Unit 2: Electrochemistry

electrochemistry

chemical reaction with the flow/presence of electrons in an electrochemical cell (look at redox reaction)

potential

an electrode’s propensity to lose/gain electrons

reference electrode

known, constant potential

indicator electrode

varying potential due to ion activity; influenced by patient sample

aperometry

measures the current when force is applied

current

flow of electrons

potentiometry

measures the difference in voltage between two electrodes in solution without a current being applied (i.e. reference vs. patient)

coulometry

measures sweat chloride; electricity is proportional to chloride concentration

anode

• ½ cell in an electrochemical cell

• oxidation reaction occurs here

• the reference electrode

• “an ox”

cathode

• ½ cell of an electrochemical cell

• reduction reaction occurs here

• the indicator electrode

• “red cat”

salt bridge

connect the 2 half cells in an electrochemical cell

potentiometer

measures the current in an electrochemical cell

what way does the current flow in an electrochemical cell?

anode to cathode

what are the types of reference electrodes?

• Ag/AgCl (silver chloride)

• calomel = Hg/HgCl2 (mercury)

ion-sensitive electrode

• type of indicator electrode

• detects individual free ions

• ex: pH, Na, K, Cl

gas sensing electrode

• type of indicator electrode

• detects specific gases

• ex: CO2

enzyme electrodes

• type of indicator electrode

• ISE covered by immobilized enzymes

• ex: urease for urea detection

principle of a pH meter

H ions (from patient sample) at the external surface attract Cl- ions from the inside electrode

• Cl leaves AgCl and creates Ag+ ions which creates the potential

principle of CO2 electrode

• pH electrode with a gas permeable membrane

• CO2 diffuses and mixes with the sodium bicarbonate solution

• dissociates and creates H+ ad HCO3-

• CO2 + H2O → H+ and HC3-

• more CO2 = more acidic pH

principle of pO2 electrode

aperometry

• fixed potential applied to the cell at the cathode

• applied potential requires the specific analyte to conduct a current

• current is proportional to pO2

• ex: drug overdose or COPD will decrease the current

what determines the type of analyte being detected?

the membrane type

• the analyte never crosses the membrane

requirements for ISE

• membrane with high sensitivity/selectivity for ion

• ion selective membrane causing separation

• stable reference electrode

advantages for ISE

• quick TAT

• few reagents

• small sample

• disposable electrodes (minimal maintenance)

direct EC measurements

• undiluted

• better and more accurate than indirect

• less reagent required

• unadultered sample (can do further testing)

• can use whole blood

indirect ISE

• diluted sample

• requires larger sample volume

• older

• hyperproteinemia can cause interference

coulometry principles

• diagnosis for cystic fibrosis

• potential is applied and current flows due to redox reaction

• first electrode - generates silver ions and creates the insoluble precipitates

• second electrode - senses free silver ions and stops timer once detected

• amount of time silver ions made = chloride concentration

osmolality

sum of moles of all dissolved ion and undissolved molecules in 1 kg of water

molality

moles of solute/ moles of solvent in kg

• dependent on temp

colligative properties

• osmotic pressure

• boiling pt elevation

• freezing point depression

• vapor pressure depression

prinicple of freezing point depression

the more substances added to a solution = the lower the freezing point

clinical uses for osmometry

• plasma = detect unmeasured substances (increases osmolality)

• in urine = assess renal concentrating ability

interpret osmol gap

compares the measured osmolality to the calculated osmolality

gap should ideally be zero

• gap increases with alcohol or ketones

calculation for osmol gap

1. determine MO

2. CO = (2 x NA mmol/L) + 0.056 (glucose mg/dL) + 0,36 (BUN mg/dL)

3. subtract (MO-CO)

4. units are mOsm/kg