Electroanalytical chemistry

Liquid membrane selective electrodes - Part 1

Prof. Dr. Ismail Khalil Al-Hiti                         Dr. Tahseen Ali Zeidan

Liquid selective electrodes based on ion exchangers were prepared before the neutral ion-carrying selective electrodes. The selective electrode prepared from the ion exchanger means that the electrode carries an active substance in the form of a charged ion exchanger and contains in its composition huge organic molecules that make it insoluble in water. The main differences between solid and liquid membrane electrodes are related to the movement of ions, as the liquid membrane electrodes have the movement of ions faster than their movement in the phases of the solid electrodes, in addition to the use of large amounts of liquid active substance in the liquid membrane electrodes, as well as changing the efficiency of the liquid membrane electrodes in the presence of interferers because they are transmitted to the electrode membrane.

All liquid ion exchangers are very poorly soluble in water, and ion exchangers can diffuse from their reservoir they are kept to the surface of the membrane in order to replace the amounts of dissolved and leaked outside the membrane.

The solubility of the ion exchanger in the given models depends on the concentration of the main substance to be determined and the pH of the solution. The higher the concentration of the substance to be estimated, the higher its solubility, and the more extreme the pH of the solution was (as if it was less than 2 or more than 10), the solubility of the substance to be determined increases, and the solubility increases dramatically if the solvent changes from water to a non-hydrophilic substance.

Membrane electrodes with liquid ion exchangers:

These membranes are liquid in nature as the ion exchanger is dissolved in an organic solvent that does not mix or does not dissolve in water. This solution is supported by an inert but porous seal, a wick (in the form of a glass filter), a porous plastic membrane, a ceramic valve, etc. Among the membranes used in this field, which serve two purposes, the first is to preserve the liquid ion exchanger with the solvent, and the second is that it is porous that allows the ions of the ion exchanger to flow through it to make the ion exchange on the outer surface of the membrane.

Figure 1 shows a typical liquid ion exchanger electrode, which consists of two chambers or two containers, one containing the ion exchanger dissolved in a suitable solvent and the other containing the internal source electrode, which is often AgCl/Ag. The electrode of the internal source is in the internal source solution consisting of the active substance from which the electrode is made, in addition to the KCl solution saturated with silver chloride solution.

The active substance in the electrode is the ion exchanger, and the other important part of the electrode is the porous membrane, which has the ability to carry the internal solution, in addition to its porosity, which works on the flow of the ion exchanger through it at a low speed. The solvent used to dissolve the ion exchanger must have the following characteristics:

          a- It should have a low vapor pressure to avoid losing it by evaporation.

           b- It should be of high viscosity to prevent its loss by rapid flow through the membrane.

            c- The solvent used should have a low solubility in aqueous solutions of the model.

             d- Most of the solvents used have a low dielectric constant that makes ionic coupling or coupling happen to a significant extent.

Figure (1):

Membrane electrode selective liquid ion exchanger.

Calcium ion selective liquid calcium electrode

The ion exchanger used in the preparation of selective calcium electrode films is derived from phosphoric acid diesters. The choice of the phosphate ester is due to the fact that calcium forms with these esters stable seats, and by introducing a long alkyl group or chain, the solubility of the liquid membrane in water decreases, and the benefit of the diester over the single ester is to overcome the problem of forming mixed complexes (such as calcium hydrogen phosphate complexes).

   The use of a solvent with highly polar substitutes such as di-n-octylphenyl phosphonate enhances the selectivity of the calcium electrode for the calcium ion relative to magnesium and other alkaline earth metals. But if the solvent d-n-octylphenyl phosphonate is replaced by the polar solvent 1-decanol, then the liquid selective calcium electrode responds to both calcium and magnesium ions and then the electrode responds to the total sum of calcium and magnesium concentrations.

This pole was used to estimate the total amount of calcium and magnesium in the water, ie, the hardness of the water. But the electrode also responds to zinc [Zn2+] and other binary cations.

The calcium electrode shows a linear response in the range (10-1 - 5 * 10-5 mo) to calcium ions and the slope of the titration curve was close to the nernesty slope and the lowest concentration to which the electrode responded was 10-5 mo calcium ions, which is related to the solubility of dikyl calcium phosphate .

   The upper bounds of the response range are extended by increasing calcium ion transport across the membrane. This transition can be determined by comparing or comparing the calcium concentration in the inner solution (usually 10-2 to 10-1 M) with the calcium concentration in the outer sample solution. In such a case, the electrode can show a neuralgic response to calcium ion concentrations greater than 1 mo.

The electrode selectivity for calcium ion is reasonable and acceptable for the alkali metals ( ) and reasonable and acceptable for the alkaline earth metals ( ).

Iron(II), lead(II), copper(II) and zinc (Zn) ions interfere. When iodide and perchlorate ions are present in concentrations higher than 3-10 mo, these ions show interference due to their dissolution in the organic phase.

When the voltage E is plotted against the pH with the stability of calcium in the solution, a special drop is observed under pH = 5. At a pH higher than 11 it is observed