360 - Electrochemistry & Electrolytes

Question 1

the proper term for the concentration of an electrolyte measured in an electrochemical cell, used
in the Nernst equation

Answer 1

activity

Question 2

the activity of an electrolyte divided by molar concentration. It is a measurement of
the interaction of the selected electrolytes with other species in the solution

Answer 2

activity coefficient

Question 3

what is potentiometry?

Answer 3

an electrochemical technique that measures the electric potential (E) between two
electrodes under equilibrium conditions

Question 4

describe potentiometric electrodes

Answer 4

• two electrodes: reference + indicator (measuring/sensing)
• reference is stable; constant potential relative to sample solution (Eref)
• reference electrode has a junction (frit) to allow electrical, ionic conductivity
  between the sample solution and the internal filling solution while preventing large-scale convective
  mixing of the solutions
• small constant electric potential across junction (Ejxn)

Question 5

two common reference electrodes

Answer 5

saturated calomel electrode
silver/silver chloride electrode

Question 6

describe the Ag/AgCl electrode

Answer 6

it is housed inside an impermeable case (glass or plastic); consists of silver wire coated in AgCl and immersed in a chloride solution

Question 7

what is the main difference in construction between a reference electrode and an indicator electrode?

Answer 7

the presence of an ion-selective membrane

Question 8

describe the Nernst equation

Answer 8

• logarithmic relationship between Ecell and activity of an electrolyte
• temperature dependent

Question 9

chemical composition of membranes of ion-selective electrode

Answer 9

must preferentially interact with
the selected ion to allow a potential difference to be created across the membrane when there is a
difference in ion activity on either side of the membrane

Question 10

this equation describes the selectivity of an ion selective electrode over interfering ions.

Answer 10

the Nicolsky-Eisenman equation

Question 11

the four classes of ion-selective membranes

Answer 11

• glass (silicon dioxide)
• liquid/polymer
• solid state
• gas sensing

Question 12

these types of electrodes are commonly used to measure H+ and Na+

Answer 12

glass membranes

Question 13

glass electrodes commonly composed of

Answer 13

SiO2, Na2O, CaO, or Al2O3
- composition of the glass determines the selectivity of the membrane

Question 14

liquid membranes

Answer 14

composed of an ion exchanger or ionophore dissolved in a viscous, water-insoluble
solvent

polymer membranes = more common; e immobilized ion exchangers or ionophores in polyvinyl chloride (PVC)

Question 15

T or F. Ion exchangers are typically lipophilic molecules

Answer 15

T!
When contact is made with the sample solution, the ion of interest establishes an equilibrium between the membrane phase and the sample solution phase

Question 16

molecules that reversibly bind ions

Answer 16

neutral ion carrier (ionophore)
Neutral carriers react with ions at the membrane
surface and bind them while counter-ions remains in solution
this interaction => electric potential

Question 17

this is a natural ionophore that binds potassium

Answer 17

valinomycin

Question 18

these membranes are composed of a single type of crystal or pressed pellets of salts of the ion of
interest which has some electrical conductivity, e.g. chloride electrode composed of silver chloride

Answer 18

crystalline (solid state) membranes
- membrane potential is created by the movement of ions in the sample solution into vacancies in the
crystal lattice

Question 19

what is a gas permeable membrane?

Answer 19

a thin outer membrane that is permeable to the gas of interest and an internal pH electrode, i.e. urea, carbon dioxide

Question 20

describe the carbon electrode in gas permeable membranes

Answer 20

CO2 diffuses through the membrane and interacts w a weak bicarbonate solution to form carbonic acid
the dissociated H+ interacts w internal electrode

Question 21

describe the urea electrode in gas permeable membranes

Answer 21

the equilibrium between ammonia and
ammonium is sensed by the pH meter

Question 22

ISE limitations/interferences:

Answer 22

• Temperature – The Nernst equation is temperature dependent
• Ionic strength – The total ionic strength of a sample affects the activity coefficient of the ion
• pH - Some ions require conversion of the analyte to one form, i.e. calcium
• Biofouling –Protein buildup on the membrane
• Cross-reacting ions – Ion not usually found in the sample react with the membrane
• Electrolyte exclusion effect

Question 23

ISEs determine the electrolyte activity

Answer 23

the number of atoms that acts as true ions in a defined volume of water
- indirect ISEs dilute patient samples with an aqueous solution; the measured values are
multiplied by a conversion factor which accounts for the dilution and the normal composition of the
sample (~ 92% water and ~ 8 % solids) to determine the actual amount of electrolytes in the undiluted
sample

Question 24

T or F. the electrolytes exclusion effect states that the sodium concentration will be falsely low in samples with an increased proportion of solids such as hyperlipidemia or hyperproteinemia

Answer 24

T
apparent decrease is due to
the increase in the fractional water content

Question 25

what is the pH ISE composed of?

Answer 25

glass
- sodium ion mobility within the glass makes the membrane slightly receptive to an electrical charge.
- ion-exchange between sodium and hydrogen occurs within the outer hydrated layer of the glass membrane
- alters the potential of the electrode; the change in
potential is correlated to the hydrogen ion activity

Question 26

T or F. pH ISE measurements are temperature dependent

Answer 26

T! if temperature increases, pH decreases; sample exposure to atmospheric air will decrease the CO2
concentration and increase pH

Question 27

sodium ISE

Answer 27

Sodium selective electrodes can have membranes made out of glass or PVC with crown ether: ETH157
and ETH227

glass membrane most common

Question 28

specimen used for sodium ISE

Answer 28

Serum or heparin plasma (lithium or ammonium)

Question 29

interference with sodium ISE

Answer 29

• Hemolysis and icterus do not cause significant interference
• Hyperlipidemia can cause a false decrease in sodium when an indirect ISE is used
• Hyperproteinemia

Question 30

Potassium-selective electrode membranes are commonly made out of

Answer 30

PVC and valinomycin

Question 31

specimen for potassium ISE

Answer 31

• Separated serum or heparin plasma
• At 4C, potassium will increase in unseparated serum or plasma due to leakage from RBCs
• At room temperature or 37C, potassium can decrease due to glycolysis
• Serum samples have significantly higher potassium concentration than plasma samples because
  platelets release potassium during clotting

Question 32

interference with potassium ISE

Answer 32

• DO NOT USE HEMOLYZED SPECIMENS
• Icterus and lipemia do not cause significant interference
• Incorrect tourniquet usage may falsely increase K+
• Excessive fist clenching or forearm exercise before venipuncture may falsely increase K+
• Leukocytosis may falsely increase K+ if the specimen is not immediately separated
• Thrombocytosis may falsely increase K+

Question 33

Chloride selective membranes are…

Answer 33

polymer based and incorporate quaternary ammonium salt anion exchanges such as propyl ammonium chloride

Question 34

specimen for chloride ISE

Answer 34

Serum or lithium heparin plasma

Question 35

chloride ISE interferences

Answer 35

• Hemolysis, icterus and lipemia do not cause significant interference
• Chloride ISE may lack selectivity in the presence of other halides and organic ions such as
  thiocyanate and lactate