A Guide to Conductivity Measuring

Resource Description

The importance of conductivity

Conductivity measurement is an extremely widespread and useful method, especially for quality control purposes. Surveillance of feedwater purity, control of drinking water and process water quality, estimation of the total number of ions in a solution, or direct measurement of components in process solutions can all be performed using conductivity measurements. The high reliability, sensitivity, and relatively low cost of conductivity instrumentation make it a potential primary parameter of any good monitoring program. Some applications are measured in units of resistivity, the inverse of conductivity. Other applications require the measurement of total dissolved solids (TDS), which is related to conductivity by a factor dependent upon the level and type of ions present. Conductivity measurements cover a wide range of solution conductivity from pure water at less than 1x10-7 S/cm to values of greater than 1 S/cm for concentrated solutions. In general, the measurement of conductivity is a rapid and inexpensive way of determining the ionic strength of a solution. However, it is a nonspecific technique, unable to distinguish between different types of ions, giving instead a reading that is proportional to the combined effect of all the ions present.

What is conductivity?

Conductivity is the ability of a solution, a metal, or a gas - in brief, all materials - to pass an electric current. In solutions, the current is carried by cations and anions whereas in metals it is carried by electrons. How well a solution conducts electricity depends on a number of factors:

• Concentration
• Mobility of ions
• Valence of ions
• Temperature

All substances possess some degree of conductivity. In aqueous solutions, the level of ionic strength varies from the low conductivity of ultra-pure water to the high conductivity of concentrated chemical samples.
 

How is conductivity measured? Conductivity may be measured by applying an alternating electrical current (I) to two electrodes immersed in a solution and measuring the resulting voltage (V). During this process, the cations migrate to the negative electrode, the anions to the positive electrode, and the solution acts as an electrical conductor.

What is a conductive solution?

Conductivity is typically measured in aqueous solutions of electrolytes. Electrolytes are substances containing ions, i.e., solutions of ionic salts or of compounds that ionize in solution. The ions formed in the solution are responsible for carrying the electric current. Electrolytes include acids, bases, and salts and can be either strong or weak. Most conductive solutions measured are aqueous solutions, as water has the capability of stabilizing the ions formed by a process called solvation.

• Strong electrolytes - are substances that are fully ionized in solution.
• Weak electrolytes - are substances that are not fully ionized in solution.

The Conductivity Meter

An electrical conductivity meter measures the electrical conductivity in a solution. It is commonly used in hydroponics, aquaculture, and freshwater systems to monitor the number of nutrients, salts, or impurities in the water.

A typical conductivity meter applies an alternating current (I) at an optimal frequency to two active electrodes and measures the potential (V). Both the current and the potential are used to calculate the conductance (I/V). The conductivity meter then uses the conductance and cell constant to display the conductivity.

Conductivity (at sample temperature) = cell constant x conductance

NOTE: the current source is adjusted so that the measured potential (V) is equal to the reference potential (Er) (approximately ± 200 mV).

Steps for Conductivity Measurement

1. Refer to the operation section of the meter manual to prepare the conductivity electrode and meter. The meter will select the range automatically. For most accurate results, calibrate the meter before use or check the accuracy of the meter with a known conductivity standard. Refer to the meter user manual for calibration and measurement options.
2. Laboratory tests: Immerse the probe in a beaker containing the sample solution. Move the probe up and down and tap it on the beaker to remove bubbles from the electrode. Field tests: Immerse the probe in the sample solution. Move the probe up and down to remove bubbles from the electrode. The vent holes should be completely submerged.
3. Turn the meter on. Make sure that the meter is set to measure conductivity.
4. Rinse the probe thoroughly with deionized water after each measurement. To display other units such as TDS (Total Dissolved Solids), salinity, or resistivity (HQd only), refer to the meter user manual.

Useful Tips

• Conductivity measurement is temperature-dependent (if the temperature increases so the conductivity value does). When performing conductivity measurements, it is advisable to choose a cell with a built-in temperature sensor or use a separate sensor.
• Remember to calibrate your conductivity cell regularly as the cell constant may vary due to changes in electrode surface due to contamination, for example.