Electrical conductivity determines a substance’s ability to carry an electrical current. This physical property is particularly relevant in liquid solutions, where the presence of dissolved ions, such as salts, acids, and bases, enables the flow of electricity. Measuring conductivity is a standard practice across various scientific and industrial fields. It offers a rapid and non-destructive way to monitor solution purity, concentration, and quality. Common applications include water quality monitoring in environmental science, ensuring consistency in chemical manufacturing, and checking purity in pharmaceutical production.
What Conductivity Measures
Conductivity measures how easily a solution allows an electrical charge to pass through it, directly related to the concentration of free ions present. This property is technically called specific conductance and is the mathematical inverse of electrical resistivity. Highly resistive materials, like pure water, have very low conductivity, while solutions with many dissolved salts have high conductivity.
The standard international unit for conductivity is the Siemens per meter (S/m). For practical applications involving water and chemical solutions, smaller units are used, such as micro-Siemens per centimeter (\(\mu\)S/cm) or milli-Siemens per centimeter (mS/cm). These measurements quantify electrolytic conductivity, which involves the movement of charged ions through a liquid medium, distinct from metallic conductivity (flow of electrons through a solid).
The Physics of Electrical Measurement
The measurement process uses a conductivity cell consisting of at least two electrodes immersed in the sample solution. To prevent polarization (ion accumulation on electrodes), the meter applies a low-level alternating current (AC) voltage, typically 1 to 3 kilohertz, instead of direct current (DC). This alternating field constantly reverses the direction of ion movement, keeping the electrode surfaces clean.
The meter measures the electrical resistance (\(R\)) of the solution section between the two electrodes. This resistance is then mathematically converted into conductance (\(G\)), since conductance is the reciprocal of resistance (\(G = 1/R\)).
To derive the specific conductivity (\(\kappa\)), which is the intrinsic property of the solution independent of the equipment’s geometry, the meter uses a predetermined geometric factor called the cell constant (\(K\)). The cell constant is calculated by multiplying the measured conductance by a value that accounts for the distance between the electrodes and their surface area.
The cell constant (\(K\)) normalizes the measurement to a standard unit of distance, ensuring the result accurately reflects the solution’s properties rather than the sensor’s size or shape. This constant is determined during calibration by measuring a reference solution with known conductivity, such as potassium chloride (KCl). Conductivity cells are manufactured with different constants to optimize the measurement range for specific applications.
Using the Conductivity Meter
A modern conductivity measurement system consists of a probe, which houses the electrodes and a temperature sensor, and a meter body that processes the signal and displays the result. The embedded temperature sensor, often a platinum resistance thermometer (RTD), is an integral part of the probe, as it accounts for the most significant variable in the measurement.
The first step in using the meter is calibration, which involves standardizing the cell constant against a certified reference solution, commonly KCl, with a known conductivity value at a specific temperature. This ensures the meter’s readings are accurate across the instrument’s measuring range. Once calibrated, the probe can be immersed directly into the sample for measurement.
Temperature compensation is automatically applied by the meter to provide a standardized, comparable result. The conductivity of a solution increases significantly as its temperature rises because the ions move faster. To account for this, the meter automatically corrects the measured conductivity value to what it would be at a standard reference temperature, which is conventionally \(25^\circ\text{C}\). This is done using an algorithm that applies a correction factor, often a linear coefficient of approximately 2% per degree Celsius difference from the reference temperature, ensuring measurements taken at various temperatures are comparable.