eDAQ has a number of options for monitoring the solution conductivity (which is related to the total ionic concentration). We supply:
- Conductivity isoPod for use with an e-corder and Chart software.
- USB Conductivity isoPod for direct connection to a computer with Pod-Vu software.
- Quad MF isoPod to measure four conductivities simultaneously, with direct USB connection to a computer using Pod-Vu software.
- e-corder systems and Chart software to connect to a third-party conductivity meter with an analog output.
and suitable electrodes:
- ET915 'Dip In' Conductivity Electrode, k=1
- ET908 'Flow-Thru' Conductivity Electrode (93 µL), k=1
- ET916 'Flow-Thru' Conductivity Electrode (17 µL), k=1
- ET901, k=0.1
- ET902, k=1
- ET903, k=10
For specialist conductivity monitoring tasks such as for capillary electrophoresis or ion chromatography, contactless conductivity (C4D) systems offer a more convenient method of recording signals as the electrodes do not touch the solution and therefore never become contaminated or need cleaning.
eDAQ will work with researchers to produce custom-made solutions for measuring the conductivity (or conductivity change) of a liquid without physical contact, please see here for details. Recent projects include the design and manufacture of contactless conductivity detectors/meters for:
- Recording the very low conductivity of arctic melt water, see the application note.
- Measuring liquid-liquid slug flow properties in tubing.
- Measuring total dissolved inorganic carbon concentration at different depths in the sea with an autonomous ocean profiling vehicle, see the application note.
- A multi-channel C4D detector, which led to the release of the ER825 C4D Detector.
- Measuring the conductivity of a liquid in a pipette tip using the Octal Contactless Conductivity System.
- Loop-mediated isothermal amplification (LAMP), a low cost, single tube method for the amplification of DNA using the ER818.
- Monitoring bacterial growth, see the application note.
The vast majority of 'conductivity cells' (the correct name for a 'conductivity electrode') comprise a pair of platinum plate electrodes coated with platinum black - do not try to 'clean' the electrodes to make them shiny! The platinum black coating (finely divided platinum) provides a catalytic surface for the rapid recombination of hydrogen and oxygen to reform water - otherwise formation of bubbles can occur because of the electrical currents flowing through the electrodes and these would contribute to inaccurate measurements.
An oscillating potential is placed on the electrodes and the resulting AC current measured, and from this the electrical resistance of the sample solution determined. Most meters have a fixed AC frequency but more expensive models may allow the user to adjust the frequency (usually between 20 - 20000 Hz) in order to optimise the signal. The cell is first calibrated by measuring a solution of known composition (say a 0.1 mol/L potassium chloride solution) at a fixed temperature.
In traditional studies the 'Cell Constant' (k) of the electrode determined. The value of 'k' is affected by the surface area of, and the distance between, the pair of platinum plate electrodes. A pair of electrodes exactly 1 cm apart, each of area 1 cm2, would have k = 1. Larger electrodes and/or smaller separation would give a smaller k value, while conversely smaller electrodes and/or a larger separation would give a larger value. The exact k value is determined by measuring a calibration solution of known conductivity.
Electrodes with a k value of approximately 1 are usually the most useful for general laboratory work. Electrodes of k = 0.1 are more useful for working in solution of very low ionic concentration (such as highly purified water) while electrodes of k = 10 are more useful when very high ionic concentrations are encountered (such as with sea water).
When using eDAQ software, it is merely necessary to enter the value of the calibration solution into the software. This will automatically calibrate the electrode without having to calculate 'k' as a separate entity.
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