INTRODUCTION
The purpose of this section is to give an overall perspective of the many electrochemical analyzers used in the processing industries. These instruments will be discussed in more detail in later sections of this chapter. Electrochemical analyzers measure composition or concentration by detecting the changes in voltage or current that occur between two electrodes in a solution (electrolyte) over time, because of the oxidation or reduction of that solution. In galvanic and polarographic analyzers (such as CO, O 2 , and toxic gas sensors discussed in Sections 8.10, 8.42, and 8.59), the electrolyte is usually a gel. The gel is separated
from the process stream by a membrane and the electrochemical reaction in this electrolyte either occurs spontaneously or is caused by a polarizing voltage. In potentiometric analyzers (such as Cl, conductivity,
ion-selective, oxidation–reduction potential (ORP), and pH probes discussed in Sections 8.11, 8.17, 8.28, 8.39, and 8.48), an electrical potential is detected, which is generated in response to the presence of dissolved ionized solids. The ORP probe detects the ratio of oxidizing to reducing agent. Conductivity
sensors measure the solution’s ability to conduct electricity, which depends on the concentration of dissolved ionized solids. In amperometric analyzers (such as Cl, Fl, HS, moisture in gas, O 3 , and toxic gases covered in Sections 8.11, 8.24, 8.26, 8.33, 8.44, 8.45, and 8.59) if a strong oxidizing agent
is present, the polarization of the measuring electrode is depolarized and a current flow is generated, which is proportional to the concentration of the oxidizing agent. In coulometric analyzers (such as CO, oxygen in gas, and SO sensors discussed in Sections 8.10, 8.42, and 8.56), the measured gas diffuses through the potassium hydroxide electrolyte to the cathode of the electrochemical cell, where it is reduced to hydroxyl ions. As an external voltage causes these ions to migrate to the anode, the resulting cell current is a  measure of concentration.