R&D Project

Enhanced Stability Online Electroanalytical Devices: Innovative pH/ISE Monitoring Systems

With three decades of experience, our engineering team at LAMBDA-ELAN Ltd., supported by the National Innovation Bureau in Hungary, has developed two new methodological concepts to enhance significantly the long-term stability of pH/ISE-based direct potentiometric measuring cells. For this purpose, we spent a lot of time with studying the stability problems and limitations connected to the pH electrode & ISE based direct-potentiometric measuring cells. The result was that in more than 90% the source of instabilities was connected to the reference electrode. These error sources are:

• the composition changes of the filling solution (reference electrolyte) within the reference electrode, as a result of double direction diffusion of ions between the sample and the reference electrolyte through the diaphragm, generating continuous shift of the double-layer potential arising on the internal reference electrode,
• the concentration growth of the reference electrolyte as a result of evaporation through the open kept filling hole,
• additional source of error is the uncertainty of the liquid-junction potential across the diaphragm, which is temperature and topology dependent and still almost unknown, in addition necessarily different in case of the sample and in case of the different calibration standard solutions (or buffer solutions), hence generating unavoidable systematic measurement error.

Both of the two new metrological concepts detailed below are aimed to avoid or reduce these effects. Our company finished in November, 2022 a very ambitious R&D project supported by a tender of the Hungarian Ministry for Innovations to exploit in practice our two new direct-potentiometric metrological concepts.

Substantive description of the two new methodological concepts for online direct-potentiometric monitoring systems:

1. Opposite polarity Twin ISE Measuring Cell without conventional reference electrode and liquid junction: This concept uses anion- & cation-selective electrode pairs connected oppositely without a conventional reference electrode and liquid junction, similar to the Titrodes patented earlier by the company Metrohm. Our concept uses them not for intelligent automatic titrations based on inflection recognition, but for high stability, accurate, direct-potentiometric ISE measurements (first of all for online monitoring and control systems), generally in clean, one-component solutions.

The basic principle is to replace the reference electrode in the ISE based conventional direct-potentiometric measuring cells with another ion-selective indicator electrode of the determinative “opposite” counter ion. This eliminates both the errors caused by the poisoning or dilution/concentration of the filling (reference) solution of the reference electrode and also the changes and instabilities of the liquid junction potential arising on the contact surface (diaphragm) between the sample (& standard solutions) and the filling- (or salt bridge-) solution of the reference electrode. According to our opinion there are a lot of industrial monitoring and control applications where the use of this principle could result much simpler cell construction, much higher stability and hence much-much lower calibration frequency.

      –  This principle can be utilized first of all in single component (clean) solutions and in their clean dilutions (e.g. intermediate reagents of different chemical manufacturing technologies, infusion solutions, dialysis solutions, titrants, ionic standard solutions, etc.). 

      –   Additionally, it is also useful in the cases when the actual sample matrix doesn’t contain disturbing ions for both of the two oppositely connected ISE’s.

      –   Thanks to the double sensitivity and the elimination of the liquid junction potential this type of measurements may ensure much more reliable operation in low or even trace concentration ranges! For instance, this type of direct-potentiometric ISE based twin measuring cell containing both cation- and anion-sensitive electrodes may improve the stability and sensitivity of the trace level online sodium- or chloride-monitors which are commonly used for the high purity turbine-water controls in energy plants, too!

      –   However, the system will be primarily utilizable for higher concentration range measurements and controls of one component acid-, base and salt solutions like NaCl, KCl, KI, LiF, HCl, H₂SO₄, NaOH, KOH, etc.

The most convenient mode for calibration of the measuring cells according to the abovementioned concept is the double known addition (DKA) to eliminate the matrix effect as much as possible.

2. Specific Differential pH/ISE Measuring Cells with underway restored reference electrode potential during continuous measurement:

There are differential pH-measuring cells manufactured by several companies, for example by EUTECH Instruments or OXYGUARD replacing the conventional Ag/AgCl or calomel reference electrode with another pH-electrode (similar type to the one used as pH-indicator electrode) immersed in a standard buffer solution instead of the sample (named external reference buffer solution). This external reference buffer solution is in ionic connection with the sample solution through a diaphragm like in case of conventional reference electrodes. Generally, they use a third, auxiliary noble metal electrode immersed in the sample solution – called as solution ground (SGND) – to reduce the grown noise collected as a result of double internal impedance of the differential pH measuring cell.

Our quality enhancing idea is to combine the so-called “SENTINEL” concept of a third company named Electro-Chemical Devices (ECD/USA) with the structure of the differential pH & ISE measuring cells, by immersing a chloride coated silver wire (Ag/AgCl electrode) as an auxiliary electrode, directly in the mentioned “reference sample” of the reference pH or ion-selective electrode. This additional electrode placed in the external reference buffer/standard solution in most cases can be used in addition instead of the solution ground immersed conventionally in the sample solution, just on the other side of the diaphragm. The level of pollution and/or dilution of the external reference buffer/standard solution by the samples can be monitored and checked detecting the changes in an additional signal, measured continuously parallel with the differential pH/ISE measurement (i.e. the voltage between the reference pH or ion-selective electrode and the mentioned “SENTINEL” Ag/AgCl auxiliary electrode). In case of long-term monitoring tasks this principle allows to begin an automatic renewal of the diluted/polluted external reference buffer/standard solution of the reference pH or ion-selective electrode during the continuous measurement, when the shift of the “SENTINEL” signal exceeds a preset limit, and finishing the renewal process when this signal gets near enough back to the value measured during the last calibration.

Finally in November of 2022, we completed our earlier mentioned very ambitious R&D project resulting in four new pH/ISE-based online direct potentiometric monitoring systems. These devices offer recalibration periods which are 3-5 times longer than the currently used systems.

Short description of the four new prototypes:

I. Turbine Water Purity Monitoring System

• Construction: Our unique flow-through cell features oppositely connected cation and anion-exchange based microcapillary PVC membrane electrodes, without the use of a reference electrode, positioned in an aluminum block-thermostat with a heat-exchanger for the streaming sample, too. The overall cell volume is less than 500 µl.
• Advantages:
  • Higher sensitivity and better detection limit due to the use of PVC membranes and two indicator electrodes instead of the single Sodium-glass ISE.
  • Greater zero-point stability by avoiding the use of a reference electrode.
  • Much lower sample volume requirements (1-5 ml/min flow rate).
  • No need for DIPA to reduce the hydrogen ion interference!
  • Avoided sample waste and environmental impact.
• Comparison: Traditional systems use dangerous reagent like DIPA, large volume flow-through vessels consuming significantly larger volume of sample, which has to be wasted as aggressive and dangerous material, and slowly drifting reference electrodes with more frequent recalibrations.
  

II. Special Precision pH Meter usable also for Online Monitoring of High Purity Water Samples

• Construction: A differential pH cell with two microcapillary pH electrodes and a chloride ion-selective electrode as „SENTINEL” electrode to attend the reference electrode potential changes, and periodic external reference buffer renewal, positioned in an aluminum block-thermostat, with a heat-exchanger for the streaming sample, too. The overall cell volume is less than 500 µl.
• Advantages:
  • Superior zero-point stability with automatic reference buffer renewal.
  • Lower sample volume needs (1-5 ml/min flow rate).
  • High precision measurements possible with minimal sample volumes.
  • Easier and more extensive electrode rinsing between successive samples.
• Comparison: Conventional systems have larger volume requirements and less stable zero points, due to the slow reference electrolyte dilution and pollution.