Module 9: Electrochemistry Flashcards
Cation
pos charged ion
Anion
neg charged ion
Basic 4 parts of electrochemical cells
electrolyte solution
2 electrodes
meter or power supply
LEO says GER
Lose of e- = oxidation at ANODE (metal ions enter solution, leaving electrons on the electrode)
Electrons travel through external connection to cathode (passing a meter)
Gain of e- = Reduction at CATHODE (metal ions come out of solution to be deposited on the electrode as solid metal)
how to rebalance a solution after redox reaction
anions must travel through salt bridge (porous plug) that connects the two solutions
If the salt bridge becomes clogged,
anions could not move through to balance the charge differences
Flow of electrons between 2 electrodes would stop
Nernst Equation
calculates potential voltage when 2 half-cells are joined
Potential is directly proportional to log of ionic activity in the solution
“0.0591/n” in equation is the SLOPE FACTOR and can be used as an indicator of the sensitivity of the electrode
2 different potential generating principles
Metallic (electrode generated potential; reference electrodes) Membrane potential (indicator electrode)
Metallic (electrode generated potential)
Reference electrodes are this type
When a metal is placed into solution of its own ions, a potential is produced that is related to the concentration of ions in solution
Low concentration = metal ions pass into solution, leaving neg charge on metal electrode
High concentration = fewer metal ions pass into solution; neg charge on metal electrode will be smaller (very little dissociation occurs therefore good as reference electrode)
Membrane Potential
2 solutions of different ionic activity (concentration) are separated by thin membrane
Oppositely charged ions will be attracted across the membrane
Potential is generated by the difference in charges in each solution
++++++++
xxxxxxxxx membrane
- - - - - - -
Which principles are employed in indicator electrodes
BOTH metallic and membrane potential
Potentiometric measurement systems consists of (4)
reference electrode
indicator electrode
meter
standards (buffers) used for calibration
Reference electrode
Provides constant and stable potential to compare the varying potential of indicator electrode to
HIGH concentration of electrolyte solution
Common reference electrodes
**Saturated calomel electrode: mercury covered in a thin coating of calomel immersed in solution of KCl
Silver/Silver chloride electrode: silver covered in a thin coating of silver chloride immersed in solution of KCl
Indicator electrode
responds to activity of ions in an unknown sample
LOW concentration of electrolyte solution (allows detectable change in potential)
Membrane electrode
Commonly designed to measure H+ in a solution
Operation of pH indicator electrode
Dip membrane into sample
H+ from sample collects on outside of membrane
Attracts negative ions to inside of membrane
Developed membrane potential
No negative ions in solution; salt moves into solution to balance charge
Metal ions left over
Meter
voltmeter
measure difference between reference and indicator electrode
displays as ionic concentration
3 parts: Amplifier, operating adjustments, display
Standards (buffers) for calibration
solution of known concentration
used to make cal curve
As temp increases, dissociation
increases
Ion selective electrodes (ISE)
membrane electrode capable of responding with great sensitivity to one kind of ion while having lower sensitivity to others
sensitivity is determined by type of membrane used
Glass membrane
Solid-state membrane
Liquid ion exchange membrane
Immobilized Enzyme
Gas-sensing
Disposable ISE (vitros potentiometric slides)
Glass membrane ISE
sensitivity depends on the formation of glass
Used for: H+, Na+, Li+, NH4+
Solid-state membrane ISE
no internal electrolyte solution
Used for : sweat chloride
Liquid Ion Exchange Membrane ISE
porous disc
consists of an inert solvent in which an ion selective carrier is embedded
One side of disc = internal reference solution with known concentration of ion
Other side of disc = unknown test solution
Used for : K+
Liquid ion exchange membrane ISE: membrane selective for K+
are made by dissolving the antibiotic valinomycin in a solvent
Immobilized Enzyme Electrodes
Combines specific enzyme bound to some form of inert support with an indicator system that responds to a by-product of the enzyme substrate reaction
Used for: urea, glucose, creat, ethanol, lactate
Gas-sensing electrodes
addition of gar-permeable membrane
Specificity is derived from the selective permeability of the membrane
Analyte gas passes through membrane into internal solution when it is sensed by indicator electrode
Common gas sensing electrode
CO2 electrode
Selectivity is provided by a gas permeable silicone rubber membrane
Disposable ISEs
AKA Vitros Potentiometric Slides
Used mainly in “dry chemistry”
Use 2 indicator electrodes (concentration cells)
One receives reference solution and other receives unknown
The difference is measured
Voltammetry (polarography)
refers to methods that measure current through a cell where voltage is controlled
Current produced is related to the concentration of the reactive ions to be measured
Amperometry
Measurement of the current flowing that is produced by a redox reaction
Common Amperometry cell
O2 electrode with platinum wire immersed in a buffer
Buffer is separated from sample by gas permeable membrane (often polypropylene)
Electrons consumed during reaction are drawn from cathode which causes more current flow to cathode to replace
Increased flow is proportional to the O2 in the sample
Can also be used to measure glucose (enzyme added as reagent)
O2 electrode can detect the consumption of O2
Conductivity
the greater the # of ions present in a solution, the better the ability to conduct current
Conductimetry
measurement of current flow between 2 electrodes immersed in a solution
Used in hematology cell counters
Dilution of whole blood drawn through aperture
1 electrode on either side of the aperture
Current flow between electrodes drops when cell passes through (based on size of cell)
Coulometry
Electrochemical titration
end point is detected by a change in current
Ex: titration of chloride using chloridometer; platinum electrode and silver electrode use to make up the generator electrodes
Applied voltage oxidized Ag electrode and Ag+ is titrated into the solution
Ag reacts with Cl- to form insoluble precipitate of AgCl
When all the Cl- from sample is consumer, the Ag+ appears in excess causing an increase in current (detected by a paid of indicator electrodes)
Direct ISE
undiluted sample
Indirect ISE
measure a diluted sample
Aliquot of sample is taken and diluted with reference solution, then mixture is measured by the system
Interference with Lipemia and high protein sample
Indirect only
Small diluted aliquot taken for measurement may have fat glob in it, making results falsely low
Interference from similar ions
membranes are selective not specific
may display some level of interference by other ions of similar structure
Ex. Cl- electrodes can display interference from high levels of bromine
Protein buildup
ISE may become coated with protein buildup from running large numbers of samples
Electrode fatigue
electrode may become fatigued with extended use or large numbers of tests in a row
Can be reconditioned by allowing it to sit in a reference solution
