U K Pergamon Press Ltd., Headington Hill Hall, Oxford. OX3 OBW, UK USA Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523, USA CANADA Pergamon of Canada Ltd., 75 The East Mall, Toronto, Ontario, Canada AUSTRALIA Pergamon Press (Aust.) Pty. Ltd., 19a Boundary Street, Rushcutters Bay, N.S.W. 2011, Australia FRANCE Pergamon Press SARL, 24 rue des Ecoles, 75240 Paris, Cedex 05, France FEDERAL Pergamon Press GmbH, 6242 Kronberg/Taunus, REPUBLIC Pferdstrasse 1, Frankfurt-am-Main, Federal Republic OF GERMANY of Germany Copyright © 1976 International Union of Pure and Applied Chemistry All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the copyright holder This report was first published in Pure & Applied Chemistry, Vol. 48, No. 1 and supplied to subscribers as part of their subscription Pure & Appl. Chem., Vol. 48, pp. 127-132. Pergamon Press, 1976. Printed in Great Britain. INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY ANALYTICAL CHEMISTRY DIVISION COMMISSION ON ANALYTICAL NOMENCLATURE RECOMMENDATIONS FOR NOMENCLATURE OF ION-SELECTIVE ELECTRODES (RECOMMENDATIONS 1975) PERGAMON PRESS OXFORD · NEW YORK · PARIS · FRANKFURT ANALYTICAL CHEMISTRY DIVISION COMMISSION ON ANALYTICAL NOMENCLATUREt RECOMMENDATIONS FOR NOMENCLATURE OF ION-SELECTIVE ELECTRODES (Recommendations 1975) INTRODUCTION The rapid expansion of activity in the field of ion-selective electrodes makes it highly desirable to achieve some standardization of nomenclature in this area. The report has been widely circulated by the Commis­ sion to other Commissions of the Analytical Chemistry Division and to experts in many countries in first, second, and third draft forms. It has been modified many times in the light of comments received over a period of two years. This final report incorporates most of the suggestions made on previous drafts, and was prepared by a subcom­ mittee consisting of G. G. Guilbault (Chairman), R. A. Durst, M. S. Frant, H. Preiser, E. H. Hansen, T. S. Light, E. Pungor, G. Rechnitz, N. M. Rice, T, J. Rohm, W. Fig. 1. Simon, and J. D. R. Thomas. I. GENERAL RECOMMENDATIONS In the present state of the art, and for the sake of (a) Definition of terms practical convenience, a simpler (and more convenient) 1. Activity, activity coefficient and concen­ definition is recommended at this time. The practical limit tration. Refer to the Manual of Symbols and Terminology of detection may be taken as the activity (or concentra­ for Physicochemical Quantities and Units (Butterworths, tion) of A at the point of intersection of the extrapolated London, 1973 edition). linear segments of the calibration curve, as shown by the 2. Calibration curve. This is a plot of the potential following illustration: (emf) of a given ion-selective electrode cell assembly (ion-selective electrode combined with an identified refer­ ence electrode) vs the logarithm of the ionic activity (concentration) of a given species. For uniformity, it is recommended that the potential be plotted on the ordinate (vertical axis) with the more positive potentials at the top of the graph and that paA (-log activity of the species measured, A) or PCA be plotted on the abscissa (horizontal axis) with increasing activity to the right. 3. Limit of detection. A calibration curve ordinarily has the shape shown in Fig. \. By analogy with definitions adopted in other fields, the limit of detection should be defined as the concentration for which, under the specified conditions, the potential Ε deviates from the average potential in region I by some arbitrary multiple of the standard error of a single meas­ urement of the potential in region I. Since many factors affect the detection limit, the ex­ perimental conditions used should be reported, i.e. com­ ^Chairman: H. M. N. H. Irving (UK); Secretary: H. Zettler position of the solution, the history and preconditioning of (FRG); Members: G. Baudin (France), H. Preiser (USA), G. G. the electrode, stirring rate, etc. Guilbault (USA), 0. Menis (USA), N. M. Rice (UK), A. J. B. Robertson (UK); Associate Members: A. C. Docherty (UK), W. 4. Drift. This is the slow non-random change with time Fischer (FRG), H. Kaiser (FRG), G. F. Kirkbright (UK), 0. in the potential (emf) of an ion-selective electrode cell Samuelson (Sweden), G. Svehla (UK), G. Tolg (FRG), T. S. West assembly maintained in a solution of constant composi­ (UK); National Representative: H. A. Tawfik (Egypt). tion and temperature. 