Electrogravimetric analysis measures the weight of an analyte deposited electrolytically onto a suitable working electrode. This method involves applying a potential to a pre-weighed electrode submerged in a solution, which results in the desired substance being deposited through reduction at the cathode or oxidation at the anode. The electrode's weight is recorded after deposition, and the difference in weight gives the analyte's weight in the solution.
To test the completeness of the deposition, the fresh electrode surface is exposed to the solution while continuing the electrolysis and checking for new deposits. The absence of new deposits signifies the completion of the reaction. Other tests include checking for color disappearance or conducting a qualitative test for the analyte in the solution.
Underpotential deposition, or monolayer formation on the electrode surface, occurs at a potential below the Nernst equilibrium potential. Conversely, overpotential (a potential above the Nernst potential) is required for bulk metal film deposition. Overpotential factors can cause unwanted reactions and affect the accuracy of the analysis.
Depolarizers can be added to prevent undesired side reactions by scavenging excess electrons into non-interfering reactions and controlling the working electrode's potential.
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.
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