No matter how carefully you work, there is no perfect way to connect a current lead to a sample in a real experimental setup. Understanding the sources of error — and minimizing them — is part of good electrochemical practice. Contact potential between dissimilar metals. Any junction between two different metals generates a contact potential. In most cases this is small (a few millivolts) and can be neglected. For example, if your circuit runs from a nickel-plated potentiostat connector through a copper wire to a magnesium sample, the Ni/Cu and Cu/Mg junction potentials do not fully cancel — they simplify to an effective Ni/Mg junction — but the residual offset is usually negligible. However, it is worth being aware of it, particularly in high-precision measurements.
Unstable mechanical contact. A alligator clip pressed against a sample is a common source of serious measurement error. The effective contact area between the clip’s teeth and the sample surface fluctuates with vibration and handling; oxide layers and surface contamination on the teeth add resistance that varies unpredictably. The result is noise and drift that can badly distort the measurement.
Electrolyte ingress at the connection point. If electrolyte reaches the junction between the current lead and the sample, you will inevitably introduce galvanic corrosion and crevice corrosion at that point. These will generate their own electrochemical signals and severely corrupt your results. Practical guidelines. Acceptable connections share a common feature: the junction is well insulated and positioned well above the electrolyte level — ideally above the cell lid, if one is present. Placing the connection merely above the liquid surface is not enough: capillary action, vapor condensation, and accidental splashing will all eventually carry electrolyte upward into the junction.
A simple and highly recommended check: measure the resistance of the assembled (dry) connection with a multimeter before immersing anything. A good metal-to-metal contact should read no more than a few ohms. A high or fluctuating reading tells you immediately that the connection needs attention before any electrochemical measurement begins.
A poor connection degrades all electrochemical measurements, but its effect is most severe in impedance spectroscopy (EIS), where even small parasitic resistances and capacitances at the junction can introduce artifacts across a wide frequency range.