Potentiometric Ion-Selective Electrodes An ion-selective electrode (ISE), is a type of electrochemical sensor that converts the activity of a specific ion dissolved in a solution into an electrical potential. The voltage of such electrode is dependent on the logarithm of the ionic activity, according to the Nernst equation. 푅푇 퐸 = 퐸 + 2.3 log 푎 (1) 표 푛퐹 Where Eo = a constant for a given cell (It is unique for the analyte, and also includes the sum of the potential differences at all the interfaces other than the membrane/sample solution interface.) R = the gas constant T = the Temperature in Kelvin n = the ionic charge F = the Faraday constant a = the activity of the ion in the sample solution and the expression 2.3RT/nF is termed the Slope Factor For example, the slope at 298K (25°C) has a value of 59.16 mV, when measuring Potassium ions, (i.e. n = +1). This is termed as the Ideal Slope Factor, and means that for each tenfold change in potassium concentration, an ideal measuring system will sense a change of 59.16 mV. The measurement of slope factor gives an indication of the performance of the electrode system. If the slope value is lower than the ideal slope factor, it signifies a loss in electrode’s performance. A number of factors such as interference from other ions, use of incorrect calibration, loss of electrolyte in the electrode or blockage of the reference junction, can be responsible for it and should be checked in such a case. The ion-selective membrane situated between the two aqueous phases, i.e., between the sample and inner solution that contain an analyte ion forms an essential part of the ISEs. It can be made of glass, a crystalline solid, or a liquid. The potential difference across the membrane is measured between the two reference electrodes positioned in the respective aqueous phases. reference electrode 2 || sample solution | membrane | inner solution || reference electrode 1 Figure 1: Schematic of a measurement setup using an Ion-Selective Electrode 1. Selectivity and interferences Ion-selective electrodes are selective but not specific. They can respond to other ions in solution although it is not designed to do so. The ion to be determined is referred as the primary ion (determinant) and other ions to which the electrode responds are known as interfering ions (interferent). The preference of an electrode for the determinant over the interferent is called the selectivity of the electrode. This preference, is expressed as a ratio called the selectivity coefficient, or ratio. Each electrode has its own set of selectivity coefficients. For example: -3 KK+/Na+ = 2.6 x 10 primary ion interfering ion Meaning that the preference for K+ (potassium) over Na+ (Sodium) for this electrode is 1 to 2.6 x 10-3 or 385:1. This means that the electrode is 385 times more selective to K+ than Na+. To take into account, the response of the electrode to an interfering ion an additional term is added to equation 1 푅푇 푛 퐸 = 퐸표 + 2.3 log(푎 + 푘 푎 푧 ) (2) 푛퐹 푖푗 푖푗 푗 Where aj is the activity of the interfering ion with charge z and kij is the selectivity coefficient, which is the response of the electrode to the interfering ion relative to its response to the primary ion. 2. Activity vs Concentration Ion-selective electrodes respond directly to the activity of an ion, rather than to its concentration. The concentration is the number of ions in a specific volume, this definition assumes that all of those ions have a similar behavior. However, ions do not always behave similar to one another: some are active i.e. exhibit properties associated with that ion, and some are not active. The number of active ions is called the activity of the solution. This means it is a relative term describing how “active” an ion is compared to when it is under standard state conditions. This characteristic is often a decided advantage, since the metabolic behavior of an ion is often more directly related to its activity. For example, the physiological effects of calcium in serum are related to the ionized Ca2+ activity rather than the total calcium concentration, which also includes protein-bound and complexed calcium species. In dilute solutions though, the ionic activity and the concentration are practically identical but in solutions containing many ions, activity and concentration may differ. This is why dilute samples are preferred for measurement with ISE's. However, it is possible to 'fix' the solution so that activity and concentration are equal. This can be done by adding a constant concentration of an inert electrolyte to the analyte solution. This is called an Ionic Strength Adjustment Buffer (I.S.A.B.). Thus in dilute solutions, the ion selective electrode will measure concentration directly. 