Module 9: Electrochemistry

Question 1

Cation

Answer 1

pos charged ion

Question 2

Anion

Answer 2

neg charged ion

Question 3

Basic 4 parts of electrochemical cells

Answer 3

electrolyte solution
2 electrodes
meter or power supply

Question 4

LEO says GER

Answer 4

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)

Question 5

how to rebalance a solution after redox reaction

Answer 5

anions must travel through salt bridge (porous plug) that connects the two solutions

Question 6

If the salt bridge becomes clogged,

Answer 6

anions could not move through to balance the charge differences
Flow of electrons between 2 electrodes would stop

Question 7

Nernst Equation

Answer 7

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

Question 8

2 different potential generating principles

Answer 8

Metallic (electrode generated potential; reference electrodes)
Membrane potential (indicator electrode)

Question 9

Metallic (electrode generated potential)

Answer 9

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)

Question 10

Membrane Potential

Answer 10

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

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Question 11

Which principles are employed in indicator electrodes

Answer 11

BOTH metallic and membrane potential

Question 12

Potentiometric measurement systems consists of (4)

Answer 12

reference electrode
indicator electrode
meter
standards (buffers) used for calibration

Question 13

Reference electrode

Answer 13

Provides constant and stable potential to compare the varying potential of indicator electrode to
HIGH concentration of electrolyte solution

Question 14

Common reference electrodes

Answer 14

**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

Question 15

Indicator electrode

Answer 15

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

Question 16

Operation of pH indicator electrode

Answer 16

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

Question 17

Meter

Answer 17

voltmeter
measure difference between reference and indicator electrode
displays as ionic concentration
3 parts: Amplifier, operating adjustments, display

Question 18

Standards (buffers) for calibration

Answer 18

solution of known concentration

used to make cal curve

Question 19

As temp increases, dissociation

Answer 19

increases

Question 20

Ion selective electrodes (ISE)

Answer 20

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)

Question 21

Glass membrane ISE

Answer 21

sensitivity depends on the formation of glass

Used for: H+, Na+, Li+, NH4+

Question 22

Solid-state membrane ISE

Answer 22

no internal electrolyte solution

Used for : sweat chloride

Question 23

Liquid Ion Exchange Membrane ISE

Answer 23

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+

Question 24

Liquid ion exchange membrane ISE: membrane selective for K+

Answer 24

are made by dissolving the antibiotic valinomycin in a solvent

Question 25

Immobilized Enzyme Electrodes

Answer 25

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

Question 26

Gas-sensing electrodes

Answer 26

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

Question 27

Common gas sensing electrode

Answer 27

CO2 electrode

Selectivity is provided by a gas permeable silicone rubber membrane

Question 28

Disposable ISEs

Answer 28

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

Question 29

Voltammetry (polarography)

Answer 29

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

Question 30

Amperometry

Answer 30

Measurement of the current flowing that is produced by a redox reaction

Question 31

Common Amperometry cell

Answer 31

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

Question 32

Conductivity

Answer 32

the greater the # of ions present in a solution, the better the ability to conduct current

Question 33

Conductimetry

Answer 33

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)

Question 34

Coulometry

Answer 34

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)

Question 35

Direct ISE

Answer 35

undiluted sample

Question 36

Indirect ISE

Answer 36

measure a diluted sample

Aliquot of sample is taken and diluted with reference solution, then mixture is measured by the system

Question 37

Interference with Lipemia and high protein sample

Answer 37

Indirect only

Small diluted aliquot taken for measurement may have fat glob in it, making results falsely low

Question 38

Interference from similar ions

Answer 38

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

Question 39

Protein buildup

Answer 39

ISE may become coated with protein buildup from running large numbers of samples

Question 40

Electrode fatigue

Answer 40

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