Conductivity meter

A conductivity meter or conductivity sensor is a device for measuring the electrical conductivity in particular of liquids and here specifically of aqueous solutions or electrolytes or water of various purities.

The electrolytic conductivity is a sum parameter for dissolved, dissociated substances. The constituents, which are dissociated as ions , give the water an electrical conductivity which, however, is one or more orders of magnitude lower than that of metallic conductors.

Conductivity sensors are used, among other things:

in liquid chromatography for the determination of ionic compounds
to control the purity of water, e.g. B. also of deionized water, e.g. B. in cooling circuits
in conjunction with glucose oxidase for blood sugar measurement
as an indicator during a titration in conductometry

A handheld device for measuring the electrical conductivity of liquids

Basics

The conductivity depends on:

the concentration of the dissolved substance, the type of its dissociation and the degree of dissociation ,
the value and mobility of the anions or cations formed,
the temperature of the water (as the temperature rises, the conductivity generally increases).

A conductivity detector consists of at least two electrodes in a parallel or coaxial arrangement. The electrodes are made of stainless steel, graphite or, more rarely, platinum or titanium. The electrodes have a defined area and are at a defined distance from one another. Together with mechanical protection and an electrical connection, this arrangement is referred to as a measuring cell.

The liquid between the electrodes behaves like an ohmic resistor ; he surrenders to ${\ displaystyle R}$

{\ displaystyle R = \ rho \ cdot Z}

and is used to calculate the electrical conductivity. This contains a variable that is characteristic of the liquid, its specific resistance , and a variable that depends on the geometric structure of the measuring cell, referred to as the cell constant. A straight conductor with a length and constant cross-sectional area through which charge carriers flow in the longitudinal direction has a resistance . In this ideal case of a conductor through which current flows uniformly . In general, the cell is adjusted to a value of the constant specified in the data sheet, or the cell constant is determined by measuring the resistance on a calibration solution with a known value . Common values are in the range = 0.01… 1 cm ⁻¹ . ${\ displaystyle \ rho}$ ${\ displaystyle Z}$ ${\ displaystyle l}$ ${\ displaystyle q}$ ${\ displaystyle R = \ rho \ cdot l / q}$ ${\ displaystyle Z = l / q}$ ${\ displaystyle \ rho}$ ${\ displaystyle Z}$

The reciprocal of the specific electrical resistance is called electrical conductivity : ${\ displaystyle \ sigma}$

{\ displaystyle \ sigma = {\ frac {1} {\ rho}} = {\ frac {Z} {R}}}

.

When using a four-electrode sensor, the measuring range is significantly higher with 1 µS / cm to 600 mS / cm. Another advantage is the higher insensitivity to interfering resistances, such as. B. from long connecting cables, dirt or polarization. These effects lead to poor results, as they reduce the voltage applied by the electrons to the measurement solution.

In addition to the conductive measuring method described here, there are also conductivity sensors based on the inductive method, which have the advantage that the measuring circuit is operated potential-free from the liquid.

The derived SI unit of electrical conductivity is S / m ( Siemens per meter). The unit S / cm is also widely used. In practice, apart from extremely high salt contents in wastewater, there are predominantly electrical conductivities in the range from mS / cm to S / cm.

For most waters, the following rule of thumb can be applied between conductivity and ion content:

2 µS / cm conductivity corresponds to approx. 1 mg / l ion concentration.

Measuring circuit

Bridge circuit for determining the conductivity of an electrolyte solution. The elements of the chemical cell are set in blue.

For measuring the conductivity of electrolyte solutions or the resistance of the filled measuring cell, direct voltage is often unsuitable because of electrolytic polarization at the electrolyte / electrode interface , which creates a counter voltage. Therefore, in this case, measuring cells are operated like in a Vienna bridge .

The electrodes in the electrolyte solution form a capacitor. Its capacity ( in the figure opposite) in one branch of the bridge circuit must be compensated for by an adjustable capacity in the other branch of the bridge circuit. ${\ displaystyle C _ {\ mathrm {Cell}}}$ ${\ displaystyle C _ {\ text {var}}}$

The ohmic resistance of the measuring cell must be compensated by setting . When the bridge is balanced ${\ displaystyle R_ {3}}$

{\ displaystyle {\ frac {R_ {2}} {R _ {\ mathrm {Cell}}}} = {\ frac {R_ {1}} {R_ {3}}}}

In the case of small changes in the salt concentration, such as in flash photolysis, measurements with bridge circuits are advantageous, as measuring bridges can be designed to be very sensitive to changes and offer a good signal-to-noise ratio (SNR) even with low photon flows.

Theoretical background

The relationships are described in the article Electrolytic conductivity . When drawing conclusions from the ion concentration on the total concentration, the influence of the dissociation equilibrium must be taken into account. Finally, with increasing concentration, an influence of the interionic interaction forces on the conductivity comes into consideration.

Hints:

${\ displaystyle \ mathrm {H} ^ {+}}$ - and ions have by far the greatest ion mobility and thus have a significant share in the conductivity in aqueous solutions. The reason for this is that the charge is transported via the water molecules connected by hydrogen bridges. However, in the range from pH 6 to pH 8, as is found in many natural waters, the contribution of these ions to the overall conductivity is small. ${\ displaystyle \ mathrm {OH} ^ {-}}$
Since the valence of the ions and their mobility are constant in natural waters, which are highly diluted electrolyte solutions, the conductivity of a water at constant temperature is only a function of its ion concentration or its ion activity.

Web links

Individual evidence

↑ Conductivity (EC) and dissolved solids (TDS). (PDF; 99 kB) In: eurotronik.de. Retrieved June 21, 2018 .

[1] Conductivity (EC) and dissolved solids (TDS). (PDF; 99 kB) In: eurotronik.de. Retrieved June 21, 2018 .