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Thursday, April 4, 2019

Electrolytic conductivity meters

An operational principle of electrolytic conductivity meters is based on the relationship between electrical conductivity and concentration of solutions.

The ability to conduct electricity is called electrical conductance, which is reciprocal of electrical resistance:

electrical conductance = 1/electrical resistance.

The unit for electrical conductance is Siemens: 1S = 1/Ohm.

Electrical conductivity is equal to electrical conductance of a volume of the material of unit length and area:

electrical conductivity = electrical conductance*length/area.

The unit for electrical conductivity is S/m. Few solutions exhibit electrical conductivities as great as 1 S/cm. So, the most commonly used units are mS/cm and mS/cm.

Electrolytic conductivity is usually defined as electrical conductance of a unit cube of solution as measured between opposite faces. It has the same units as electrical conductivity.

In conductive or electrolytic solutions positive ions (cations) move toward the cathode, and negative ions (anions) move toward the anode. Reduction and oxidation take place on the cathode and anode, respectively. Electrolytic conductivity of a solution mostly depends on the concentration and mobility of all ions in the solution. The latter depends on the ion size, charge, dielectric constant of the solvent, temperature and viscosity of the solution. Electrolytic conductivity of a mixture of solutions is proportional to the sum of relative concentration of each components and the mobility of ions. Therefore, conductivity meters are used for electrolytic conductivity measurements of one component solutions only. Fig. 7.4 shows typical conductivity curves for NaCl solution in water.

Electrolytic conductivity is usually measured by placing electrodes in contact with an electrolytic solution. In this case electrical conductance between electrodes is related to electrolytic conductivity of the solution. Since the conductivity cell has unchanged dimensions, so by measuring electrical conductance of the solution in this cell, and thus determining the cell constant, we can relate thus measured electrical conductance to the actual value of electrolytic conductivity.

Fig. 7.5 schematically shows an electrolytic conductivity meter, which employs an alternating current Wheatstone bridge in order to avoid polarisation of measuring electrodes. A conductivity cell is immersed in the solution 1. This cell consist of an insulating shield 2 made of either glass or epoxy, or polystyrene, or Teflon. Two metal electrodes 3 are placed inside this shield. These electrodes are made of either stainless steel or nickel, or platinum, or gold, or platinum-plated metals. The shield is perforated to provide good contact of solution with these electrodes. The operational principle of a Wheatstone bridge is described in 3.5. We measure an electrical resistivity (r) of the electrolytic solution between cell electrodes. An electrical resistivity is defined as an electrical resistance of a conductor of unit cross-sectional area and unit length, as follows: r=R*A/L, (Ohm*m), where, R is the electrical resistance of the conductor (Ohm), A is the cross-section area of the conductor (m2), and L is the length of the conductor (m).

Figure 7.4. Electrolytic conductivity of NaCl solutions. 

Since temperature of an electrolytic solution has influence on its electrolytic conductivity, therefore, we should be able to introduce temperature correction (compensation). For this purpose an electrode sensor filled with a reference liquid, which has a thermal coefficient of electrolytic conductivity approximately equal to that of the measuring solution, is employed. This sensor is immersed in the measuring electrolytic solution near the measuring conductivity cell, and through conductors is connected to the side of the bridge adjacent to that side of the bridge which is connected to the measuring conductivity cell. Since temperatures of the conductivity measuring cell and the sensor cell are equal, then variations of temperature of the electrolytic solution will not have influence on the results of electrolytic conductivity measurements.

Figure 7.5. Electrolytic conductivity meter.

Article Source:: Dr. Alexander Badalyan, University of South Australia


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