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Electrolytical conductivity of reference solutions

Range of services
Working Group 3.41 offers a calibration service in order to measure the conductivity values of reference solutions that are traceable to the International System of Units (SI). The table below gives an overview of the range of services.

Measuring range

Standard measurement uncertainty *

2 - 150 µS⋅cm-1

0.01 - 0.2 µS⋅cm-1

150 - 1500 µS⋅cm-1

0.03 - 0.3 µS⋅cm-1

1.5 - 15 mS⋅cm-1

0.0003 - 0.005 mS⋅cm-1

1.5 -25 S⋅m-1

0.0005 - 0.009 S⋅m-1

* The values indicated each refer to the measuring range limits.

We generally calibrate aqueous KCl or NaCl solutions of manufacturers of reference solutions or of calibration laboratories in the range between 50 µS∙cm-1 and 25 S∙m-1 at 25 °C. In addition, we are able to measure the conductivity of any solution between 0.1 µS∙cm-1 and 25 S∙m-1 and between 5 °C and 40 °C with primary traceability to the SI. We also welcome calibrations of extraordinary solutions. Such a solution must, however, be sufficiently stable and harmless to health and it must not corrode the components of the measuring cell (essentially borosilicate glass, pt electrodes, aluminum oxide ceramics). Please contact us for further information.

For the calibration of conductivity measuring cells that are intended to be used in water with conductivities below 150 µS∙cm-1, we offer a Opens internal link in current windowcalibration service in a closed circuit.

Measuring principle
The measurement of the electrolytical conductivity of electrolytical solutions is conducted using a primary piston-type cell that was developed at PTB. The cylindrical measuring cell has a platinum electrode at the bottom end. A second platinum electrode is positioned on a movable piston made of aluminum oxide ceramics that can be precisely moved within the measuring cell by means of a positioning system. The entire measuring cell is located in a temperature-controlled air bath by means of which the measurement temperatures can be adjusted stably between 5 °C and 40 °C.


At two different positions of the movable electrode, impedances are measured by means of a precision measuring bridge at several frequencies ƒ. The DC resistances R1 and R2 of the solution, which correspond to the electrode positions, can usually be determined through extrapolation of the real parts of the measured impedances for 1/ƒ towards 0. The selection of the frequency range thereby depends on the type of solution and its concentration. From the resistance values gained from this measurement as well as from the cell diameter D and the distance Δl of the two measurement positions of the movable electrode, the conductivity κ is calculated:

κ(t) = 4 Δl / πD2 (R2 (t) - R1 (t))

 

Furthermore, the temperature t within the measuring cell is measured by means of a Pt100 platinum resistance sensor. If the temperature measured deviates from the aimed-at nominal temperature t0, the conductivity κ(t) measured at t is mathematically corrected to the conductivity κ(t0) at t0 by means of a solution-specific temperature coefficient ακ.

 
κ(t0) = κ(t) (1 + ακ (t - t0))-1


Usually, the deviations are in a range of 30 mK at maximum. All quantities are traceable to the SI units so that the conductivity determined in this way is a quantity that is traceable to the SI.