Conductivity (or specifically electrolytic conductivity) is defined as the ability of a substance to conduct electric current. It is the reciprocal of the more commonly encountered term, resistivity. All substances possess conductivity to some degree, but the amount varies widely, ranging from extremely low (insulators such as benzene, glass) to very high (silver, copper, and metals in general). Most industrial interest is in the conductivity measurement of liquids. Electric current will readily flow through some liquids. The less ordered arrangement of the liquid molecules is not conducive to free electron movement. Therefore, another sort of charged particle must serve this purpose if any current is to flow at all. In solvents where electrical conductance occurs, notably in water, ionization will provide the needed carriers. Ionization refers to the tendency of most soluble inorganic compounds to partially or completely separate into two or more elemental components, called ions, having opposite electrical charges. These charged particles, or ions, act as current carriers producing electrolytic current flow. It is the physical characteristics of the carriers as much as that of the medium that determines electrical conductance of a solution. These solutions have conductivities approximately midway between insulators and metallic conductors. This conductivity can be measured quite easily by electronic means, and this offers a simple test which can tell much about the quality of the water, or the makeup of the solution. A broad line of conductivity equipment is available to measure liquids ranging from ultra-pure water (low conductivity) to concentrated chemical streams (high).
Advantages and Disadvantages of Conductivity Measurement
In general, conductivity offers a fast, reliable, nondestructive, inexpensive and durable means of measuring the ionic content of a sample. Reliability and repeatability are excellent.
The principle drawback of conductivity is that it is a nonspecific measurement; it cannot distinguish between different types of ions, giving instead a reading proportional to the combined effect of all ions present. Therefore it must be applied with some pre-knowledge of the solution composition or used in relatively pure (single solute) solutions to be successful.
Units of Conductivity
The units of measurement used to describe conductivity and resistivity are quite fundamental and are frequently misused. Once the units are known, various waters can be quantitatively described.
The basic unit of resistance is the familiar ohm. Conductance is the reciprocal of resistance, and its basic unit is the siemens, formerly called mho. In discussions of bulk material, it is convenient to talk of its specific conductance, now commonly called its conductivity. This is the conductance as measured between the opposite faces of a 1-cm cube of the material. This measurement has units of siemens/cm. The units microsiemens/cm (¥ìS/cm) and millisiemens/cm (mS/cm) are most commonly used to describe the conductivity of aqueous solutions. The corresponding terms for specific resistance (or resistivity) are ohm-cm (¥Ø-cm), megaohm-cm (M¥Ø -cm) and kilohm-cm (k¥Ø -cm).
Users of ultra-pure water prefer to use resistivity units of ¥Ø-cm, because measurement in these units tends to spread the scale out in the range of interest. These same users frequently use k -cm when dealing with less pure water such as tap water. Others, however, use the units of ¥ìS/cm and mS/cm when dealing with any stream from quite pure to very concentrated chemical solutions. In these applications, the use of conductivity has the advantage of an almost direct relationship with impurities, especially at low concentration. Hence, a rising conductivity reading shows increasing impurities, or a generally increasing concentration in the case of a chemical stream (with some exceptions in concentrated solutions). See Table 1 for a comparison of resistance and conductivity.