The get the most out of measurement technologies by adopting it’s use and develop new methods is our biggest strengths. We talked about the many advantages of electrodes(1) compared to other online measurement technologies. While others use the electrodes in online measurements in the same way as in the laboratory, they forget the basic difference between laboratory and online measurement. The online measurement instruments – the analyser, in best case the Seibold analyser series COMPOSER – has a great advantage in repeatability and precision compared to laboratory use(2).
The electrode online advantages
- Measurements based on Ion-Selective Electrodes are inexpensive and have an extremely wide concentration range.
- ISE are very robust and durable and ideal for use in on-line measurement analysers.
- Interfering ions, pH levels, or high concentrations are handled by specialised experimental methods and special reagents.
- ISE are unaffected by sample colour or turbidity.
- ISE can be used in aqueous solutions over a wide temperature range; typically in the range 0°C to 80°C.
The electrode online disadvantages
- Because of electrodes logarithmic profile, the resolution in the high range is very low.
- The lower limit of tested electrodes is in the range of 0.1 ppm. For many applications this is no good enough.
- Sensitive to interferences and limited influence on their elimination. But real world experiences have shown that with the help of chemistry (i.e. masking/complexing agents) added to the buffer and measurement tactics as standard addition and multiple standard addition this situation can be handled nicely.
The electrode technology basics
The electrode consists of a sensing element bonded to the measurement assembly. When the sensing element is in contact with a solution containing measured ions, an electrode potential develops across the sensing element. This potential, which depends on the level of free measured ions in solution, is measured against a constant reference potential in mV. The measured potential corresponding to the level of the measured ion in solution is described by the Nernst equation.
E = measured electrode potential
Eo = reference potential (a constant)
A = fluoride ion activity level in solution
S = electrode slope
The Idea
Follow the development of electrode signal while adding sample to the measurement chamber. The measurement chamber initially is filled with a know concentration of the measured ion and will be replaced continuously by the sample. Flush IN phase. Than after the sample is replaced by the storage solution with a known concentration. Flush OUT phase.
The results of our new Measurement Theory for Electrodes and ISE
- Measurement limit is now well below 0.05 ppm.
- Increased resolution in all concentrations.
- Great resolution in the high measurement range, because of dilution effects.
- Less stress to the electrode, because of the safe controlled environment.
- Less influence of interfering ions, because of dilution effects.
- More operational reliability because of calculating result based on flush IN and flush OUT cycle.
The New Measurement Theory for Electrodes and ISE decreases the error to less than 2%!
(1) Please have a look about technology comparison stated earlier. About Electrode Measurements ISE.
(2) Natural Resources Management and Environment Department on Quality of Analytical Procedures.