Cation-selective polymeric electrodes

Details Cation-selective polymeric electrodes Cation-selective polymeric electrodes

1994-04-28

1995-03-07

Niebling, John

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204435, 128668, 128670, 128696, 128700, G01N 2726, A61B 502, A61B 504

Patent

active

053955056

DESCRIPTION:

BRIEF SUMMARY
CATION-SELECTIVE POLYMERIC ELECTRODES

The present invention relates to improvements in cation-selective electrodes and, in particular, to a method of stabilizing the internal or reference potential of a cation-selective electrode to reduce the disadvantage of a large temperature dependent voltage.
Cation-selective electrodes are generally membrane-based devices, involving cation-responsive membrane materials which are selective for the passage of particular cations through the membrane. They may be used for the measurement of active species in liquids.
Cation-selective electrodes are generally used in the potentiometric mode and the cation-responsive membrane establishes a voltage difference between the two solutions in contact with its surfaces, i.e. the test solution and the internal reference solution. The voltage difference is related to the difference in the logarithms of the concentration of the cation to which the membrane is selective in the two solutions. The cation-selective electrode is normally incorporated into an electrochemical cell with an internal standard reference electrode contacting the internal reference solution and a separate reference electrode contacting the analyte or test solution, usually via a salt bridge. In order for the voltage of the cell not to drift with time it is important that the concentration of the cations in the internal reference solution is kept stable. A convenient way of stabilizing the cation concentration in the internal reference solution is to use a saturated solution with the cations in solution to contact with crystals of their salt. This method has the advantage that variations in the water content of the electrolyte do not affect the concentration of the cation. This method has the disadvantage that if the electrode is used for measuring low concentrations of the cation, a large voltage is generated across the cation-selective membrane. Furthermore, voltage is dependent upon the temperature of the cell and the temperature coefficient that results is inconvenient for many applications. The temperature compensation of the measurements made with cation-sensitive electrodes is becoming of increasing importance as more measurements are made outside the laboratory e.g. in environmental work and in potential biochemical applications.
All of the ligands or ionophores which are used to make cation-selective electrodes show varying degrees of cross-sensitivity with cations other than the particular cation to which they are primarily sensitive. This is normally a disadvantage. We have now used this particular property in devising a method of stabilizing the internal or reference potential of a cation-sensitive electrode reducing the disadvantage of the large temperature dependent voltage.


SUMMARY OF THE INVENTION

Accordingly, the present invention provides a cation-sensitive electrode which comprises an internal reference electrode, an internal reference solution and a cation-responsive membrane, characterized in that the internal reference solution contains a saturated solution of a salt of a cation other than the cation for which the cation-responsive membrane is primarily selective, the internal reference solution being formed in situ by hydration of a combination of a hygroscopic material with the salt of the cation other than the cation for which the cation-responsive membrane is primarily selective, either by absorption of water from the atmosphere and/or by hydration of the hygroscopic material when the electrode is put into use, thereby forming the internal reference solution.
In the present invention the cross sensitivity of the cation-responsive membranes is used to allow the internal reference solutions to comprise saturated solutions of cations which are normally considered to be interferents. The high concentration of the interfering cation is constant in the saturated solution which comprises the internal reference solution, but is only equivalent to a low concentration of the cation for which the cation-responsive membrane is selective. The mem

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