Conductivity (electrolytic)

The 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 the water purification systems.

In many cases, conductivity is linked directly to the total dissolved solids (T.D.S.). High quality deionized water has a conductivity of about 5.5 μS/m, typical drinking water in the range of 5-50 mS/m, while sea water about 5 S/m (i.e., sea water's conductivity is one million times higher than deionized water).

Conductivity is traditionally determined by measuring the AC resistance of the solution between two electrodes. Dilute solutions follow Kohlrausch's Laws of concentration dependence and additivity of ionic contributions. Onsager gave a theoretical explanation of Kohlrausch's law by extending Debye–Hückel theory.

Units

The SI unit of conductivity is S/m and, unless otherwise qualified, it refers to 25 °C (standard temperature). Often encountered in industry is the traditional unit of μS/cm. 10⁶ μS/cm = 10³ mS/cm = 1 S/cm. The numbers in μS/cm are higher than those in μS/m by a factor of 100 (i.e., 1 μS/cm = 100 μS/m). Occasionally a unit of "EC" (electrical conductivity) is found on scales of instruments: 1 EC = 1 μS/cm. Sometimes encountered is so-called mho (reciprocal of ohm): 1 mho/m = 1 S/m. Historically, mhos antedate Siemens by many decades; good vacuum-tube testers, for instance, gave transconductance readings in micromhos.

The commonly used standard cell has a width of 1 cm, and thus for very pure water in equilibrium with air would have a resistance of about 10⁶ ohm, known as a megohm, occasionally spelled as "megaohm". Ultra-pure water could achieve 18 megohms or more. Thus in the past megohm-cm was used, sometimes abbreviated to "megohm". Sometimes, a conductivity is given just in "microSiemens" (omitting the distance term in the unit). While this is an error, it can often be assumed to be equal to the traditional μS/cm. The typical conversion of conductivity to the total dissolved solids is done assuming that the solid is sodium chloride: 1 μS/cm is then an equivalent of about 0.6 mg of NaCl per kg of water.

Molar conductivity has the SI unit S m² mol⁻¹. Older publications use the unit Ω⁻¹ cm² mol⁻¹.

Measurement

Main article: Electrical conductivity meter

Principle of the measurement

A conductivity meter and probe

The electrical conductivity of a solution of an electrolyte is measured by determining the resistance of the solution between two flat or cylindrical electrodes separated by a fixed distance. An alternating voltage is used in order to avoid electrolysis. The resistance is measured by a conductivity meter. Typical frequencies used are in the range 1–3 kHz. The dependence on the frequency is usually small, but may become appreciable at very high frequencies, an effect known as the Debye–Falkenhagen effect.

A wide variety of instrumentation is commercially available. There are two types of cell, the classical type with flat or cylindrical electrodes and a second type based on induction. Many commercial systems offer automatic temperature correction.

Definitions

Resistance, R, is proportional to the distance, l, between the electrodes and is inversely proportional to the cross-sectional area of the sample, A. Writing ρ (rho) for the specific resistance (or resistivity),

$R={\frac {l}{A}}\rho .$

In practice the conductivity cell is calibrated by using solutions of known specific resistance, ρ^*, so the quantities l and A need not be known precisely. If the resistance of the calibration solution is R^*, a cell-constant, C, is derived.

$R^{*}=C\times \rho ^{*}$

The specific conductance, κ (kappa) is the reciprocal of the specific resistance.

$\kappa ={\frac {1}{\rho }}={\frac {C}{R}}$

Conductivity is also temperature-dependent.

Theory

The conductivity of a solution containing one electrolyte depends on the concentration of the electrolyte. Therefore it is convenient to divide the conductivity by concentration. This quotient is termed molar conductivity, is denoted by Λ_m

$\Lambda _{m}={\frac {\kappa }{c}}$

Strong electrolytes

Strong electrolytes are believed to dissociate completely in solution. The conductivity of a solution of a strong electrolyte at low concentration follows Kohlrausch's Law

$\Lambda _{m}=\Lambda _{m}^{0}-K{\sqrt {c}}$

where $\Lambda _{m}^{0}$ is known as the limiting molar conductivity, K is an empirical constant and c is the electrolyte concentration. (Limiting here means "at the limit of the infinite dilution".)

Moreover, Kohlrausch also found that the limiting conductivity of anions and cations are additive: the conductivity of a solution of a salt is equal to the sum of conductivity contributions from the cation and anion.

