The concept of a strong collision has been bound intimately with the development of the theory of unimolecular reactions ever since its inception in the 1920s. In 1927, Rice & Ramsperger introduced the approximation that an activated molecule would be deactivated whenever it suffered a collision, although they were well aware of the limitations in making such an assumption [27.R]; they supposed that it would be possible at a later stage to determine the extent to which this approximation was untrue, but I doubt if it was envisaged that it would take more than 50 years for us to achieve a realistic understanding of the problem. The principal reason for this delay has been the failure of kineticists to formulate a proper definition of a strong collision; the usual description has been a rather intuitive one [72.R; 73.F], generally adequate within the framework of a steady state treatment of the reaction process, but not always consistent with the principle of detailed balancing. Two useful (and equivalent) verbal definitions of a strong collision taken from the literature are as follows: a strong collision is one in which so much energy is transferred that the subsequent condition of the molecule may be chosen at random (with appropriate weighting factors for energy) from all its possible states [66.B2]; strong collisions are Markovian events whose outcome follows the probabilities given by the Boltzmann distribution, without reference to the initial conditions [77.T1].

Because of an editorial policy discouraging the presentation of experimental results in both graphical and tabular form, the primary rate constant data for the thermal isomerisation of cyclopropane in the fall-off region [53.P2] are only available in thesis form. One might have expected these results to have been superseded by now, but that has not happened, and Sowden's rather inaccessible thesis [54. S] remains the only source of these key data. In view of their continuing importance in the testing of unimolecular reaction theories, I am reproducing those results here (and also those of the cyclobutane reaction) for the convenience of future users.

The general perception, all these years, of the course of a unimolecular reaction has been that the reactant molecule receives the requisite amount of energy by collision, but that it can only react to form products after a time delay during which it has to rearrange that energy and reassemble it into some more appropriate fashion. This reorganisation of the internal energy has almost always been regarded as an intramolecular process [30. R], with a rate which is independent of all external conditions; except for this, the nature of the reorganisation process has been rather poorly defined. Broadly speaking, older treatments of the problem tend to visualise the molecule comprising two domains, one where the energy is collected from external sources by collision, and the other where it is needed to bring about the reaction, with a time delay for communication of internal energy between the two. On the other hand, many modern treatments use the language of radiationless processes: as such, they tend to be unnecessarily complicated from the point of view of treating bulk unimolecular reactions, and they also tend to neglect other possible causes which could have the same end result. The treatment given below is an elaboration of one which I have evolved [80.P2] and which I believe is helpful in clarifying the nature of the unimolecular reaction process.

The reader who has followed the development of this book from the beginning will recognise a progressive increase in the degree of speculation, chapter upon chapter; after describing the basic experimental phenomena, we began with a treatment of relaxation in simple molecules, which is virtually irrefutable, and ended up with an attempt to treat in a semiquantitative manner such concepts as those of randomisation and of activation bottlenecks, about which we still know really very little. For the sake of brevity, I have tried to keep the speculation to the minimum required to form a consistent foundation for the treatment of unimolecular reaction manifestations, wherever they may occur. My discussion has concentrated almost solely on the shape of the fall-off curve for a few thermal unimolecular reactions: this, despite the fact that I enumerated other interesting properties of simple thermal reactions in Chapter 1, and omitted entirely to mention the wide range of other experimental properties usually encompassed within the general topic of unimolecular reaction theory [72.R; 73.F]. In conclusion, therefore, I would now like to hold a brief inspection of each of those topics raised in Chapter 1, together with a few others so far not mentioned, to see how successful the present theory appears to be.

Measurement with ion-selective electrodes appears at first sight extremely simple and this is one of the main reasons for the great popularity of the method. However, to obtain meaningful results a number of conditions must be met, conditions which are sometimes contradictory and difficult to fulfil. A severe limitation is imposed on measurements with ISEs because of a fact inherent to most electrochemical methods: namely, that the measurement depends on heterogeneous reactions occurring at the electrode-solution interface. Consequently, the reproducibility and long-term constancy of the conditions at the interface is of paramount importance for accurate and reproducible measurements. There is no general solution to this problem and an ideal state can be approached more or less closely only by judicious selection of the experimental conditions, starting with the sample preparation, through the actual measurement, to the handling of the results. This is the main reason for the fact that of the extensive literature on ISEs, only a small part deals with real practical applications rather than with the laboratory study of the electrodes and methods, and that routine use of the ISEs in chemical analysis is less common than it would appear from the literature.

