Units and Measurement of Conductivity
– The SI unit of conductivity is S/m.
– The traditional unit of conductivity is μS/cm.
– Megohm-cm is sometimes used to express conductivity.
– Conductivity can be given in microsiemens, which is equal to μS/cm.
– The conversion of conductivity to total dissolved solids depends on the chemical composition of the sample.
– The electrical conductivity of a solution is measured by determining the resistance between two electrodes.
– An alternating voltage is used to minimize water electrolysis.
– Conductivity meters are used to measure the resistance.
– Different types of electrode sensors are used for different conductivities.
– Conductivity sensors are typically calibrated with KCl solutions.
– Resistance (R) is proportional to the distance between the electrodes and inversely proportional to the cross-sectional area of the sample (A).
– Specific conductance (κ) is the reciprocal of specific resistance.
– Conductivity is temperature-dependent.
– The conductance (G) is the reciprocal of the resistance.
– The specific conductance (κ) can be calculated by multiplying the conductance (G) with the cell constant (C).
Theory of Conductivity
– The specific conductance of a solution depends on the concentration of the electrolyte.
– Molar conductivity (Λm) is the specific conductance divided by concentration.
– Strong electrolytes follow Kohlrausch’s Law at low concentration.
– The conductivity of strong electrolytes becomes directly proportional to concentration at low concentrations.
– The limiting molar conductivities of individual ions can be determined.
– Weak electrolytes are never fully dissociated.
– The relationship between conductivity and concentration is not linear for weak electrolytes.
– Weak electrolytes become more fully dissociated at weaker concentrations.
– For low concentrations of well-behaved weak electrolytes, the degree of dissociation increases.
– The behavior of weak electrolytes can be described using the Debye-Hückel-Onsager theory.
Higher Concentrations and Mixed Solvent Systems
– Kohlrausch’s law and the Debye-Hückel-Onsager equation break down as the concentration of the electrolyte increases.
– As concentration increases, the average distance between cation and anion decreases, leading to more interactions between close ions.
– Ion association is often assumed to occur at higher concentrations, forming ion pairs.
– Ion-association constant (K) can be derived to describe the equilibrium between ions.
– The inclusion of an ion-association term extends the range of agreement between theory and experimental conductivity data.
– The limiting equivalent conductivity of solutions based on mixed solvents depends on the nature of the alcohol.
– For methanol, the minimum conductivity is observed at 15 molar% water.
– For ethanol, the minimum conductivity is observed at 6 molar% water.
Conductivity versus Temperature
– The conductivity of a solution generally increases with temperature due to increased ion mobility.
– Reference values are reported at an agreed temperature, usually 298K (≈ 25°C or 77°F).
– Compensated measurements are made at a convenient temperature, but the reported value is calculated as if measured at the reference temperature.
– The Arrhenius equation is used to determine the activation energy (Ea) based on conductivity measurements versus temperature.
– The change in conductivity due to the isotope effect for deuterated electrolytes is significant.
Applications of Conductivity Measurement
– Measured conductivity is a good indicator of the presence or absence of conductive ions in solution.
– Conductivity measurements are used to monitor water quality in public water supplies, hospitals, and industries.
– Conductivity measurements can be used to determine the amount of total dissolved solids (TDS) if the composition of the solution and its conductivity behavior are known.
– TDS measurements are used to assess water purity and are important in aquariums for maintaining specific dissolved solids levels.
– Conductivity measurements can be combined with other methods to increase sensitivity for detecting specific types of ions, such as in boiler water technology. Source: https://en.wikipedia.org/wiki/Conductivity_(electrolytic)
Conductivity (or specific conductance) of an electrolyte solution is a measure of its ability to conduct electricity. The SI unit of conductivity is siemens per meter (S/m).
Conductivity measurements are used routinely in many industrial and environmental applications as a fast, inexpensive and reliable way of measuring the ionic content in a solution. For example, the measurement of product conductivity is a typical way to monitor and continuously trend the performance of water purification systems.
In many cases, conductivity is linked directly to the total dissolved solids (TDS).
High quality deionized water has a conductivity of
at 25 °C.
This corresponds to a specific resistivity of
.
The preparation of salt solutions often takes place in unsealed beakers. In this case the conductivity of purified water often is 10 to 20 times higher. A discussion can be found below.
Typical drinking water is in the range of 200–800 μS/cm, while sea water is about 50 mS/cm (or 0.05 S/cm).
Conductivity is traditionally determined by connecting the electrolyte in a Wheatstone bridge. Dilute solutions follow Kohlrausch's Laws of concentration dependence and additivity of ionic contributions. Lars Onsager gave a theoretical explanation of Kohlrausch's law by extending Debye–Hückel theory.