129 PAC, Vol. 48. No. 1-J 130 COMMISSION ON ANALYTICAL NOMENCLATURE 5. Hysteresis (electrode memory). Hysteresis is said to often added to minimize the effects of certain interfer­ have occurred if, after the concentration has been ences. changed and restored to its original value, there is a 12. Nemstian response. An ion-selective electrode is different potential observed. The reproducibility of the said to have a Nernstian response over a given range of electrode will consequently be poor. The systematic error activity (or concentration) in which a plot of the potential is generally in the direction of the concentration of the of such electrode in conjunction with a reference elec­ solution in which the electrode was previously immersed. trode vs the logarithm of the ionic activity of a given 6. Membrane. This refers to a continuous layer cover­ species (aA) is linear with a slope of 2.303 x 10^ RT/ZAF ing a structure or separating two electrolytic solutions. mV/decade (59.16/ZA mV per unit of paA at 25T). The membrane of an ion-selective electrode is responsible 13. Practical response time. The length of time which for the potential response and selectivity of the electrode elapses between the instant at which an ion-selective (see II for listing of membranes). electrode and a reference electrode are brought into 7. Ion-selective electrodes. These are electrochemical contact with a sample solution (or at which the concentra­ sensors, the potentials of which are linearly dependent on tion of the ion of interest in a solution in contact with an the logarithm of the activity of a given ion in solution. ion-selective electrode and a reference electrode is Such devices are distinct from systems which involve changed) and the first instant at which the potential of the redox reactions (Class I and II electrodes). cell becomes equal to its steady-state value within I mV. Comment: The term "ion-specific electrode" is not The experimental conditions used should be stated, i.e. recommended. The term "specific" implies that the elec­ the stirring rate, the composition of solution of which the trode does not respond to additional ions. Since no response time is measured, the composition of the solu­ electrode is truly specific for one ion, the term "ion- tion to which the electrode was exposed prior to this selective" is recommended as more appropriate. "Selec­ measurement, the history and preconditioning of the tive ion-sensitive electrode" is a little used term to electrode, and the temperature. describe an ion-selective electrode. 14. Combination electrode. An electrochemical ap­ The potential response has as its principal component paratus which incorporates an ion-selective electrode and the free-energy change associated with mass transfer (by a reference electrode in a single assembly, thereby avoid­ ion-exchange, adsorption, solvent extraction or some ing the need for a separate reference electrode. other mechanism) across a phase boundary. 15. Potentiometnc selectivity coefficient. /c^% defines 8. Metering substance. This is any species, other than the ability of an ion-selective electrode to distinguish the ion being measured, whose presence in the sample between different ions in the same solution. It is not solution affects the measured potential of a cell. identical to the similar term used in separation processes. Interfering substances fall into two classes: "electrode" The selectivity coefficient is evaluated by means of the interferences and "method" interferences. Examples of ion-selective electrode emf response, in mixed solutions the first class would be those substances which give a of the primary ion. A, and interfering ion, B, (or less similar response to the ion being measured and whose desirably, in separate solutions). The activities of the presence generally results in an apparent increase in the primary ion. A, and the interfering ion, B, at which /c^% is activity (or concentration) of the ion to be determined determined should always be specified, as the value of (e.g. Na^ for the Ca^^ electrode), those species which /c5,3 is defined by the modified Nernst equation. The interact with the membrane so as to change its chemical smaller the value of the greater the electrode's composition (i.e. organic solvents for the liquid or preference for the principal ion. A, as described later. polyvinylchloride (PVC) membrane electrodes) or elec­ Comment: The terms selectivity constant and selectiv­ trolytes present at a high concentration giving rise to ity factor are frequently used instead of selectivity coeffi­ appreciable liquid-junction potentials. The second class of cient. However, in