Introduction to ISE Three Methods of Analysis Potentiometric ion analyses with ionselective electrodes (ISEs) are performed by use of one of three methods, each entailing its own advantages: direct potentiometry, incremental methods, and potentiometric titration. Hanna offers a solution for each of these methods. Direct Potentiometry Direct potentiometry is a widely used method of performing ion analysis with ISEs. This method is highly effective when the user must quickly measure large batches of samples at varying concentrations. Our direct reading meters, such as the HI98191, display concentration of the unknown sample by a direct reading after calibration of the instrument with two or more standards; ionic strength adjustments are made to both samples and standards. In some applications, quick and reliable measurements can be made on-site without taking samples back to the laboratory. Incremental Methods Incremental methods are useful techniques used to determine ion concentration in samples whose constituents are variable or concentrated. Incremental methods have some inherent advantages over direct potentiometry. The techniques can reduce errors from variables such as temperature, viscosity, pH or ionic strength. The electrodes remain immersed throughout the process, thus reducing sample carry over and possible liquid junction changes in the reference. Known addition, known subtraction, analyte addition, and analyte subtraction methods are four of these incremental techniques. All four techniques involve adding a standard to the sample, or sample to the standard; the meter then calculates the ion concentration of the sample. Potentiometric Titration A potentiometric titration can increase the precision of ISE measurements and also the number of ionic species that can be determined. ISEs are commonly used as indicators for the titrant or sample species to follow the progress of a precipitation or complexometric titration. A small change in reactant addition corresponds to a large change in electrode potential at the stoichiometric endpoint. An example of a precipitation titration is the determination of chloride using silver nitrate. A silver ISE can be used to follow this titration. A complexometric titration is used for the determination of calcium. A calcium solution is titrated with the complexing agent, EDTA. During the titration there is a gradual decrease in the free Ca2+ ion concentration as more EDTA is added. The endpoint corresponds to the point at which all of the Ca2+ is complexed. The progress of this titration can be monitored using a calcium ISE. Ion Selective Electrode Types Hanna’s ISEs can be grouped into three general categories based upon construction. Solid-state Solid-state electrodes are available as both single half-cells or as combination electrodes complete with reference electrode. These electrodes incorporate a solid sensing surface made of compressed silver halides or solid crystalline material. Hanna’s offering includes sensors for the determination of bromide, cadmium, chloride, cupric, cyanide, fluoride, iodide, lead and silver ions. Rugged, solid body construction ensures a long life. Theory: A solid-state electrode develops a voltage due to ion-exchange occurring between the sample and the inorganic membrane. An equilibrium mechanism occurs due to the very limited solubility of the membrane material in the sample. Liquid Membrane Liquid membrane electrodes are available as single half-cells or as combination electrodes complete with reference electrode. The sensing surfaces of these electrodes are comprised of a homogeneous polymer matrix containing organic ion exchangers that are selective for the determined ion. These sensors incorporate easily replaceable membrane modules and are available for measurements of nitrate, potassium and calcium. Theory: The potassium electrode was one of the earliest liquid membrane sensors developed. The membrane is usually in the form of a thin disc of PVC impregnated with the antibiotic valinomycin. The exchanger, also known as an ionophore, is a ring structure that fits potassium ions inside, functioning as a lock and key mechanism. This type of membrane is not as rugged as the solid-state type so they are designed for easy replacement of the sensing module. 3 ISE 3.2 | www.hannainst.com introduction
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