Questions: Voltammetry and Electrochemical Stripping Analysis

Question 1

Why does stripping analysis achieve far lower detection limits than standard voltammetry applied to the same sample?

• A. A more selective electrode material is used that reacts only with trace metals
• B. The analyte is concentrated at the electrode during a deposition step before the measurement is made
• C. A slower scan rate gives more time for current to flow at trace concentrations
• D. Differential-pulse waveforms are applied instead of linear sweeps

Answer: (B)

The key innovation of stripping analysis is the preconcentration step: trace analyte from a large solution volume is electrodeposited onto a tiny electrode surface, then stripped in a single burst. This concentration effect amplifies the signal by orders of magnitude. Differential-pulse waveforms (option D) also improve sensitivity, but they are an enhancement layered on top of the preconcentration — standard voltammetry with a differential-pulse waveform still cannot approach the detection limits of stripping analysis without the deposition step.

Question 2

In anodic stripping voltammetry (ASV), which sequence of events correctly describes the two-phase measurement?

• A. First, metal deposits are cathodically stripped into solution; then, fresh metal is oxidized from solution onto the electrode
• B. First, metal ions are reduced and deposited onto the electrode; then, the electrode potential is swept anodically to oxidize and dissolve the deposit
• C. First, the solution is stirred vigorously to accumulate metal at the electrode surface; then, current is measured at open circuit
• D. First, an oxidizing potential dissolves surface contaminants; then, a reducing potential plates the analyte

Answer: (B)

ASV works in two phases: (1) deposition — holding a sufficiently negative potential reduces dissolved metal cations (e.g., Pb²⁺ → Pb⁰) and plates them onto the electrode; (2) stripping — sweeping the potential in the positive (anodic) direction oxidizes the deposited metal back into solution. The resulting oxidation current peak identifies the metal (via its characteristic stripping potential) and quantifies it (via peak height or area).

Question 3

A longer deposition time in stripping analysis generally improves sensitivity because more analyte accumulates on the electrode.

• T. True
• F. False

Answer: False

While longer deposition time does increase analyte accumulation — improving sensitivity — it can also lead to electrode saturation or, when multiple metals are present, to intermetallic compound formation between co-deposited metals. Intermetallics shift or distort stripping peaks, degrading selectivity. In practice, deposition time must be optimized rather than simply maximized.

Question 4

In stripping analysis, the potential at which the stripping current peak occurs identifies which metal analyte is present.

• T. True
• F. False

Answer: True

Different metals strip (re-oxidize or re-reduce) at characteristic potentials determined by their standard reduction potentials. Just as in standard voltammetry, the position of the current peak on the potential axis is a qualitative identifier, while the peak magnitude or area provides quantitative information about concentration.

Question 5

Explain why anodic stripping voltammetry can detect metals at parts-per-trillion levels while standard voltammetry on the same sample cannot.

Model answer: In standard voltammetry, you measure the small current from analyte dissolved at trace concentration directly. In ASV, a long deposition step accumulates trace metal from a large volume of solution onto a tiny electrode surface — essentially running an electrochemical preconcentration. When the concentrated deposit is then stripped, it produces a current signal many orders of magnitude larger than the original dissolved analyte could generate, pushing detection limits from ppb (standard voltammetry) down to ppt.

The principle is analogous to concentrating a dilute solution before measuring it: the measurement itself has not changed, but the amount of analyte being measured has increased enormously. Detection limit is ultimately governed by how well the signal can be distinguished from background noise; by amplifying the signal through preconcentration, stripping analysis clears this bar for concentrations that would otherwise be unmeasurable.