Module 9 Flashcards
(37 cards)
What are electrodes?
Half cells
What occurs at the location of cathodes and anodes?
Cathode- reduction, positive charge
Anode- oxidation, negative charge
What happens in an electrochemical cell?
At the anode oxidation occurs, metal ions enter the solution. Electrons are left on the electrode.
Electrons travel to the cathode, reduction takes place and ions are deposited on the electrode.
Solution is unbalanced so anions travel through the salt bridge.
What is the net reaction in an electrochemical cell?
The sum of the two half cell reactions.
How is formed potential measured?
As voltage by a voltmeter.
Current (actual flow of electrons) is measured by an ammeter.
What does the Nernst equation calculate?
Potential voltage.
One half cell can be maintained at a constant potential, the potential of the other can be used to determine the ionic activity of the solution.
What does potentiometry measure?
The concentration of electrolytes in clinical samples.
What is metallic potentiometry?
Metal is placed in solution of its own ions and produces a potential related to concentration.
How does solution concentration used for potentiometry affect measurement?
Low- metal ions pass into solution, negative charge on electrode
High- fewer ions in solution, smaller negative charge
Very high- little dissociation, little potential generated, used for reference electrode
How does membrane potential potentiometry work?
Two solutions of different ionic concentration are separated by a membrane.
Potential is generated by difference in charges.
What are the components of the potentiometric measurement system?
Reference electrode- constant potential (high electrolyte concentration)
Indicator electrode- responds to ion activity in the sample (low concentration of electrolytes)
Meter- voltmeter measures the difference in potential
Standards for calibration- solutions of known concentrations used to calibrate the potentiometer
What types of reference electrodes are there?
Saturated calomel electrode- Hg coated with calomel (Hg2Cl2) immersed in saturated KCl
Ag/AgCl- Ag coated with AgCl immersed in saturated KCl
What types of indicator electrodes are there?
Membrane- pH and ion analysis
pH- Ag/AgCl salt electrode immersed in dilute HCl
How do potentiometers work?
Insert electrode into sample, H displaces Na.
H build up attracts Cl from dilute solution.
Change in Cl near electrode, more Cl dissociates.
More Ag remain, produces a change in potential proportional to the H in sample.
What do the calibrate and slope functions on electrodes do?
Calibrate- sets response to a certain standard concentration
Slope- sets the gain of the amplifier for a given change in concentration
How does temperature affect dissociation?
As temperature increases so does dissociation.
What are ion selective electrodes?
Capable of responding with great sensitivity to one kind of ion, exhibiting lower sensitivities to others.
What types of ISEs are there?
Glass membrane- selectivity depends on the glass composition
Solid state membrane- crystalline or solid membrane and lack an internal electrolyte solution, internal element in direct contact with inner surface of the membrane, used for sweat chlorides
How do liquid ion exchange membrane ISEs work?
A porous disc with an ionophore embedded in an inert solution (PVC).
Membrane holds the solvent.
Ionophore has a high affinity for the ion to be measured.
One side is the internal reference electrode and the other is the test solution.
Dissociation equilibrium results in potential.
How are membranes made selective for K?
Dissolving valinomycin
How do immobilized enzyme electrodes work?
An enzyme bound to inert support is combined with an indicator system.
Enzyme catalyzes a reaction involving the analyte, produces a charged product that interacts with the membrane and changes the potential.
How do gas-sensing electrodes work?
A gas permeable membrane and buffer/electrolyte system measures a sample in a mix of gases or solution with the analyte gas dissolved.
Gas passes through the membrane into the internal solution where it’s sensed by indicator electrodes.
How does a CO2 electrode work?
Silicon rubber membrane
Gas passes into the bicarbonate buffer.
Gas is converted to bicarbonate and H.
H is measured with a pH electrode.
How do vitros slides work?
Disposable ISEs, dry chemical instruments.
One electrode gets reference solution the other gets the sample.
Creates a difference in potential.
