Conductivity (electrolytic)
Measure of a solution's ability to conduct electricity via ions.
Conductivity, also called specific conductance, measures how well an electrolyte solution can carry an electric current. Its SI unit is the siemens per meter (S/m). This measurement is a common, quick, and low-cost method for determining a solution’s ionic content in industrial and environmental settings—for instance, it is routinely used to track the performance of water purification systems.
Conductivity often correlates directly with total dissolved solids (TDS). At 25 °C, high-quality deionized water has a conductivity of 0.05501 ± 0.0001 μS/cm, which corresponds to a specific resistivity of 18.18 ± 0.03 MΩ·cm. However, when salt solutions are prepared in unsealed beakers, the conductivity of the purified water can be 10 to 20 times higher. Typical drinking water ranges from 200 to 800 μS/cm, while seawater measures about 50 mS/cm (0.05 S/cm). Electrolytic conductivity spans from roughly 10⁻¹⁰ S/m for purified toluene to about 10 S/m for recently discovered highly concentrated “water-in-salt” solutions.
Traditionally, the conductivity of aqueous and other polar solutions is measured by placing the electrolyte in a Wheatstone bridge. Dilute solutions obey Kohlrausch’s law, which describes concentration dependence and the additivity of ionic contributions. Lars Onsager later provided a theoretical basis for this law by extending the Debye–Hückel theory. Low- and non-polar solutions have very low conductivity, also due to ion motion in an electric field, but the nature and solvation of the ions differ; these are discussed under conductivity (non-aqueous). Such measurements typically use probes with a low cell constant and a low-frequency electric field.
Notation in this field can be confusing. Rho (ρ) reliably stands for specific resistivity, usually in ohm-meters. Kappa (κ) and sigma (σ) are inversely proportional to resistivity, with κ preferred in classical electrochemistry and σ in Maxwellian electrodynamics. However, both symbols have other meanings that can mislead: in electrochemistry, κ also denotes the inverse Debye screening length, while in electrodynamics, σ can refer to double-layer charge density.
The SI unit for conductivity is S/m, typically referenced to 25 °C unless stated otherwise. The more traditional unit is μS/cm.
- Si unit
- S/m
- Traditional unit
- μS/cm
- High quality deionized water conductivit
- 0.05501 ± 0.0001 μS/cm at 25 °C
- High quality deionized water resistivity
- 18.18 ± 0.03 MΩ⋅cm
- Typical drinking water range
- 200–800 μS/cm
- Sea water conductivity
- about 50 mS/cm (0.05 S/cm)
- Conductivity to tds conversion factor ra
- 0.54 to 0.96 (mg/kg per μS/cm)
Lore & Background
Conductivity of aqueous and other polar solutions is traditionally determined by connecting the electrolyte in a Wheatstone bridge. Dilute solutions follow Kohlrausch's law of concentration dependence and additivity of ionic contributions, with Lars Onsager giving a theoretical explanation by extending Debye–Hückel theory. Notation in this domain is treacherous: ρ reliably denotes specific resistivity, while κ and σ are inversely proportional to resistivity, with κ preferred in classical electrochemistry and σ in Maxwellian electrodynamics, though both have confounds—κ also denotes inverse Debye screening length, and σ also denotes double-layer charge density.
The electrical conductivity of a solution is measured by determining the resistance between two flat or cylindrical electrodes separated by a fixed distance, using an alternating voltage to minimize water electrolysis. Typical frequencies used are in the range 1–3 kHz. A wide variety of instrumentation is available, including electrode-based sensors (static or flexible design) and inductive sensors, with calibration typically done using KCl solutions of known conductivity. Conductivity is highly temperature-dependent, and many commercial systems offer automatic temperature correction.
Reader's Guide
Conductivity is a fundamental parameter in aquarium water chemistry because it directly reflects the ionic content of the water, which affects fish health, biological filtration, and the effectiveness of additives. The article notes that conductivity is linked directly to total dissolved solids (TDS), with a conversion factor that varies between 0.54 and 0.96 depending on chemical composition, though a typical assumption using sodium chloride gives about 0.64 mg of NaCl per kg of water per μS/cm. For aquarium purposes, understanding the conductivity range—from high-quality deionized water at 0.05501 μS/cm to typical drinking water at 200–800 μS/cm and sea water at about 50 mS/cm—helps aquarists maintain appropriate conditions for freshwater or marine systems. The measurement technique, using alternating voltage and calibrated cells, ensures reliable monitoring. The article also emphasizes that conductivity is temperature-dependent, so automatic temperature correction is common in commercial meters. The legacy of conductivity measurement lies in its fast, inexpensive, and reliable nature, making it a standard tool for tracking water quality trends in aquariums, just as it is in industrial and environmental applications.
Did You Know?
- Typical drinking water ranges from 200–800 μS/cm, while sea water is about 50 mS/cm.
- The conversion from conductivity to total dissolved solids varies between 0.54 and 0.96 mg/kg per μS/cm.
- Conductivity is traditionally measured using a Wheatstone bridge with alternating voltage to minimize electrolysis.
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