How Electrodes Detect Cadmium in Water

Electrode-based detection of cadmium in water is most commonly built around anodic stripping voltammetry (ASV), a technique that turns a dilute cadmium concentration into a measurable electrical current through a two-step accumulation-and-release sequence.

The measurement takes place in a three-electrode cell: a working electrode, a reference electrode, and a counter electrode. The working electrode is the site where cadmium ions are captured and later stripped; its surface chemistry largely determines sensitivity and selectivity. Glassy carbon electrodes coated with a thin bismuth or mercury film are widely used, while bismuth-film electrodes have gained favour as a less toxic alternative to mercury.

Detection begins with the preconcentration step. A sufficiently negative deposition potential is applied to the working electrode, typically in the range of −1.0 to −1.4 V versus Ag/AgCl, while the solution is stirred. Under these conditions, dissolved Cd(II) ions are reduced to metallic cadmium and accumulate on the electrode surface. The amount deposited increases with deposition time, commonly 60 to 300 seconds, which is why longer accumulation can lower the detection limit up to the point of surface saturation.

Once accumulation is complete, the potential is swept in the positive direction. The deposited cadmium is oxidized back into Cd(II), releasing electrons and producing a sharp stripping peak current. Because different metals oxidize at characteristic potentials, the position of the cadmium peak provides qualitative identification, while the height or area of the peak is proportional to the amount of cadmium originally present in the sample. Square-wave or differential-pulse waveforms are often used during stripping to enhance peak resolution and signal-to-noise ratio.

Solution chemistry critically shapes the result. A mildly acidic buffer, frequently an acetate buffer near pH 4.5 to 5.5, is used to stabilize the stripping signal and improve peak separation. Copper is a notable interferent because it can form intermetallic compounds with cadmium on the electrode surface; masking agents or careful potential selection are used to suppress this effect. Organic matter and surfactants can also distort the signal, so water samples may require filtration or wet-ashing before measurement.