Why secondary electrode is preferred over primary electrode?
Answers
Answer:
An electrode with an invariant potential. In electrochemical methods, where it is necessary to observe, measure, or control the potential of another electrode (denoted indicator, test, or working electrode), it is necessary to use a reference electrode, which maintains a potential that remains practically unchanged during the course of an electrochemical measurement. Potentials of indicator or working electrodes are measured or expressed relative to reference electrodes.
One such electrode, the normal hydrogen electrode, has been chosen as a reference standard, relative to which potentials o...
Explanation:
The difference is one of arbitrary definition. The standard reference point for electrochemical potentials is the hydrogen electrode (H+/H2). This is the primary reference electrode. The problem with this reference is that the proton concentration should be 1 Molar, i.e. a highly acid solution. This is not desirable for many practical applications. You also need to have a continuous supply of hydrogen (there are some elegant products out there to allow that).
A secondary reference electrode, such as the ubiquitous Ag/AgCl reference electrode, is applicable across a wider range of pH values, cheap and robust. However, for pH dependent redox reactions (e.g. hydrogen or oxygen evolution) it is critical to understand that a typical secondary reference will require a correction of around 50 mV for each pH unit. It is also necessary to correct for the potential difference between the secondary reference and and the primary reference based on the hydrogen electrode (which is about 200 mV and temperature dependent).
A good electrochemist will acquire their data relative to whichever secondary reference electrode makes sense for their application. Using simple arithmetic, they will correct their their data to conform to the primary reference electrode so when they present those results they are readily comparable to others.