & faster
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POTENTIOMETRY
 Indicator (measuring) electrodes
 The electrode, whose half-cell potential
responds to
changes in the activity or concentration of the
substance in the solution that is being measured.

The response of the electrode to the substance
(species) being measured and not to other substances
present is defined as the selectivity of the electrode.

The selectivity of an electrode for a particular species
is dependent on the type of membrane that
separates the electrode and the solution containing
the species to be measured.
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ELECTROCHEMISTRY

Liquid junction – also known as a salt bridge
are required to complete the circuit (between the
reference and without contaminating anything.

KCl (potassium chloride) is the most common
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POTENTIOMETRY

Ion Selective Electrodes
Also called Ion Specific Electrodes
 Indicator electrode
 Modified electrochemical half cells
 Advantages

No reagent preparation
 No standard curve preparation
 Cost effective
 Fast analysis times
 Very sensitive and selective for an analyte
 Easy to maintain

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ION SELECTIVE ELECTRODES



Different membranes that are selectively sensitive
to the electrical effects of different electrolytes can
be placed over the tips of the measuring electrodes,
making them susceptible only to the effects of
these particular electrolytes
Differences in the measured potential of the circuit
can be calibrated with known concentrations of
electrolytes
Because ISE measures electrical potential, it is an
example of potentiometry
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ION SELECTIVE ELECTRODES
 Common
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substances measured by ISE
Sodium (Na) –
 membrane composed of selective glass
Potassium (K)
 valinomycin ( example of liquid layer membrane)
Chloride (Cl) –
 crystal membrane
Ionized Calcium (Ca)
Hydrogen ions (H)
 glass membrane different composition than Na’s
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ION SELECTIVE ELECTRODES ( ISE )

Consist of two main components to an
electrochemical half cell
Reference Electrode ( generates a known,
constant voltage )
 Measuring Electrode ( generates a variable
voltage )
 These components must be connected together,
but can not contaminate each other. The joining
is done using a “salt bridge”

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POTENTIOMETER MEASUREMENT

pH Electrode
 based on the measurement of a potential
(voltage) difference between two electrodes
immersed in a solution under the condition
of zero current.
 Each cell has a half-cell reaction
 The potential difference between the two
electrodes is usually measured using a
pH/milli-volt meter.
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PH ELECTRODES

Combination electrode

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Internal reference
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
Both the indicator &
reference electrodes are
housed in the same
assembly.
Ag/AgCl type bathed
in KCl
The measuring or
indicator electrode has
a “glass membrane”
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BLOOD GAS ELECTRODES

pCO2 Electrode
Measurement of PCO2 in
routine blood gases is
done using the
Severinghaus electrode
 A modified pH electrode
with a CO2 permeable
membrane covering the
glass membrane surface
 A bicarbonate buffer
separates the membranes
 Change in pH is
proportional to the
concentration of dissolved
CO2 in the blood

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BLOOD GAS ELECTRODES
 pO2
Electrode
 Clark PO2 (polarographic) electrode.
Measurement of PO2 is done using a
Clark electrode which works on the
principles of polarography

principle of polarography- involves
measurement of gain or loss of
electrons in a chemical reaction, by
detecting a change in potential.
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PO2


ELECTRODE
The cathode is a platinum wire and the anode is a
silver wire in AgCl
both electrodes are in contact with electrolyte
solution
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Oxygen in the sample diffuses through the semipermeable
membrane and is reduced (gains electrons) at the cathode.
The flow of electrons stops when no more oxygen is
available.
Current generated in the system is proportional to the
amount of oxygen
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ELECTROCHEMISTRY COULOMETRIC/AMPEROMETRIC
CHLORIDOMETERS
 Coulometry



a titration in which the titrant is
electrochemically generated
Silver (Ag) is the most common titrant.
Amperometry

Measurement of current flow produced by an oxidationreduction reaction
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COULOMETRIC/AMPEROMETRIC
CHLORIDOMETERS

The basic components consist of two pair of
electrodes immersed in an electrolyte diluent
(containing acetic and nitric acids)
E q u iva le n ts A g + g e n e ra t ed =
–
e q u iva le n t C l
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COULOMETRIC/AMPEROMETRIC
CHLORIDOMETERS
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Procedure

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Sample added to
diluent
Electrodes releases
silver ions into the
diluent
Ions combine with
chloride to form a
precipitate
When chloride is gone,
excess silver causes a
change in current flow
Flow change signals
the end of titration
REFERENCES
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
Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical
Chemistry: Techniques, principles, Correlations. Baltimore:
Wolters Kluwer Lippincott Williams & Wilkins.
Sunheimer, R., & Graves, L. (2010). Clinical Laboratory
Chemistry. Upper Saddle River: Pearson .
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