Electrochemistry Lecture Review
Galvanic vs Electrolytic Cells
Electrochemistry in the clinical laboratory underpins many analytical methods for measuring electrolytes and blood gas analytes.
Core definition: electrochemistry is the study of electricity by chemical action due to the presence or flow of electrons; serves as the basis for many analytical methods in chemistry analyzers.
Common electrochemical methods in the clinic include potentiometry, amperometry, coulometry (the transcript uses the term “coolimetry” which is likely a mis-spelling of coulometry), and polarography.
Two basic electrochemical cells:
Galvanic (voltaic) cell: spontaneous flow of electrons; energy is from chemical energy; external power source not required.
Electrolytic cell: current is forced to flow by an external electromotive force (EMF).
Configurations of electrochemical cells:
Two half-cells connected by a salt bridge and a voltmeter (external measuring device).
A single cell can be formed in a beaker by immersing two electrodes in a salt solution (the salt bridge concept is demonstrated by a solution like silver nitrate serving as the salt bridge in a depiction).
Purpose of the salt bridge: maintains electrical neutrality and permits ion flow to sustain the current between the half-cells.
Key distinction: galvanic cells generate electrical energy from chemical reactions; electrolytic cells require external energy to drive the reaction.
Relationship between half-cells and measurement:
One half-cell is the reference electrode with a reference solution of an analyte for calibration.
The other half-cell is the indicator (measuring) electrode containing the sample with the analyte to be measured.
You cannot measure electrochemical activity of a single half-cell alone; measurements always involve a pair of half-reactions.
Potentiometry
Potentiometry measures the electrical potential (voltage) between two electrodes (indicator vs reference) due to ion activity, with no net current flowing through the cell.
Applications: ISE (ion-selective electrode), pH, PCO2 (gas sensing), and general electrolyte measurements.
The measured potential is in volts (V).
Ion Selective Electrodes (ISE): designed to be selective for particular ions; membranes are perm-selective, allowing either anions or cations to pass, yielding a signal dominated by the ion of interest.
Membrane selectivity and interaction:
Some interaction with other ions may occur, but the electrode is designed so the target ion’s activity dominates the measured potential.
Main ISE membrane types:
Glass membranes: used for pH and sodium measurements; membrane composition can be varied for selectivity toward specific ions.
Polymer membranes: used for potassium (K+), sodium (Na+), chloride (Cl−), and bicarbonate (HCO3−); most prevalent in modern instrumentation.
Gas ISE: used for gases such as PCO2; membrane is sensitive to gas permeation.
A commonly used ion-selective electrode in clinical labs: the pH meter.
pH indicator electrode (indicator electrode):
Contains a silver wire coated with silver chloride (AgCl).
The indicator electrode is encased in a glass membrane sensitive to hydrogen ions (H+).
Insertion into the test solution allows diffusion of H+ into the membrane, generating a potential proportional to the difference between sample [H+] and the reference solution [H+] at the electrode, i.e., the pH.
pH reference electrode options:
Calomel electrode (Hg2Cl2 paste in contact with metallic mercury; KCl electrolyte): provides a stable reference voltage as long as electrolyte concentration and temperature are consistent; stability up to 80°C.
Silver/silver chloride (Ag/AgCl) reference electrode: compact and stable to higher temperatures.
Liquid junction: a tiny opening at the bottom allowing electrical connection between reference and indicator electrodes; typically filled with KCl because potassium and chloride have similar mobilities.
Commonly used reference electrolytes and junction materials:
Potassium chloride (KCl) is widely used due to similar ionic mobilities of K+ and Cl−.
Modern pH meters typically use a combined (pregnant) probe that contains both indicator and reference electrode in a single glass-encased probe; measurement is read by a voltmeter or readout meter.
Potentiometric sensors for specific electrolytes:
Sodium electrode: glass-capillary ion-selective membrane; sample flows past the membrane, ions interact with the membrane, a potential develops, and the voltage is proportional to Na+ concentration when compared to the reference electrode.
Potassium electrode: improved selectivity for K+ achieved via incorporation of the neutral antibiotic valinomycin, yielding higher selectivity for potassium over sodium.
Gas-sensing electrode (PCO2): gas-permeable membrane allows CO2 diffusion; a thin bicarbonate film inside reacts with CO2 to form carbonic acid and H+; the resulting pH change is detected by the pH electrode; the measured signal is proportional to dissolved CO2 partial pressure (PCO2).
Direct vs indirect ion-selective electrode (ISE) measurement (ISE methodologies):
Direct ISE (often called direct): undiluted whole blood or plasma; common in blood gas analyzers and point-of-care devices.
Indirect ISE (often called indirect): diluted sample before measurement; generally results are similar to direct ISE except where dilution interacts with sample turbidity (lipemia) or high protein levels, causing potential measurement errors.
Errors and maintenance in potentiometry/ISE:
Protein buildup on the membrane over time reduces selectivity and reproducibility.
If the measured potential is out of the instrument’s range, recalibration or dilution may be required.
Membrane failure may necessitate replacement of the ISE.
Air bubbles on the membrane should be removed by gentle agitation to maintain accurate readings.
Amperometry and Coulometry (coined as “coolimetry” in the transcript)
Amperometry: measure current (I) at a fixed potential between the working electrode and the reference electrode in a dead cell (no net charge transfer from the solution except the redox process at the electrode).
Principle for PO2 measurement (Clark electrode):
A fixed potential is applied between the electrode (anode and cathode) to drive the reduction of oxygen at the cathode.
