10.12 : Electrogravimetric Analysis: Overview

Electrogravimetric analysis determines an analyte's mass in a sample solution by electrolytically depositing it onto a suitable pre-weighed working electrode.

Applying the appropriate potential deposits the analyte through metal cation reduction at the cathode or oxidation at the anode, or even through oxidation of the anode metal itself.

The electrode's increase in weight after deposition gives the analyte's weight in the solution.

The deposition completeness can be evaluated by exposing the fresh electrode surface to the used solution while continuing the electrolysis and checking for new deposits.

Other methods include observing the disappearance of a colored analyte or quantitatively testing for the analyte using a spectrophotometer.

Quantitative analyte deposition requires an overpotential—a potential past the Nernst equilibrium potential—because an underpotential often favors the formation of a monolayer on the electrode surface over bulk metal deposition.

In a constant-current method, the overpotential may cause unwanted reactions that affect the accuracy of the analysis. Depolarizers prevent these reactions by scavenging excess electrons into non-interfering reactions.

In addition, controlling the working electrode's potential can help avoid unwanted side reactions that change the mass of the analyte deposited.