A Brief History of Ion Selective Electrodes
*Ionic selective electrode. In 1934, B. Renger et al. observed that glass electrodes containing aluminum oxide or boron trioxide also responded to sodium. In the late 1950s, G. Eisenman et al. developed glass electrodes that exhibited a Stokes response to cations other than hydrogen ions. In 1936, H.J.C. Tandero observed the response of fluorite membranes to Ca2+, and in 1937, I.M. Kortov developed a halide ion electrode using silver halide flakes. In 1961, the E. Pongo system in Hungary developed a precipitation membrane electrode that responded to various ions, including Ag+, S2-, and halide ions, using inert substrates such as silicone rubber. In 1966, M.S. Frank and J.W. Ross from the United States made highly selective fluoride ion electrodes using lanthanum fluoride single crystals, which was an important contribution to the development of ion selective electrodes; The following year, Ross made * types of liquid ion exchange calcium ion electrodes. At the same time, the Swiss Simon School began research on another important type of electrode, namely neutral carrier membrane electrodes, by preparing potassium electrodes from antibiotics. By the end of the 1960s, there were about 20 types of products for ion selective electrodes, and this analytical technique began to become an independent branch of electrochemical analysis.
Basic Theory of Ion Selective Electrode
When an electrochemical membrane separates two electrolyte solutions, if the membrane only serves to prevent rapid mixing of the two solutions by allowing any ions to pass through uniformly, a diffusion potential is generated between the solutions on both sides of the membrane due to the difference in concentration and mobility of each ion in the solution, known as the liquid interface potential. Another scenario is that if the membrane prevents at least one of the ions from passing through, it generates the so-called Tangnan potential. The sensitive membrane of an ion selective electrode is a selective penetrating membrane that exhibits relative selectivity rather than specificity for the penetration of different ions. Therefore, the membrane potential falls between the two situations mentioned above.
The basic assumption of TMS theory is that the total potential of the membrane consists of three parts, which is equal to the sum of the two phase interface potentials on both sides of the membrane and the diffusion potential inside the membrane. On this basis, the membrane potential equation containing the Tangnan term and the Henderson term was derived. Eisenman et al. derived membrane potential equations for different types of electrodes by solving the Nernst Planck flow equation.
Under the premise that the electrode membrane only allows ions with the same charge to pass through and does not allow ions with opposite charges and solvent molecules to pass through, and that all ions passing through the membrane have ideal behavior and the membrane current is zero, a unified formula can be used to represent the membrane potential Em：(1) in the formula a 媴 and 媴 a For i The activity of ions in solutions 1 and 2; a Wanhe a The total number is N Planting of j The activity of ions in solutions 1 and 2; Zi and Zj For ions i and j The number of charges; T For temperature; R Gas constant; F Faraday's constant; Kij For electrode pairs with major ions i Compared to other ions j The selectivity coefficient. When the composition of solution 2 remains unchanged, a 徎 and a If all values are constant, then the above equation is the Nicolsky Eisenmann equation, which is the fundamental equation in ion selective electrode analysis.
If the electrode has a high selectivity for the i ion, i.e. all Kij are close to zero, then the above equation becomes: its form is consistent with the Nernst formula *. This is why people are accustomed to using the Nernst relationship to describe the response characteristics of ion selective electrodes.