Conductivity measurement of water: A quick look

Salinity and conductivity measure the water's ability to conduct electricity, which provides a measure of what is dissolved in water. A higher conductivity value indicates that there are more chemicals dissolved in the water. It is the opposite of resistance. Pure, distilled water is a poor conductor of electricity. When salts and other inorganic chemicals dissolve in water, they break into tiny, electrically charged particles called ions. Ions increase the water’s ability to conduct electricity. Common ions in water that conduct electrical current include sodium, chloride, calcium, and magnesium. Because dissolved salts and other inorganic chemicals conduct electrical current, conductivity increases as salinity increases. Organic compounds, such as sugars, oils, and alcohols, do not form ions that conduct electricity.

The SI unit of conductivity is Siemens per meter (S/m). In many cases, conductivity is linked directly to the total dissolved solids (T.D.S.). High-quality deionized water has a conductivity of about 0.5 μS/cm at 25 °C, typical drinking water is in the range of 200 - 800 μS/cm, while seawater is about 50,000 μS/cm. The conversion of conductivity to the total dissolved solids depends on the chemical composition of the sample and can vary between 0.54 and 0.96. Typically, the conversion is done assuming that the solid is sodium chloride, i.e., 1 μS/cm is then equivalent to about 0.64 mg of NaCl per kg of water.

Measurement

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. Electrical resistivity called specific electrical resistance. Electrical conductivity or specific conductance is the reciprocal of electrical resistivity. Electrical conductivity is a fundamental property of a material that quantifies how strongly it resists or conducts electric current. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-meter (Ω⋅m). For example, if a 1 m × 1 m × 1 m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then the resistivity of the material is 1 Ω⋅m. The process follows the use of an alternating voltage between electrodes. The resistance is measured by a conductivity meter. Typical frequencies used are in the range of 1–3 kHz.

A wide variety of instrumentation is commercially available.

Fundamental facts

Temperature vs EC

The conductivity process in aqueous solutions is by means of ionic motion. The conductivity invariably increases with increasing temperature. It is affected by the nature of the ions, and by the viscosity of the water. In low ionic concentrations (very pure water), the ionization of the water furnishes an appreciable part of the conducting ions. All these processes are quite temperature-dependent, and as a result, the conductivity has substantial dependence on temperature. This dependence is usually expressed as a relative change per degc at a particular temperature, commonly as percent/degc at 25 degc, and this is called the slope of the solution.

Ultra-pure water has by far the largest slope, 5.2%/degc, while ionic salts run about 2%/degc in the middle ranges. Acids, alkalis, and concentrated salt solutions run somewhat lower, typically 1.5%/degc. Non-aqueous materials such as oleum (H2SO4 with dissolved SO2) and conducting organics have quite different temperature dependences. From these numbers, it is obvious that a small difference in temperature makes a large difference in conductivity, and the effect is very troublesome when a high degree of accuracy is required. In making conductivity readings at high and low temperatures, the data is usually normalized to 25 degc, i.e., it is stated as what the reading is with a 25 degc solution. Fortunately, temperature sensors are available which have characteristics similar to those of the solutions to be tested. A modern technique uses a microprocessor and an associated "lookup table" which contains the temperature response data of the solution.

TDS vs EC

Researchers have done various investigations to find out the precise mathematical correlation between these two parameters, so TDS concentration can be simply calculated from the EC value. The correlation of these parameters can be estimated by the following equation: TDS (mg L) = k x EC (μS cm). The value of k will increase along with the increase of ions in water. However, the relationship between conductivity and TDS is not directly linear; it depends on the activity of specific dissolved ions average activity of all ions in the liquid and ionic strength

Limitations

[1] Complete dissociation. Ion association may take place, particularly with ions of higher charge.

[2] Weak electrolytes. A weak electrolyte will not fully dissociate.

[3] Ions are spherical, not point charges, and are not polarized. Many ions such as the nitrate ion, NO3−, are not spherical. Polyatomic ions are also polarizable.

[4] Role of the solvent. The solvent is not a structureless medium but is made up of molecules. The water molecules in an aqueous solution are both dipolar and polarizable.

[5] The ionic radius is not the same. It is specific to every ion and does not occupy the same space and space depends on how they arrange themselves. This ionic space every time you measure the conductivity of the same solution is different.

[6] Research results have found that the correlation between TDS and EC is not always linear. The ratio is not only strongly influenced by salinity contents, but also by materials contents

Credit: Google