An environmental chemist needs to measure lead (Pb²⁺) at parts-per-trillion concentrations in drinking water. Which electroanalytical branch is best suited to this task, and why?
APotentiometry, because ion-selective electrodes can detect any metal ion with extreme selectivity
BConductometry, because total ionic content is directly proportional to trace metal concentration
CVoltammetry (specifically stripping voltammetry), because analytes can be pre-concentrated at the electrode surface before measurement, achieving trace-level sensitivity
DCoulometry, because it yields absolute amounts without a calibration curve, eliminating matrix effects
Stripping voltammetry achieves sub-ppb and even ppt detection limits for trace metals by exploiting a two-step process: the analyte is electrodeposited (concentrated) onto the working electrode surface during a deposition step, then stripped off in a voltage scan that produces a sharp current peak. This pre-concentration step amplifies the analytical signal by orders of magnitude compared to measuring the dilute solution directly. Potentiometry (A) lacks the sensitivity for ppt levels and ion-selective electrodes are not available for all metals. Conductometry (B) cannot distinguish Pb²⁺ from other ions and is insensitive to trace levels. Coulometry (D) is accurate but not inherently trace-sensitive — it measures everything that is electrolyzed, not trace amounts pre-concentrated at an electrode.
Question 2 Multiple Choice
What property makes coulometry a 'primary' analytical method — one that can yield accurate results without a calibration curve?
ACoulometry uses multiple electrodes that cross-validate each other, eliminating systematic error
BCoulometry measures total charge passed during complete electrolysis, and since Q = nFN, the amount of analyte is calculated directly from charge using Faraday's constant without reference to any standard
CCoulometry is performed at equilibrium, so the Nernst equation relates charge directly to analyte activity
DCoulometry is the only method that requires the analyte to undergo a redox reaction, ensuring specificity
The relationship Q = nFN (charge = electrons per molecule × Faraday's constant × moles of analyte) is a fundamental physical law. If you measure the total charge passed during the complete electrolysis of an analyte, you can calculate the exact number of moles from the charge and Faraday's constant — no comparison to a standard solution is needed. This makes coulometry a primary method in the metrological sense: its accuracy is grounded in a physical constant rather than in the accuracy of a prepared standard. This stands in contrast to potentiometry and voltammetry, which require calibration curves using known standards to convert their signals (voltage or current) into concentrations.
Question 3 True / False
A pH meter measures hydrogen ion activity using potentiometry, even though no current flows through the solution and no oxidation or reduction occurs at the glass membrane.
TTrue
FFalse
Answer: True
This is the defining feature of potentiometry — it is a non-faradaic technique. The glass membrane develops a voltage (potential difference) in response to the difference in hydrogen ion activity on its two sides, and this equilibrium potential is measured without passing current through the sample. No redox reaction occurs at the membrane; ion exchange equilibrium across the glass generates the signal. This directly contradicts the common misconception that all electroanalytical methods require the analyte to undergo a redox reaction. Potentiometry listens to the system's equilibrium electrical state rather than driving a reaction.
Question 4 True / False
Most four branches of electroanalytical chemistry require the analyte to be oxidized or reduced at an electrode surface to generate the analytical signal.
TTrue
FFalse
Answer: False
Potentiometry and conductometry are non-faradaic methods — they do not require or involve electrode reactions. Potentiometry measures the equilibrium potential that develops across a selective membrane based on the chemical activity of the target ion, with no current flowing. Conductometry measures the ability of the solution to conduct an AC current, which depends on ion mobility and concentration, not on any redox chemistry. Only voltammetry and coulometry are faradaic methods that rely on electron transfer at an electrode. Confusing all electroanalytical methods with redox reactions is one of the most common conceptual errors in this area.
Question 5 Short Answer
A researcher needs to determine the concentration of fluoride (F⁻) in a complex environmental sample containing many other ions (Na⁺, Cl⁻, SO₄²⁻, Ca²⁺, etc.). Explain why potentiometry with a fluoride-selective electrode is more appropriate than conductometry for this task.
Think about your answer, then reveal below.
Model answer: Conductometry measures the total ionic conductance of the solution — the combined contribution of all ions present, weighted by their concentration and mobility. In a complex matrix containing many ions, a change in F⁻ concentration would produce a negligible change in total conductance that could not be distinguished from changes in the other ionic species. Conductometry therefore cannot selectively detect F⁻ in the presence of a background of other ions at similar or higher concentrations. A fluoride-selective electrode (a lanthanum fluoride crystal membrane) responds specifically to fluoride ion activity with very little response to most other common anions, generating a voltage that varies logarithmically with fluoride activity according to the Nernst equation. This selectivity arises from the crystal structure of the membrane, which admits only F⁻ at its surface. The measurement is thus resistant to interference from the complex matrix, providing the ion-specific information that conductometry cannot.
The core principle is that potentiometry's selectivity comes from the electrode membrane, which acts as a chemical filter — responding to one ion while ignoring others. Conductometry has no such filter and cannot distinguish among ionic species. Matching the right electroanalytical technique to the analytical question (here: selective single-ion measurement in a complex matrix) is the practical skill that this overview is designed to build.