The conductivity of an electrolyte solution is a measure of its ability to conduct electricity. Thus, the electrolytic conductivity of a solution is directly dependent upon the concentration of its solute(s). The SI unit of conductivity is siemens per metre (S/m).
Conductivity measurements are performed routinely in many laboratory and field applications as a fast, inexpensive and reliable way of determining 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 water purification systems.
Conductivity measurements are temperature dependent with conductivity increasing with temperature. Thus the conductivity measurement must be reported with the temperature, either the actual temperature at which it was measured, or a temperature corrected result. The accepted standard temperature for conductivity measurements is 25°C.
To ensure quality results conductivity meters require frequent calibration against reference solutions of certified conductivity.
Whilst a variety of solutes may be chosen for conductivity reference solutions, standards organisations such as ISO and ASTM specify the use of potassium chloride and have established the values for conductivity at specified concentrations. In turn, national metrology institutes (NMIs) provide primary reference materials derived from the standards set by these organisations.
Within the ROMIL-PrimAg® scheme potassium chloride is readily traceable to primary silver and hence ultimately to the SI unit, the mole. Using the published and widely agreed data on potassium chloride aqueous solutions ROMIL offer reference solutions for electrolytic conductivity at the concentrations specified by the standards organisations. Such solutions are considered as primary calibrants, on a par with those offered by NMIs.
However, the primary calibrants can be inconvenient to use, being neither round numbers nor extensive enough in range for routine conductivity measurements.
Secondary reference solutions address such restrictions. They tend to be convenient round numbers ranging in values typically from 5 to 500,000 µS/cm @ 25°C and are certified by measurement on a precision conductivity meter that has been calibrated through use of the primary calibrants.
ROMIL-PrimAg® conductivity primary calibrants are accredited to ISO 17025 calibration and produced under ISO Guide 34. All ROMIL-PrimAg® conductivity reference solutions are supplied with a Certificate of Calibration reporting the conductivity value and associated uncertainty.
PrimAg is a registered trademark of ROMIL Ltd.