3. Types of electrode There are four types of ion selective electrode whose construction and mode of operation differ considerably. These are: 1. Glass body electrode 2. Solid state (crystalline membrane) 3. Liquid ion exchange (polymer membrane) 4. Gas sensing type 1. Glass body electrodes The most common ISE is the glass-bodied pH electrode. Glass membranes are made from an ion- exchange type of glass (silicate or chalcogenide). This type of ISE has good selectivity, but only for several single-charged cations; mainly H+, Na+, and Ag+. 2. Solid state ion selective electrode In these type of electrode, the electrode potential of the standard and the sample solutions is measured across a solid, polished crystalline membrane. The crystalline material is prepared from a single compound or a homogeneous mixture of compounds (for example, the fluoride ISE has a Lanthanum Fluoride crystal) 3. Polymer membrane ion selective electrode These electrodes contain a polymeric membrane containing a selective ion exchanger, the liquid membranes are hydrophobic and immiscible with water. They are most commonly made of plasticized poly (vinyl chloride). By doping the membranes with a hydrophobic ion (ionic site) and a hydrophobic ligand (ionophore or carrier) that selectively and reversibly forms complexes with the analyte, these membranes are made selective. The electrode potential of solutions is measured by their effect on the ion exchange material. This type of ion-selective electrode is subject to more interferences than other ISEs due to complex properties of ion exchangers. Figure 2: Schematic view of the equilibrium between sample, ion-selective membrane, and inner filling solution (cell 1). The cation-selective membranes are based on (A) cation exchanger (R-), (B) electrically neutral ionophore (L) and anionic sites (R-), and (C) charged ionophore (L-) and cationic sites (R+). The aqueous solutions contain an analyte cation (I+) and its counter anion (X-). 4. Gas sensing type The gas sensing type of ISE use a membrane separating a sample and a filling solution. The gas from the sample solution (for e.g., liberation of ammonia by adding a caustic solution to it) permeates through the membrane and changes the pH of the filling solution. The change in pH is proportional to the concentration. This gives a quantitative measurement of the analyte gas in the sample solution. Figure 3: Schematic of the sensing portion on the 4 main types of Ion-selective/gas sensing electrodes 4. Reference Electrodes The potential of an Ion Selective Electrode can only be measured against an appropriate reference electrode in contact with the same test solution. Reference electrodes are electrodes with a stable and well-known electrode potential. By employing a redox system with constant (buffered or saturated) concentrations of each participant, a chemical equilibrium can be maintained inside them which is responsible for a constant value of the electrode potential. A silver chloride electrode is among the most commonly used reference electrodes, it is usually the internal reference electrode in pH meters. The electrode functions as a redox electrode by maintaining the equilibrium between the silver metal (Ag) and its salt—silver chloride (AgCl). The corresponding half-reactions inside electrode are as follows: - Ag+ + e- ⇋ Ag (s) AgCl (s) + e- ⇋ Ag (s) + Cl- This reaction is characterized by fast electrode kinetics, meaning that a sufficiently high current can be passed through the electrode with the 100% efficiency of the redox reaction (i.e., dissolution of the metal or cathodic deposition of the silver-ions). The reaction obeys these equations in solutions ranging from pH 0 to 13.5. The standard electrode potential E0 against standard hydrogen electrode (SHE) is 0.230 V ± 10 mV. 5. Methods of analysis 5.1 Potentiometry In potentiometry, the potential of a solution between two electrodes is passively measured, affecting the solution very little in the process. One of the electrode which has constant potential is called the reference electrode, while the other electrode whose potential changes with the composition of the sample is called indicator electrode. Therefore, the potential difference between the two electrodes is indicative of the composition of the sample. Potentiometry is a non-destructive technique, assuming that the electrode is in equilibrium with the solution we are measuring the potential of the solution. Potentiometry usually uses an ion-selective electrode, so that the potential solely depends on the activity of this ion of interest. The time taken by the electrode to establish equilibrium with the solution will affect the sensitivity or accuracy of the measurement. 5.2 Calibration Curve Method This is the simplest and most widely used method of obtaining quantitative results using Ion Selective Electrodes. Standard solutions are prepared by serial dilution of a concentrated standard. The recommended Ionic Strength Adjustment Buffer (ISAB), is added to each standard as well as to the unknown samples.