Conductivity Measurement Theory Guide

Conductivity Measurement - the Theory and Practice

A Guide of Conductivity Applications in the Laboratory Environment

This guide provides all the important basics that are necessary for a good understanding of conductivity measurement. Furthermore, all the important factors that influence the measurement and possible sources of errors are discussed. This booklet is not limited to theoretical aspects. It also contains a substantial practical part with step-by-step tutorials and guidelines for reliable calibration and measurements, descriptions of specific applications, and a section with answers to frequently asked questions.

Table of Content:

• Introduction to conductivity
  

• Theory, Basic Information and Definition
  

• Code of Best Practice

• Frequently Asked Questions

• Glossary

• Appendix (Temperature Correction Factors)

Electrical conductivity has been measured in practice for more than 100 years and it is still an important and widely used analytical parameter today. The high reliability, sensitivity, fast response, and the relatively low cost of the equipment make conductivity a valuable, easy to use tool for quality control. Electrical conductivity is a non-specific sum parameter over all dissolved ionic species (salts, acids, bases, and some organic substances) in a solution. This means that this technique is unable to differentiate between diverse kinds of ions. The reading is proportional to the combined effect of all ions in the sample. Therefore, it is an important tool for monitoring and surveillance of a wide range of different types of water (pure water, drinking water, natural water, process water, etc.) and other solvents. It is also used to determine the concentrations of conductive chemicals.

Electrical Conductivity – Basic Information

Electrical conductivity is the ability of a material to carry an electrical current. The term conductivity can also be used in other contexts (e.g., thermal conductivity). For simplicity, in this guide the term “conductivity” is always used in the sense of electrical conductivity.

The transport of electricity through matter always requires the presence of charged particles. Conductors can be classified into two main groups based on the nature of the charged particle. Conductors in the first group consist of a lattice of atoms with an outer shell of electrons. The electrons in this ‘electron cloud’ can dissociate freely from their atom and transport electricity through the lattice and therefore also through
the material. Metals, graphite, and a few other chemical compounds belong to this group.

The conductors in the second group are so-called ionic conductors. In contrast to the conductors of the first group the current flow is not caused by freely moving electrons but by ions. Thereby the charge transfer in electrolytes is always linked to the transport of matter. Conductors in the second group consist of electrically charged and moveable ions and are called electrolytes.Ionization occurs by dissolving in a polar solvent (such as water) or through melting.

Conductivity is measured in a wide range of different applications. The second part of this guide provides a lot of application know-how. First, a general operation mode for calibration, verification, and conductivity measurements including the special case of low conductivity measurement is described. Furthermore, the maintenance and storage of conductivity sensors is discussed. In the following chapters, the most important applications are described in detail.

All METTLER TOLEDO conductivity meters provide further measurement modes beside conductivity measurements. Table 7 gives an overview of the measurements modes which are supported by a meter. TDS, salinity, conductivity ash, and bioethanol measurements are described in detail in section 3.6.

How do I select the right sensor?

Checking the following three criteria will help you to choose the right sensor.

1. Chemical stability:

• There must be no chemical reaction between the sensor material and the sample.

2. Construction type:

• 2-pole sensor: Best for low conductivity measurements

• 4-pole sensor: Best for mid to high conductivity measurements

3. Cell constant:

• Use a sensor with a low cell constant (0.01–0.1 cm-1) for low conductivity measurements
  and a sensor with a higher cell constant (0.5–1.0 cm-1) for mid to high conductivity measurements.

Alternating current (AC):  Flow of electric charge which periodically reverses direction.

Anion:                               A negatively charged ion.

Calibration:                       Empirical determination of the cell constant by measuring a standard solution.

Cation:                              A positively charged ion.

Cell constant K [cm-1]:    Theoretical: K = l / A; The ratio of the dis­tance between the electrodes (l) to the effective cross-sectional
                                         area of electrolyte between the poles (A).
                                        The cell constant is used to transform the conductance into the conductivity and is determined by calibra­tion.
                                        The difference between the theoretical and real cell constant is caused by field lines.

Conductance G [S]:        The ability of material to conduct electricity.