Chemistry
Using Square Wave Voltammetry for Trace Heavy Metal Detection
Quick fact
Square wave voltammetry can detect metals at concentrations as low as parts per billion, often in less than a minute.
Why this is interesting
Imagine detecting a single drop of lead in an Olympic-sized swimming pool. How can chemists find such tiny amounts of heavy metals?
Read the full explanation
Understanding Using Square Wave Voltammetry for Trace Heavy Metal Detection
Square wave voltammetry (SWV) is an electrochemical technique that measures current as a function of applied potential. Instead of a simple linear sweep, SWV applies a staircase-shaped potential with a square wave superimposed on each step. The current is sampled twice during each square wave cycle—once at the end of the forward pulse and once at the end of the reverse pulse. By subtracting the reverse current from the forward current, background capacitive currents are largely eliminated, yielding a clean, peak-shaped response. This differential measurement dramatically improves sensitivity and speed, making SWV ideal for trace analysis. In practice, a sample containing heavy metals is placed in an electrochemical cell with a working electrode (often mercury or a mercury film). As the potential is scanned, metal ions in the sample are reduced and deposited onto the electrode (preconcentration step). Then, during the stripping step, the deposited metals are oxidized back into solution, producing a current peak at a potential characteristic of each metal. The peak height is proportional to the concentration, allowing quantification.
A deeper explanation
The mechanism behind SWV's sensitivity lies in its unique waveform and current sampling. The applied potential consists of a staircase, where each step has a superimposed square wave of small amplitude (typically 10-50 mV) and a fixed frequency (e.g., 100 Hz). The current is sampled twice per cycle: at the end of the forward pulse (where the potential is more positive) and at the end of the reverse pulse (where the potential is more negative). The difference between these two currents (Δi) is plotted against the base staircase potential. This differential approach cancels the charging current (capacitive current) that arises from the electrical double-layer capacitance, which is largely independent of the square wave frequency and nearly identical in both forward and reverse pulses. Meanwhile, the faradaic current (from the redox reaction) changes with potential, so the difference reflects the net faradaic process. The result is a highly sensitive, low-noise signal. Additionally, the use of a fast square wave frequency (tens to hundreds of Hz) means that each measurement is quick, enabling scans in seconds. SWV is often combined with a preconcentration step, such as in anodic stripping voltammetry, where metals are deposited onto a mercury electrode at a negative potential, then oxidized during the square wave sweep. This preconcentration step enhances detection limits even further. The peak potential identifies the metal, and the peak current correlates with concentration via calibration curves. SWV has become a standard method for monitoring heavy metals like lead, cadmium, and zinc in environmental waters, food, and biological samples, because it combines rapidity, sensitivity, and relatively low cost equipment.