The basic principles of potentiometric measurement of ion concentration using the ion-selective electrode chain are discussed in greater detail elsewhere. Similarly, the marvellous properties of ion-selective electrode membranes are interesting enough to merit their own chapter. Additionally, as a main reference for this topic, I refer the readers to Nallanna Lakshminarayanaiah's Membrane Electrodes (2012), as well as Martin Frant's two articles.

Structure of the potassium-sensitive electrode

One cannot speak too broadly, having experience of only one blood gas analyser. The locally available unit features a potassium-sensitive electrode with a ionophore-impregnated PVC membrane.

potassium selective electrode with ionophore-impregnated PVC membrane

The protective cellophane membrane is semipermeable, and its purpose seems to be keeping the larger molecules from befouling the delicate PVC inside. Water and ions get through.

Inside, the PVC membrane awaits, loaded with valinomycin. This is an interesting development.

Prior to impregnated plastics, ion-selective electrode membranes were composed of a porous wick which was soaked with some sort of hydrophobic ion carrier, typically in the form of a thick oily organic solvent. The wick was usually clay (which was experimentally convenient), and the solvent could be any sort of organic liquid. This historical aside is of little relevance (but was the topic of a digression anyway). Suffice to say, the potassium-selective electrode features a PVC membrane doped with a potassium-selective ionophore. Radiometer do not specify which, but valinomycin is historically a popular choice. It is a macrolide antibiotic with a ring-shaped molecule (the image below, Figure 4 from Chapter 8 in Lakshminarayanaiah's Membrane electrodes, is used repeatedly in these chapters precisely because valinomycin is the prototypical model ionophore).

potassium-valinomycin complex

The potassium ion measures about 1.33 angstrom across, and just about fits the cavity as depicted, which is between 2.7 and 3.3 angstrom across. An electrode with a valinomycin-impregnated PVC membrane can be up to 5000 times more selective for potassium when compared to sodium.

For the potassium electrode in the local blood gas analyser, the detection limits are between 0.5 and 25mmol/L. History has afforded the author exposure to blood gas results which approach this limit.

References

Device-specific information in all these ABG pages refers to the ABG machine used in my home unit.

Other machines may have different reference ranges and different symbols.

For my ABG analyser, one can examine this handy operations manual.

There is also an even more handy reference manual, but one needs to be an owner of this equipment before one can get hold of it. Its called the "989-963I ABL800 Reference Manual"

Kurzweil, Peter. "Metal oxides and ion-exchanging surfaces as pH sensors in liquids: state-of-the-art and outlook." Sensors 9.6 (2009): 4955-4985.

Breathnach, C. S. "The development of blood gas analysis." Medical history 16.01 (1972): 51-62.

Lakshminarayanaiah, Nallanna. Membrane electrodes. Elsevier, 2012.

Buck, RICHARD P., and Erno Lindner. "Recommendations for nomenclature of ionselective electrodes (IUPAC Recommendations 1994)." Pure and Applied Chemistry 66.12 (1994): 2527-2536.

Frant, Martin S. "Historical perspective. History of the early commercialization of ion-selective electrodes." Analyst 119.11 (1994): 2293-2301.

Frant, Martin S. "Where did ion selective electrodes come from? The story of their development and commercialization." Journal of chemical education 74, no. 2 (1997): 159.

Young CC."Evolution of blood chemistry analyzers based on ion selective electrodes."Journal of chemical education 74, no. 2 (1997): 177.

Yim, Hyoung-Sik, et al. "Polymer membrane-based ion-, gas-and bio-selective potentiometric sensors." Biosensors and Bioelectronics 8.1 (1993): 1-38.

Oesch, Urs, Daniel Ammann, and Wilhelm Simon. "Ion-selective membrane electrodes for clinical use." Clinical Chemistry 32.8 (1986): 1448-1459.

Sollner, Karl. "Membrane electrodes." Annals of the New York Academy of Sciences 148.1 (1968): 154-179.

Bloch, René, Adam Shatkay, and H. A. Saroff. "Fabrication and evaluation of membranes as specific electrodes for calcium ions." Biophysical journal 7.6 (1967): 865-877.

Oesch, Urs, and Wilhelm Simon. "Lifetime of neutral carrier based ion-selective liquid-membrane electrodes." Analytical Chemistry 52.4 (1980): 692-700.