What is the principle of potentiometry?
It measures electrical potential based on the activity of free ions.
This technique is essential for determining ion concentrations in various solutions.
Potentiometry governed by what equation
Nernst Equation
How is ion concentration determined in potentiometry?
Ion concentration is determined by measuring the potential difference between two electrodes.
This method relies on the Nernst equation to relate potential to ion activity.
What does a change in voltage indicate in Potentiometry?
It corresponds to a change in analyte activity.
This relationship is crucial for understanding how potentiometric measurements reflect ion concentration.
Potentiometry components
• Measuring electrode
• Reference electrode (constant and stable potential)
Measures potential difference across a glass membrane permeable only to H⁺ ions
pH Electrode (Glass pH Electrode)
What are the types of electrodes used in potentiometry?
- Standard Hydrogen Electrode
- Saturated Calomel Electrode:
mercury + mercurous chloride (calomel) with a known
concentration of potassium chloride - Silver–Silver Chloride Electrode:
silver electrode in KCl saturated with AgCl, suitable for
temperatures > 60°C, reacts with more sample
components than calomel
These electrodes are essential for accurate potentiometric measurements in electrochemistry.
What are the structural components of a glass pH electrode bulb?
Composed of:
1. hydrated
2. non-hydrated glass bulb
*Internal Solution:
> Chloride ion buffer.
How is pH determined in potentiometry, and what is the “gold standard” electrode?
Calculated from measured voltage;
the Glass pH Electrode is the most common.
it is a modified pH electrode
pCO₂ Electrode (Severinghaus Electrode)
pCO₂ Electrode is composed of
- pH electrode inside a plastic jacket
- Sodium bicarbonate buffer
- Gas-permeable membrane
In a Severinghaus electrode, how is pCO2 determined?
CO2 diffuses through a membrane, reacts with a buffer to change the pH, and pCO2 calculated indirectly from that pH change.
What is the function and selectivity basis of an ION-SELECTIVE ELECTRODE ISE?
- electrochemical transducer that responds selectively to one specific ion
- Selectivity depends on membrane composition
What are the three primary characteristics of ISE measurements?
- Highly sensitive
- Measures ion activity, not absolute concentration
- Nonspecific in that other ions may still influence voltage
What components and mechanism define an ISE measurement?
Components
- Measures potential
- reference electrode.
Mechanism
- Semipermeable membrane
How are ISE results expressed?
mmol/L of plasma water
Type of ION-SELECTIVE ELECTRODE
- Direct ISE
- Indirect ISE
key features of Direct ISE?
- No dilution;
- measures plasma water activity directly;
- unaffected by high lipids/proteins.
key features of indirect ISE
- Sample is diluted (1:16 to 1:34)
- ssumes plasma water content of 93%
- affected by high protein or lipid levels
Direct ISE used in
- POCT
- Blood gas analyzers
Indirect ISE used in
Routine electrolyte testing
Examples of ISE membranes
- Glass electrodes (H⁺, Na⁺)
- Liquid membrane electrodes
- Precipitate-impregnated membranes
- Solid-state electrodes
- Gas electrodes
- Enzyme electrodes
Which specific membrane materials are used for Potassium and Sodium?
- Valinomycin gel = potassium
- Glass aluminum silicate = sodium
common physical interference in ISE and effects
Protein coating on membrane, affecting response error
