Electrochemistry Flashcards
Gold standard for measurement of chloride in serum/ plasma?
Coulometry
Why do potentiometers require frequent calibration?
Signals from the electrodes change constantly due to, for example, protein build up, worn-out membranes, aging electrodes etc
Protein coating of ion-sensitive membranes
Contamination of membrane/salt bridge by ions that compete/react with selected ion and thus alter the electrode response
Types of electrodes used for potentiometry?
- Redox
- Ion-selective: glass and polymer
- PCO2
Advantages of electrochemical techniques
- Wide dynamic range
- Low detection limits
- Required sample volumes are small
- Can be used in vivo
3 categories of electrochemical techniques
- Potentiometry
- Voltammetry
- Coulometry
Define potentiometry
Measurement of a cell potential under equilibrium conditions
Draw a silver/silver chloride electrode
A silver wire, coated in a thin film of silver chloride, immersed in a solution of known chloride concentration, which fixes the half cell potential.
What are the characteristics of an ideal reference electrode?
- Stable half cell potential
- Reversible half cell reaction
- Easy to manufacture and use
- Components chemically stable
Name three common/important reference electrodes
- Standard hydrogen electrode
- Saturated calomel electrode
- Silver/silver chloride electrode
Draw/describe a saturated calomel electrode
Mercury, covered by relatively insoluble mercury chloride, dispersed within a chloride solution. Chloride concentration fixes the half cell potential.
Draw/describe a standard hydrogen electrode
The reference cell on which tables of half cell reduction potentials are based. Hydrogen gas is bubbled over platinum immersed in an acid solution with an H+ activity of 1. The potential of this half cell is defined as 0.0V at ALL temperatures. Not in use practically because other reference electrodes (saturated calomel and silver/silver chloride) are easier to construct and use.
What is the liquid junction potential?
The potential that develops at the tip of the reference electrode, where it is immersed in sample solution. Arises because the types and concentrations of ions in the reference electrode solution are different from those in the sample. A potential source of error in potentiometric measurement, but can be minimised and controlled (made constant) by adjusting the sample ionic strength.
What methods are used in your lab for pO2, pCO2, pH?
pO2 - optical chemical sensor (red light phosphorescence after green LED light excitation of an oxygen sensor)
pCO2 - glass pH electrode senses change in H+ due to diffusion of CO2 into aqueous solution
pH - glass pH electrode
What methods are used in your lab for glucose and lactate (ABL Flex)?
Enzyme-based biosensor with amperometric detection
Describe an oxygen (Clark) electrode
A complete electrochemical cell. Cathode is a platinum disk and anode is Ag/AgCl electrode. Isolated from sample by a membrane permeable only to oxygen. Platinum disk held at a negative potential compared to Ag/AgCl electrode (so electrons flow towards it). As oxygen is reduced at the platinum disk (half equation: O2 + 2e- + 2H+ –> H2O2), the current that flows through the system is measured, and is proportional to the oxygen concentration (partial pressure) in the sample.
