How Ion-Selective Electrodes Work

An ion-selective electrode responds preferentially to one species in a solution. Differences in concentration of the selected ion inside and outside the electrode produce a voltage difference across the membrane.  

Hydrophobic water hating (does not mix with water) 

Excess negative charge Excess positive charge 

Valinomycin has a cyclic structure containing six amino acids and six carboxylic acids. Isopropyl and methyl substituents are not shown In this diagram. [From L. Stryer, Biochemistry, Ath ed. (New York W. H. Freeman and Company, 1995).] 

The electrode really responds to the oct/V/fy of analyte (Section 12-2), not the concentration. In this book, we will write concentrations instead of activities. 

L has some ability to bind other ions besides C so those other ions interfere to some extent with the measurement of C. An ion-selective electrode uses a ligand with a strong preference to bind the desired ion. 

The region of charge imbalance extends just a tew nanometers into the membrane and Into the neighboring solution. 

When C+ diffuses from a region of activity dAm in the membrane to a region of activity JAa in the outer solution, the free-energy change is 

The driving force for diffusion of C+ from the membrane to the aqueous solution is the favorable solvation of the ion by water. As C+ diffuses from the membrane into the water, there is a buildup of positive charge in the water immediately adjacent to the membrane. The charge separation creates an electric potential difference ( ou,cr) across the membrane. The free-energy difference for C+ in the two phases is AG = —nFE(Mcr, where F is the Faraday constant and n is the charge of the ion. At equilibrium, the net change in free energy for diffusion of C+ across the membrane boundary must be 0  

---

The modification of electrodes with PVC membranes has found applicability in ion selective electrode work [99] (so-called "coated wire electrode ). The molecular motion of species within such electrodes has been investigated by Compton and Waller [100]. Using a range of derivatives of the nitroxide spin probe TEMPO, they were able to show how the rotational correlational time was dependent upon the molecular volume of the probe and, by use of variable-temperature apparatus, how this varied with temperature. The effect of various plasticizers upon the molecular motion within the PVC membrane was investigated, rotational correlational times being dependent upon the nature of the plasticizer and the loading level. The effect of loading level upon the correlation time was shown to correlate with data obtained by Compton Maxwell [101] for the response times of K+ ion selective electrodes based upon PVC modified electrodes. 

An extension of artificial membranes for ion selective electrochemical work was the construction of biomimetic ion channel sensors [65]. These devices were based on Langmuir-Blodgett deposition of charged lipid membranes onto a glassy carbon electrode. This work indicated that a conductive zone can be opened reversibly by a stimulant-membrane interaction by surface charge alterations. This work has demonstrated how the concept of the conductivity measurement could be extended to the more common and useful technique of cyclic voltammetry. 

Figure 15-6b shows how the electrode works. The key in this example is the ligand, L (called an ionopkore), which is soluble inside the membrane and selectively binds analyte ion. In a potassium ion-selective electrode, for example, L could be valinomycin, a natural antibiotic secreted by certain microorganisms to carry ion across cell membranes. The ligand L is chosen to have a high affinity for analyte cation and low affinity for other ions. 

A micropipet H ion-selective electrode similar to the NH4 electrode in Box 15-2 was constructed to measure the pH inside large, live cells by impaling a cell with the electrode (and with a similarly small reference electrode). The ion exchanger at the tip of the H ion-selective electrode was made from 10 wt% tri(dodecyl)amine [(Ci2H25)3N] and 0.7 wt% sodium tetraphenylborate, dissolved in o-nitrophenyl octyl ether. The selectivity for H" relative to Na, K", Mg ", and Ca " was sufficient for intra- and extracellular measurements without significant interference from these metal ions. Explain how this electrode works. 

---