To attain satisfactory performance with most electrochemical methods, including potentiometry with ISEs in routine analytical work, a certain amount of information on electrochemistry and experimental experience is required. In this respect, electrochemical methods often compare unfavourably with spectral methods.

In physics, an elastic two-dimensional plate is termed a membrane (Latin membrana = parchment) but in chemistry the term denotes a body, usually thin, which serves as a phase separating two other bulk phases. If this body is permeable to the same degree for all components of the adjacent phases and does not affect their mobility, then its only function is to prevent rapid mixing of the two phases. This is then termed a diaphragm. A real membrane must exhibit a certain selectivity, based on different permeability for the components of the two phases, and is then termed a semipermeable membrane. Membranes separating two electrolytes that are not permeable to the same degree for all ions are called electrochemical membranes. It is with these that we are concerned here.

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined.

The second edition of Ion-Selective Electrodes contains a survey of the theory and applications of ion-selective electrodes based on the literature published up to mid-1981. Because of the rapid progress in the whole field and the very large amount of diverse data, a compact and unified treatment of the theory has been attempted. The technology and the applications have also been updated. In view of these facts we have had to write practically a new book. In contrast to the first edition, only a selective list of references could be included in the book, because otherwise we would have to deal with more than four thousand references.

We are grateful to our colleagues Professor J. Dvořák (Faculty of Science, Charles University, Prague), Dr J. Bureš (The Institute of Physiology, Czechoslovak Academy of Sciences, Prague), Dr J. Veselý (The Geological Survey, Prague) and to Dr Z. Samec (The J. Heyrovský Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Prague) for valuable suggestions during the preparation of both editions of the book. The Czech manuscript has been translated into English by Dr M. Hyman-Štulíková. We would also like to thank Dr A. Kejharová, Mrs M. Kozlová, Mrs D. Tůmová and Mrs L. Korytová for their help with the preparation of the manuscript.

As stated on p. 28, an analytically ideal sensor would determine the determinand both specifically and quantitatively. In potentiometry, this would require an electrode sensitive to one single substance among all the components of the system.

Cationic metal electrodes definitely do not have this property. They attain (though not always) a potential that is a function of the activity of the corresponding ion according to the Nernst equation; however, in the presence of the ions of nobler metals these electrodes become covered with a layer of the deposited nobler metal and thus have quite different properties. Again, in the presence of components of oxidation-reduction systems in the test solution, mixed potentials develop at these electrodes (see for example [86a], and p. 000). The dependence of this potential on the solution composition is complex and thus such electrodes are unsuitable for analytical purposes. Some cation electrodes do not obey the Nernst equation with respect to the corresponding ions and their potentials are simultaneously affected by the formation of an oxide film and by reactions with various solution components (hydrogen ions, oxygen, etc.). Therefore, cation electrodes are not suitable sensors for specific estimation of the determinand concentration. In optimal cases, redox electrodes sometimes yield thermodynamic potentials corresponding to components in the solution, but mixed potentials are also often encountered.

Anion electrodes of the second kind have better properties in this respect. They respond to the corresponding anions over wide activity ranges and the only major interference with their function comes from anions forming salts less soluble than the salt covering the electrode.

There is a tendency in many fields of contemporary physical and chemical science to seek impulses from biology for solving problems. This tendency shows promise, not only because it leads to new artificial tools, materials or processes, but also because it forms models permitting deeper understanding, at least in certain areas, of the actual processes occurring in nature. Efforts to imitate nature thus facilitate its deeper understanding.

Ion-selective electrodes are a remarkable product of this approach. Their development can be followed in several circular pathways from natural bioelectric phenomena to artificial membrane systems and back again, to attempts to explain processes at a cellular level.

The relationship between electric and physiological processes was discovered in 1791 by L. Galvani [27] in his classic experiments with frog muscles and nerves. In spite of these exciting results, the study of electrophysiological phenomena did not progress for several decades. M. Faraday [99] studied the electricity produced by the torpedo, but only in order to prove that this ‘animal’ electricity is the same as other kinds of electricity, for example ‘voltaic’ electricity from an electric machine or ‘galvanic’ electricity from a galvanic cell.

In 1848 du Bois-Reymond [21] suggested that the surfaces of biological formations have a property similar to the electrode of a galvanic cell and that this is the source of bioelectric phenomena observed in damaged tissues. The properties of biological membranes could not, however, be explained before at least the basic electrochemistry of simple models was formulated.