Measurement and Control Basics Fourth Edition

Chapter 8 - Analytical Measurement and Control

Introduction

This chapter discusses the basic principles of chemical analytical measurement
and control, with an emphasis on the following areas: conductivity,
pH, density, humidity, turbidity, and gas analysis. We introduce the basic
principles of electromagnetic (EM) radiation and describe several common
phototransducers that use EM radiation to measure analytical variables.

Conductivity Measurement

Measuring conductivity means determining a solution's ability to conduct
electric current. This ability is referred to as specific conductance-or, simply,
conductivity-and is expressed in "mhos," which is the reciprocal of
ohm (the unit used to express resistance).

Aqueous solutions of acids, bases, or salts are known as electrolytes; they
are conductors of electricity. Conductivity measurements are generally
made to detect electrolytic contaminants around water and waste-treatment
areas. The degree of electrical conductivity of such solutions is
affected by three factors: the nature of the electrolyte, the concentration of
the solution, and the temperature of the solution. A measurement of the
conductivity at a fixed temperature can be a measurement of the solution's
concentration, which can be expressed in percentage terms by weight,
parts per million, or other applicable units.

If you know the conductivity values of various concentrations of an electrolyte,
you can then determine the concentration by passing current through a solution
of known dimensions and measuring its electrical resistivity or conductivity.

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