Oxygen must diffuse through an oxygen-permeable membrane into the electrolyte; the resulting current produced by the reduction of O2 is proportional to the concentration or partial pressure of oxygen in the sample.
Common sources of error in PO2 amperometry:
Protein buildup on the membrane can slow diffusion of oxygen to the electrode.
Bacterial contamination can consume oxygen, altering the signal.
Poor or incorrect calibrations.
Coulometry (often called coulimetry in the transcript):
Involves applying a constant potential and measuring the total charge (Q) passed over time; Q = I × t.
The amount of electricity (charge) is directly proportional to the amount of oxidized/reduced substance; the measurement is in coulombs (Q), with current (I) and time (t).
Historically used for chloride detection; modern practice relies more on chloride ISE for chloride measurement.
Co-Oximetry and Pulse Oximetry
Co-oximetry (often referred to as co-oximetry): a spectrophotometric method that uses multiple wavelengths to distinguish different forms of hemoglobin based on their absorbance spectra.
Measures oxyhemoglobin (HbO2), deoxyhemoglobin (Hb), carboxyhemoglobin (HbCO), and methemoglobin (HbMet).
Provides a more complete assessment of oxygen-carrying capacity than pulse oximetry alone.
Pulse oximetry (part of co-oximetry family): a noninvasive, continuous method using multi-wavelength spectrophotometry to estimate the proportion of HbO2 and Hb (and to some extent HbCO/HbMet) by analyzing light absorption through tissue (commonly a finger).
Interferences: light-absorbing substances on the skin (e.g., dyes, pigments), motion, poor calibration, and sensor placement issues can affect readings.
Henderson-Hasselbalch reference in blood gas analysis:
The bicarbonate/ carbonic acid ratio informs the acid-base status when measured pH and PCO2 are known.
The relationship is commonly expressed as:
\mathrm{pH} = 6.1 + \log{10}\left(\frac{[\mathrm{HCO}3^-]}{0.03 \times P{CO2}}\right)
Here, PCO2 is in mmHg and [HCO3−] is in mmol/L.
Base excess (BE) and base deficit:
BE/Basedeficit quantify the metabolic component of an acid-base disorder.
BE is calculated from pH, PCO2, and hemoglobin; a positive BE indicates metabolic alkalosis (excess base or bicarbonate), while a negative BE indicates metabolic acidosis (base deficit).
BE is used to separate metabolic from respiratory components of acid-base disorders.
Blood gas preanalytical and analytical considerations:
Specimen exposed to air: PCO2 decreases, pH increases, and PO2 increases.
Specimen at room temperature for > 30 minutes: PO2 decreases, pH decreases, and PCO2 increases.
Instrumental/analytical errors: protein buildup on electrodes or poor calibrations.
Blood Gas and Chemical Impedance (IAC) Measurements: Direct vs Indirect Methods
Direct ISE measurement (direct ISE):
Uses undiluted whole blood or plasma;
Common in blood gas analyzers and point-of-care devices.
Indirect ISE measurement (indirect ISE):
Samples are diluted prior to measurement;
Generally similar results to direct ISE but more susceptible to interference from lipemia (high lipid content) or high protein levels, which can displace plasma water and cause falsely low measurements.
Coulometry vs Amperometry: Key Equations and Concepts
Basic charge-current relationship:
Q = I \cdot t
Charge (Q) in coulombs equals current (I) in amperes times time (t) in seconds.
Current proportionality in amperometry:
The measured current at a fixed potential is proportional to the concentration of the electroactive species in solution (e.g., O2 in Clark electrode).
Practical, Practical Implications and Summary
The choice of electrochemical method (potentiometry, amperometry, coulometry) depends on the analyte and measurement context (electrolytes, blood gases, oxygenation, etc.).
Understanding electrode types and configurations is essential for interpreting results and troubleshooting:
Reference electrode stability and junctions affect measurement accuracy.
Membrane fouling and sample quality influence selectivity and signal stability.
Dilution effects in indirect ISE can introduce biases with lipemic or protein-rich samples.
Analytical interferences and preanalytical variables can significantly affect results, underscoring the importance of careful sample handling and calibration.
The Henderson-Hasselbalch relationship provides a clinical bridge between measured pH, PCO2, and bicarbonate for understanding acid-base status and calculating base excess/deficit.
Modern clinical practice: combine potentiometric ISE (for ions) with optical/co-oximetry methods (for Hb species) and amperometric sensors (for PO2) to obtain a comprehensive metabolic and respiratory profile.
Key Takeaways
Electrochemical cells come in galvanic (spontaneous, chemical energy) and electrolytic (externally driven) forms; salt bridges and reference/indicator electrodes enable measurement.
Potentiometry measures voltage without current; ISEs, pH, and gas sensors fit here.
Ion-selective membranes (glass, polymer, gas) determine selectivity; potassium selectivity is enhanced with valinomycin.
pH measurement hinges on a glass indicator electrode and a stable reference electrode (calomel or Ag/AgCl); liquid junction and KCl electrolyte are common.
Oxygen measurement via Clark amperometry uses a fixed potential and a diffusion membrane; current relates to PO2.
Coulometry quantifies charge to determine analyte amount; historically used for chloride detection but largely replaced by ISE methods.
Co-oximetry and pulse oximetry together provide detailed information about hemoglobin species and oxygenation, with attention to potential interferences.
Base excess/deficit reflects metabolic contributions to acid-base status and requires integration of pH, PCO2, and Hb.
Preanalytical variables (air exposure, temperature, time) and analytical variables (calibration, membrane fouling) critically influence accuracy and precision in electrochemical measurements.