$\Lambda _{m}^{0}=\nu _{+}\lambda _{+}^{0}+\nu _{-}\lambda _{-}^{0}$

where:

• $\nu _{+}$ and $\nu _{-}$ are the number of moles of cations and anions, respectively, which are created from the dissociation of 1 mole of the dissolved electrolyte;
• $\lambda _{+}^{0}$ and $\lambda _{-}^{0}$ are the limiting molar conductivities of the individual ions.

The following table gives values for limiting molar conductivities for selected ions.

Limiting ion conductivity in water at 298 K

Cations	λ+0 /mS m2mol-1	anions	λ-0 /mS m2mol-1
H+	34.96	OH-	19.91
Li+	3.869	Cl-	7.634
Na+	5.011	Br-	7.84
K+	7.350	I-	7.68
Mg2+	10.612	SO42-	15.96
Ca2+	11.900	NO3-	7.14
Ba2+	12.728	CH3CO2-	4.09

A theoretical interpretation of these results was provided by the Debye–Hückel.

$\Lambda _{m}=\Lambda _{m}^{0}-(A+B\Lambda _{m}^{0}){\sqrt {c}}$

where A and B are constants that depend only on known quantities such as temperature, the charges on the ions and the dielectric constant and viscosity of the solvent. As the name suggests, this is an extension of the Debye–Hückel theory, due to Onsager. It is very successful for solutions at low concentration.

Weak electrolytes

A weak electrolyte is one that is not fully dissociated . Typical weak electrolytes are weak acids and weak bases. The concentration of ions in a solution of a weak electrolyte is less than the concentration of the electrolyte itself. For acids and bases the concentrations can be calculated when the value(s) of the acid dissociation constant(s) is(are) known.

For a monoprotic acid, HA, with a dissociation constant K_a, an explicit expression for the conductivity as a function of concentration, c, known as Ostwald's dilution law, can be obtained.

${\frac {1}{\Lambda _{m}}}={\frac {1}{\Lambda _{m}^{0}}}+{\frac {\Lambda _{m}c}{K_{a}(\Lambda _{m}^{0})^{2}}}$

Higher concentrations

Both Kolrausch's law and the Debye-Hückel-Onsager equation break down as the concentration of the electrolyte increases above a certain value. The reason for this is that as concentration increases the average distance between cation and anion decreases, so that there is more inter-ionic interaction. Whether this constitutes ion-association is a moot point. However, It has often been assumed that cation and anion interact to form an ion-pair. Thus the electrolyte is treated as if it were like a weak acid and a constant, K, can be derived for the equilibrium

A⁺ + B^- ⇌ A⁺B^-; K=[A⁺][B^-]/[A⁺B^-]

Davies describes the results of such calculations in great detail, but states that K should not necessarily be thought of as a true equilibrium constant, rather, the inclusion of an "ion-association" term is useful in extending the range of good agreement between theory and experimental conductivity data. Various attempts have been made to extend Onsager's treament to more concentrated solutions.

The existence of a so-called conductance minimum has proved to be a controversial subject as regards interpretation. Fuoss and Kraus suggested that it is caused by the formation of ion-triplets, and this suggestion has received some support recently.

Applications

Notwithstanding the difficulty of theoretical interpretation, conductivity measurements are used extensively in many industries. For example, conductivity measurements are used to monitor quality in public water supplies, in hospitals, in boiler water and industries which depend on water quality such as brewing. This type of measurement is not ion-specific; it can sometimes be used to determine the amount of total dissolved solids (T.D.S.) if the composition of the solution and its conductivity behavior are known.

Sometimes, conductivity measurements are linked with other methods to increase the sensitivity of detection of specific types of ions. For example, in the boiler water technology, the boiler blowdown is continuously monitored for "cation conductivity", which is the conductivity of the water after it has been passed through a cation exchange resin. This is a sensitive method of monitoring anion impurities in the boiler water in the presence of excess cations (those of the alkalizing agent usually used for water treatment). The sensitivity of this method relies on the high mobility of H⁺ in comparison with the mobility of other cations or anions.

Conductivity detectors are commonly used with ion chromatography.

See also

• Debye–Falkenhagen effect
• Svante Arrhenius
• Alfred Werner - coordination chemistry
• Conductimetric titration - methods to determine the equivalence point

External links

• Cole-Parmer Technical Library Conductivity
• Conductivity measurement PowerPoint presentation