Introduction to Electroanalytical Chemistry
Electroanalytical chemistry encompasses a group of techniques that use electrical measurements — potential, current, charge, or conductance — to determine chemical information about a sample. These methods exploit the fundamental relationship between chemical reactions and electrical energy: oxidation and reduction processes transfer electrons, and this electron transfer can be measured with extraordinary precision and sensitivity.
Electroanalytical methods offer several advantages: they can be highly sensitive (detecting analytes at femtomolar concentrations), they require minimal sample volume, they can monitor reactions in real time, and many can be miniaturized into portable or implantable sensors.
Fundamentals: The Electrochemical Cell
All electroanalytical measurements occur in an electrochemical cell containing: - A working electrode (WE): where the analytical reaction of interest occurs - A reference electrode (RE): maintains a constant, known potential (e.g., standard hydrogen electrode, saturated calomel electrode SCE, or Ag/AgCl) - A counter electrode (CE): completes the circuit and carries the current (in three-electrode cells) - An electrolyte solution: provides ionic conductivity
The cell potential is the difference in electrical potential between electrodes. When controlled experimentally, it determines which redox reactions occur and at what rate.
Potentiometry
Potentiometry measures the equilibrium potential of an electrochemical cell at essentially zero current. The measured potential is related to activity (effective concentration) of ions through the Nernst equation:
E = E° − (RT/nF) ln Q
At 25°C this simplifies to:
E = E° − (0.05916/n) log Q
Where: - E° = standard electrode potential - n = number of electrons transferred - F = Faraday's constant (96,485 C/mol) - Q = reaction quotient (ratio of product to reactant activities)
Ion-Selective Electrodes (ISE)
Ion-selective electrodes (ISEs) are potentiometric sensors that respond selectively to specific ions. The most famous example is the glass pH electrode:
- A thin glass membrane (containing ~20% Al₂O₃ + SiO₂) generates a potential proportional to the H⁺ activity difference across it.
- The Nernst response: ΔE ≈ 59.16 mV per pH unit (at 25°C)
- Modern pH meters display readings to 0.001 pH units
Other ISEs detect: - Fluoride (F⁻): LaF₃ crystal membrane; used in dental and environmental monitoring - Nitrate (NO₃⁻): liquid membrane; used in soil and water analysis - K⁺ and Na⁺: valinomycin or crown ether ionophores in polymer membranes; critical in blood gas analyzers - Ca²⁺: used in water hardness monitoring and clinical blood analyzers - NH₄⁺: ammonium ISE for wastewater and aquaculture
Blood gas analyzers in hospitals use an array of ISEs (pH, pCO₂, pO₂, K⁺, Na⁺, Ca²⁺, Cl⁻) to analyze a 200 μL blood sample in 60 seconds — providing critical information for patient management in emergency and intensive care settings.
Calibration and Standard Addition
ISEs require calibration with standards of known activity: - External calibration curve: Plot E vs. log[analyte] — should be linear (Nernst slope ~59/n mV per decade) - Standard addition method: Add known amounts of analyte to the sample, monitor potential change. Less affected by complex sample matrices.
Voltammetry
Voltammetry measures the current that flows through the working electrode as the applied potential is scanned or pulsed. The current arises from faradaic processes — the actual oxidation or reduction of the analyte at the electrode surface:
O + ne⁻ → R (reduction, cathodic current)
R → O + ne⁻ (oxidation, anodic current)
Cyclic Voltammetry (CV)
Cyclic voltammetry scans the potential from an initial to a final value and back, recording the resulting current. The output is a cyclic voltammogram — a current vs. potential plot revealing the redox potentials and kinetics of electroactive species.
Key features of a CV: - Cathodic peak (ipc) and anodic peak (ipa): Current peaks corresponding to reduction and oxidation of the analyte - Half-wave potential (E₁/₂ ≈ (E_pc + E_pa)/2): Related to the formal reduction potential E°' - Peak current (Randles-Ševčík equation): i_p = 0.4463 n F A C (nFDν/RT)^(1/2)
For a reversible redox couple: |ipa/ipc| = 1 and ΔE_p = |E_pa − E_pc| ≈ 59/n mV.
CV is used extensively in research to characterize new materials, study reaction mechanisms, and screen electrocatalysts.
Differential Pulse Voltammetry (DPV) and Square Wave Voltammetry (SWV)
These pulse techniques superimpose potential pulses onto a staircase ramp, measuring current only at times that minimize non-faradaic (background) charging current. They achieve: - Detection limits of 10⁻⁸ to 10⁻¹⁰ mol/L - Peak-shaped output directly proportional to concentration - Discrimination between closely spaced redox couples
Stripping Voltammetry
Anodic stripping voltammetry (ASV) is among the most sensitive electroanalytical techniques, capable of detecting heavy metals (Pb²⁺, Cd²⁺, Zn²⁺, Cu²⁺) at sub-ppb levels. The method has two stages:
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Preconcentration: Apply negative potential for a fixed time (e.g., 5 min) to reduce and accumulate metal from solution onto the working electrode (mercury film or bismuth film): M²⁺ + 2e⁻ → M(amalgam)
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Stripping: Scan potential positive; accumulated metals reoxidize in order of increasing reduction potential, producing sharp peaks.
The large preconcentration factor (100–1000×) enables detection in contaminated water, blood, and food samples.
Amperometry and Biosensors
Amperometry holds the potential constant and measures the current as a function of time — current is proportional to analyte concentration. The most famous amperometric device is the glucose biosensor:
- Glucose oxidase (GOx) enzyme oxidizes glucose → gluconolactone + H₂O₂
- H₂O₂ is electrooxidized at the electrode: H₂O₂ → O₂ + 2H⁺ + 2e⁻
- Current measured proportional to glucose concentration
Over 1 billion glucose test strips are sold annually for diabetes management. Modern continuous glucose monitors (CGM) use similar amperometric principles with subcutaneous sensors operating for 10–14 days.
Conductometry and Coulometry
Conductometry measures solution electrical conductance — proportional to total ion concentration. Conductometric titrations detect endpoints as sharp changes in slope (conductance vs. volume added) without indicators.
Coulometry measures total charge consumed in a redox reaction. By Faraday's law: Q = nFm/M, where Q is charge, m is mass, M is molar mass. With 100% current efficiency, coulometric methods can be extremely accurate (primary standard quality).
Real-World Applications
Environmental monitoring: ASV monitors heavy metal contamination in rivers, drinking water, and sediments. Portable electrochemical analyzers enable field measurements without a laboratory.
Clinical diagnostics: Point-of-care blood gas/electrolyte analyzers, handheld glucose meters, lactate monitors in sports medicine and critical care — all electroanalytical.
Food quality: Electrochemical sensors detect antioxidants, preservatives, and freshness indicators in food products.
Corrosion science: CV and impedance spectroscopy characterize protective coatings and corrosion inhibitors.
Energy storage: CV, EIS, and galvanostatic cycling characterize battery electrode materials and fuel cell catalysts.
Electroanalytical methods continue to expand, driven by the development of new electrode materials (carbon nanotubes, graphene, molecularly imprinted polymers) and the demand for miniaturized, portable chemical sensors in medicine, environmental protection, and the Internet of Things.