360 - Electrochemistry & Electrolytes Flashcards
the proper term for the concentration of an electrolyte measured in an electrochemical cell, used
in the Nernst equation
activity
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
activity coefficient
what is potentiometry?
an electrochemical technique that measures the electric potential (E) between two
electrodes under equilibrium conditions
describe potentiometric electrodes
- 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)
two common reference electrodes
saturated calomel electrode
silver/silver chloride electrode
describe the Ag/AgCl electrode
it is housed inside an impermeable case (glass or plastic); consists of silver wire coated in AgCl and immersed in a chloride solution
what is the main difference in construction between a reference electrode and an indicator electrode?
the presence of an ion-selective membrane
describe the Nernst equation
- logarithmic relationship between Ecell and activity of an electrolyte
- temperature dependent
chemical composition of membranes of ion-selective electrode
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
this equation describes the selectivity of an ion selective electrode over interfering ions.
the Nicolsky-Eisenman equation
the four classes of ion-selective membranes
- glass (silicon dioxide)
- liquid/polymer
- solid state
- gas sensing
these types of electrodes are commonly used to measure H+ and Na+
glass membranes
glass electrodes commonly composed of
SiO2, Na2O, CaO, or Al2O3
- composition of the glass determines the selectivity of the membrane
liquid membranes
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)
T or F. Ion exchangers are typically lipophilic molecules
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
molecules that reversibly bind ions
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
this is a natural ionophore that binds potassium
valinomycin
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
crystalline (solid state) membranes
- membrane potential is created by the movement of ions in the sample solution into vacancies in the
crystal lattice
what is a gas permeable membrane?
a thin outer membrane that is permeable to the gas of interest and an internal pH electrode, i.e. urea, carbon dioxide
describe the carbon electrode in gas permeable membranes
CO2 diffuses through the membrane and interacts w a weak bicarbonate solution to form carbonic acid
the dissociated H+ interacts w internal electrode
describe the urea electrode in gas permeable membranes
the equilibrium between ammonia and
ammonium is sensed by the pH meter
ISE limitations/interferences:
- 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
ISEs determine the electrolyte activity
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
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
T
apparent decrease is due to
the increase in the fractional water content
what is the pH ISE composed of?
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
T or F. pH ISE measurements are temperature dependent
T! if temperature increases, pH decreases; sample exposure to atmospheric air will decrease the CO2
concentration and increase pH
sodium ISE
Sodium selective electrodes can have membranes made out of glass or PVC with crown ether: ETH157
and ETH227
glass membrane most common
specimen used for sodium ISE
Serum or heparin plasma (lithium or ammonium)
interference with sodium ISE
- Hemolysis and icterus do not cause significant interference
- Hyperlipidemia can cause a false decrease in sodium when an indirect ISE is used
- Hyperproteinemia
Potassium-selective electrode membranes are commonly made out of
PVC and valinomycin
specimen for potassium ISE
- 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
interference with potassium ISE
- 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+
Chloride selective membranes are…
polymer based and incorporate quaternary ammonium salt anion exchanges such as propyl ammonium chloride
specimen for chloride ISE
Serum or lithium heparin plasma
chloride ISE interferences
- 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
