Electrogravimetric and Coulometric Analysis

Electrolysis is a chemical reaction in which we apply a voltage to drive a redox reaction that would not otherwise occur. An electroactive species is one that can be oxidized or reduced at an electrode. 

One of the oldest electrolytic methods in quantitative analysis is electrogravimetric analysis, in which analyte is plated out on an electrode and weighed. For example, an excellent procedure for the measurement of copper is to pass current through a solution of a copper salt to deposit all of the copper on the cathode  

The increase in mass of the cathode tells us how much copper was present in the solution. 

How do you find out when electrolysis is complete One way is to observe the disappearance of color in a solution from which a colored species such as Cu is removed. Another way is to expose most, but not all, of the surface of the cathode to the solution during electrolysis. To test whether the reaction is complete, raise the beaker or add water so that fresh surface of the cathode is exposed to the solution. After an additional period of electrolysis (15 min, say), see whether the newly exposed electrode surface has a deposit. If it does, repeat the procedure. If not, the electrolysis is finished. A third method is to remove a small sample of solution and perform a qualitative test for analyte. 

Electrogravimetric analysis would be simple if there were only a single analyte in an otherwise inert solution. In practice, there may be other electroactive species that interfere. Water decomposes to H2 at the cathode and to O2 at the anode at sufficiently high voltage. Gas bubbles at an electrode interfere with deposition of solid. Because of these complications, control of electrode potential is important for successful analysis. 

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A major branch of analytical chemistry uses electrical measurements of chemical processes at the surface of an electrode for analytical purposes. For example, hormones released from a single cell can be measured in this manner. Principles developed in this chapter provide a foundation for potentiometry, redox titrations, electrogravimetric and coulometric analysis, voltammetry, and amperometry in the following chapters.1-2... 

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