Conductivity in chemistry describes a material’s ability to transmit an electrical current. This property is fundamental to understanding how different substances interact with an electric field. Conductivity, represented by the Greek letter kappa (\(\kappa\)), is an intrinsic property of the material itself. It normalizes the measured conductance by accounting for the physical dimensions of the substance, such as the distance between electrodes and their surface area. Conductivity allows for direct comparison of the inherent ability of various chemical substances to carry charge regardless of their shape or size.
The Movement of Charge Carriers
The mechanism by which electrical charge moves through a chemical substance determines the material’s conductive nature. Charge transfer occurs via the movement of charged particles known as charge carriers, through two distinct processes.
Electronic conduction is typical in metals and some solid-state materials. It involves the flow of highly mobile, delocalized electrons that are not bound to any single atom. These free electrons move easily when an external voltage is applied, leading to high conductivity. Since the atoms remain fixed, electronic conduction involves no mass transport.
The other primary mechanism is ionic, or electrolytic, conduction, common in solutions and molten salts. Here, the charge carriers are positive and negative ions, which must physically migrate toward the electrode of opposite charge for current to flow. This ionic movement inherently involves the transport of matter, which can lead to chemical changes at the electrodes. The speed and number of these moving ions dictate the measured conductivity.
Types of Chemical Conductors
Chemical substances are broadly categorized based on their charge carrier mechanism. Metallic conductors, such as copper or silver, rely exclusively on electronic conduction. They exhibit high conductivity because of the high concentration and low mass of electrons.
Electrolytic conductors are typically solutions of dissolved salts, acids, or bases, or ionic substances in a molten state. Their ability to conduct current depends entirely on the dissociation of the compound into mobile ions. Their conductivity is generally much lower than that of metals due to the larger size and slower movement of the ions.
A third category includes semiconductors, which have conductivity between that of conductors and insulators. Materials like silicon can be chemically treated, or “doped,” to control the number of charge carriers. Some solid-state materials can also exhibit mixed conduction, utilizing both electronic and ionic charge movement simultaneously, a feature used in battery technology.
Variables that Affect Conductivity
Several factors influence the conductivity of electrolytic solutions, which are frequently analyzed chemically.
Concentration
The concentration of the electrolyte is the most straightforward variable. A greater number of dissolved ions per unit volume means more charge carriers are available, resulting in a proportional increase in conductivity.
Ion Mobility
The mobility of the individual ions is another determining factor, as conductivity depends on how quickly the ions can move through the solvent. Smaller ions tend to have higher mobility, but this is complicated by hydration in aqueous solutions. Smaller ions often bind to more water molecules, forming a larger, heavier hydrated sphere that moves more slowly, decreasing its effective mobility.
Temperature
Temperature also significantly impacts conductivity. As the temperature rises, the kinetic energy of the ions increases, causing them to move faster. Furthermore, increased temperature generally lowers the viscosity of the solvent. This combined effect leads to a noticeable increase in electrolytic conductivity.
Conductivity in Chemical Measurement
Measuring conductivity is a widely used analytical technique in laboratory and industrial settings. This measurement is performed using a specialized instrument called a conductometer, or conductivity meter. The device works by applying an alternating current to two electrodes immersed in the sample solution and measuring the resulting conductance. The standard unit for conductivity is the Siemens per meter (S/m), though micro- and milli-Siemens per centimeter (\(\mu\text{S}/\text{cm}\) or \(\text{mS}/\text{cm}\)) are often used for solutions.
Applications
Conductivity measurement is used for determining the purity of water, as trace amounts of dissolved ionic impurities drastically increase conductivity. Monitoring is also used to track the efficiency of water treatment processes, such as reverse osmosis and desalting operations, by measuring the reduction of ionic solids.
Conductometric titration is another important application used to find the endpoint of a chemical reaction. During the titration, the total ion concentration changes, causing a measurable change in conductivity. Plotting the conductivity against the volume of titrant added reveals a sharp change in the slope, which indicates the reaction’s completion point.