pH electrode

Details pH electrode pH electrode

1992-10-13

1994-10-11

Niebling, John

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204435, 204418, 204421, 204422, G01N 2726

Patent

active

053544498

DESCRIPTION:

BRIEF SUMMARY
This invention relates to pH electrodes.
A conventional pH electrode consists of a membrane of glass of special composition, commonly in the form of a bulb, fused to a hollow insulating glass stem. The inside of the membrane is in contact with an internal reference solution which contains a buffering agent to maintain a constant pH. The internal reference solution is contacted by an internal reference electrode, commonly a silver or platinum conductor coated with silver/silver chloride. When the membrane is immersed in a solution, a voltage is established across the glass that is the sum of the two interface potentials which are determined by the Nernst equation and any asymmetry potential existing across the thickness of the glass membrane. The potential of this half cell is measured against an external reference electrode. In order for the electrode to be used as a pH electrode, the inner interface potential must be kept constant and the asymmetry potential calibrated out by measuring the voltage of the electrode in solutions of known pH. The function of the internal reference solution is to maintain a constant pH at the inner interface and to maintain a constant chloride activity to stabilize the internal reference electrode potential. The composition of the internal reference solution is maintained by the sealed construction of the electrode and the relative impermeability of glass to water vapor and ions.
Glass electrodes have many applications; however, their use in vivo has been restricted by a number of limitations, the most important of which is the tendency for proteins and other constituents of biological materials to coat and poison the outer sensing surface of the membrane. The fragility of the glass and the cost of manufacture can also be limitations on their use in practice.
Plastic pH membranes that are pH sensitive, either directly through the properties of the polymer itself, or through the incorporation of a ligand or ionophore selective for the hydrogen ion, have considerable advantages for biological measurements, particularly in vivo. They show biocompatibility and freedom from poisoning, are robust and cheap to make. They have, however, one important disadvantage compared with the glass electrode; the low resistance plastic membranes are relatively permeable to water vapor and to the passage of electrolytes. The dry shelf life is therefore limited by the evaporation of the internal reference solution. In use, a relatively large bulk of internal reference solution containing a buffering agent must be provided to minimize drifts in the electrode potentials due to changes in the pH of the internal reference solution caused by the movement of water and electrolytes across the membrane. Storage and miniaturization are therefore problems that have limited their applications.
In the pH sensing electrode disclosed in European Application No. A-0133 536, a silicone rubber membrane containing tridodecylamine as the ionophore is used in conjunction with a pH 4 buffer containing NaCl as an electrolyte. This pH 4 buffer corresponds substantially to the pH at which the chosen ionophore (tridodecylamine) starts to respond by exhibiting an inner membrane potential which varies with pH.
Similarly, the use of a citrate buffer in a pH monitoring sensor including a plastic membrane containing tri-n-dodecylamine as the ionophore, is disclosed in "Ion-selective sensors for assessment of the fetus" by M. O'Dowd et al, J. Biomed. Eng. 1988, Vol. 10, April, 165-169. The pH electrode disclosed in Analytical Chemistry, 1985, 57, 1155 likewise uses a tridodecylamine-containing PVC membrane in conjunction with an internal electrolyte buffered to pH 4.6. This technique of maintaining a constant internal pH requires the use of a relatively large volume of buffered electrolyte. This is not only expensive but also precludes miniaturization of the pH electrode.
Such plastic pH membranes differ from glass membranes in that the pH range over which the membranes behave according to the Nernst equation is limited and is

REFERENCES:
patent: 3676319 (1972-07-01), Kirsten
patent: 4294258 (1981-10-01), Bernard
patent: 4708776 (1987-11-01), Roth et al.
patent: 4734184 (1988-03-01), Burleigh et al.
patent: 4871439 (1989-10-01), Enzer et al.
patent: 5223124 (1993-06-01), Ege
Patent Abstracts of Japan, vol. 6, No. 41 (P-106) (919), published Mar. 13, 1982.
Patent Abstracts of Japan, vol. 6, No. 252 (P-161) (1130), published Dec. 10, 1982.
Eric J. Fogt et al., Simplified Procedure for Forming Polymer-Based Ion-Selective Electrodes, 57 Analytical Chemistry, 1155-1157 (1985).
EPO Search Report for PCT/GB92/00049 published May 6, 1992.

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