order to standardize the terminology interfering substance is that which interacts with the ion associated with ion-selective electrodes, use of the term being measured so as to decrease its activity or apparent selectivity coefficient is recommended, as is the fixed concentration, but where the electrode continues to report interference method for its evaluation (see III. D. 2). the true activity (i.e. CN~ present in the measurement of 16. Standard addition or known addition method. This Ag^). is a procedure for the determination of the concentration 9. Reference electrode. An electrode which maintains a of a particular species in a sample by adding known virtually invariant potential under the conditions prevail­ amounts of that species to the sample solution and ing in an electrochemical measurement, and which serves recording the change in potential of an ion-selective to permit the observation, measurement or control of the electrode vs a suitable reference electrode. potential of the indicator (or test) or working electrode. 17. Standard subtraction or known subtraction. This is (Comment: Practical reference electrodes are generally a variation of the standard addition method. In this constructed so that their electrolyte solutions serve as salt procedure changes in the potential resulting from the bridges to the solutions under investigation). addition of a known amount of a species which reacts 10. Internal reference electrode. This is a reference stoichiometrically with the ion of interest (e.g. a complex­ electrode which is contained inside an ion-selective elec­ ing agent) are employed to determine the original activity trode assembly. Comment: The system frequently con­ or concentration of the ion. sists of a silver-silver chloride electrode in contact with an 18. Isopotential point. For a cell containing an ion- appropriate solution containing chloride and a fixed con­ selective electrode and a reference electrode there is often centration of the ion for which the membrane is selective. a particular activity of the ion concerned for which the 11. Ionic'Strength adjustment buffer. A pH buffered potential of the cell is independent of temperature. That solution of high ionic strength added to samples and activity, and the corresponding potential, define the isopo­ calibration solutions before measurement in order to tential point. The identity of the reference electrode, and achieve identical ionic strength and hydrogen ion activity. the composition of the filling solution of the measuring In addition, complexing agents and other components are electrode, must be specified. Recommendations for nomenclature of ion-selective electrodes 131 Π. CLASSIFICATION OF ION-SELECTIVE ELECTRODES classification is the hydrogen gas electrode which A. Primary electrodes responds both to the partial pressure of hydrogen and to 1. Crystalline electrodes. May be homogeneous or pH. The oxygen electrode fits under this classification heterogeneous. although, in contrast to all other sensors, it is an a. Homogeneous Membrane Electrodes are ion- amperometric and not a potentiometric device). selective electrodes in which the membrane is a crystal­ 2. Enzyme substrate electrodes are sensors in which an line material prepared from either a single compound or a ion-selective electrode is covered with a coating contain­ homogeneous mixture of compounds (i.e., AgiS, ing an enzyme which causes the reaction of an organic or Agl/Ag^S). inorganic substance (substrate) to produce a species to b. Heterogeneous Membrane Electrodes are formed which the electrode responds. Alternatively, the sensor when an active substance, or mixture of active sub­ could be covered with a layer of substrate which reacts stances, is mixed with an inert matrix, such as silicone with the enzyme to be assayed, rubber or PVC, or placed on hydrophobized graphite, to form the sensing membrane which is heterogeneous in in. CONSTANTS AND SYMBOLS nature. A. The modified Nemst equation for ion-selective elec­ 2. Non-crystalline electrodes. In these electrodes a trodes and definition of IC^B support, containing an ionic (either cationic or anionic) species or an uncharged species, forms the ion-selective Ε = constant + ^^^^^log [AA + kl%{aBY^''- membrane which is usually interposed between two ZAF aqueous solutions. The support used can be either porous +kñ{^cy^'^c ----] (e.g. Millipore filter, glass frit, etc.) or non-porous (e.g. glass or inert polymeric material such as PVC, yielding Ε is the experimentally observed potential of a with the ion-exchanger and the solvent a "solidified" cell (in millivolts) homogeneous mixture). These electrodes exhibit a re­ R is the gas constant and is equal to 8.31441 sponse due to the presence of the ion-exchange material JK-^ mol-' in the membrane. Τ is the thermodynamic temperature (in °K) a. Rigid matrix electrodes {e.g. glass electrodes) are F is the Faraday constant and is equal to ion-selective electrodes in which the sensing membrane is (9.648670 ± 0.000054) X 10^ C mol' a thin piece of glass. The chemical composition of the a A is the activity of the ion, A glass determines the selectivity of the membrane. In this AB and flc are the activities of the interfering ions, Β and group are: C, respectively hydrogen ion-selective electrodes ^5,% is the potentiometric selectivity coefficient monovalent cation-selective electrodes 2A is an integer with sign and magnitude corres­ b. Electrodes with a mobile carrier: ponding to the charge of the principal ion, A (1) Positively charged—bulky cations (e.g. those of ZB and Zc are integers with sign and magnitude corres­ quaternary ammonium salts or salts of transition metal ponding to the charge of interfering ions, Β complexes such as derivatives of 1, 10-phenanthroline) and C, respectively which, when dissolved in a suitable organic solvent and held on an inert support (e.g. Millipore filter or PVC), The "constant" term includes the standard or zero provide membranes which are sensitive to changes in the potential of the indicator electrode, EJSE, the reference activities of anions. electrode potential, ERef, and the junction potential, Ej (all (2) Negatively charged—Compkxing agents (e.g. of in millivolts). type (RO)2P02") or bulky anions (e.g. tetra-p- chlorophenylborate anions) which, when dissolved in a B. Ionic strength of a solution is defined by 1=\I2 Xc,z,^ suitable organic solvent and held in an inert support (e.g. / is the ionic strength; CI is the concentration in mole Millipore filter or PVC), provide membranes which are per liter of an ion, i, Zi is the charge of the ion, i. sensitive to changes in the activities of cations. (3) Uncharged Carn^r—Electrodes based on solutions C. Other symbols of molecular carriers of cations (e.g. antibiotics, mac- Sign conventions should be in accord with lUPAC rocyclic compounds or other sequestering agents) which recommendations {Manual of Symbols and Terminology can be used in membrane preparations which show for Physicochemical Quantities and Units, Butterworths, sensitivity and selectivity to certain cations. London, 1973 edition, p. 27). B. Sensitized ion-selective electrodes D. Methods for determining 1. Gas sensing electrodes are sensors composed of an 1. Fixed interference method. The potential of a cell indicating and a reference electrode which use a comprising an ion-selective electrode and a reference gas-permeable membrane or an air-gap to separate the electrode is measured with solutions of constant level of sample solution from a thin film of an intermediate interference, AB, and varying activity of the primary ion, solution, which is either held between the gas membrane AA. The potential values obtained are plotted vs the and the ion-sensing membrane of the electrode, or placed activity of the primary ion. The intersection of the on the surface of the electrode using a wetting agent (e.g. extrapolation of the linear portions of this curve will air-gap electrode). This intermediate solution interacts indicate the values of A A which are to be used to calculate with the gaseous species in such a way as to produce a k^Afi from the equation: change in a measure value (e.g. pH) of the intermediate solution. This change is then sensed by the ion-selective kn = aAl{aBy^'\ electrode and is proportional to the partial pressure of the gaseous species in the sample. {Note: An exception to this 2. Separate solution method. The potential of a cell 132 COMMISSION ON ANALYTICAL NOMENCLATURE comprising an ion-selective electrode and a reference logfcp°* E2-E1 ZA\ . ^ electrode is measured with each of two separate solutions, ^'^ 2,303RTIZAF \ ζ J one containing the ion A at the activity AA (but no B), the other containing the ion Β at the same activity AB = «A This method is recommended only if the electrode (but no A). If the measured values are E, and E2, exhibits a Nernstian response. It is less desirable because respectively, the value of )t^°B may be calculated from the it does not represent as well the actual conditions under equation: which the electrodes are used.