The Basics of the ISEs
In this chapter, we will discuss the formalism of the practically relevant representation of the signals obtained from an “ideal” electrode. We will do this using a macroscopic, thermodynamic approach. We will not go into the microscopic details on why an
- PDF / 2,085,933 Bytes
- 22 Pages / 439.37 x 666.142 pts Page_size
- 35 Downloads / 205 Views
The Basics of the ISEs
In this chapter, we will discuss the formalism of the practically relevant representation of the signals obtained from an ‘‘ideal’’ electrode. We will do this using a macroscopic, thermodynamic approach. We will not go into the microscopic details on why and when the electrodes respond in this particular way, leaving this discussion, and also the discussion of the ‘‘real-world electrodes’’, which are not that ideal, for subsequent chapters. The consideration of the mechanism of ISE response relies on two types of electric potentials: boundary potential and diffusion potential. We will start the discussion of these two potentials with the description of their physical origin and then turn to the respective thermodynamical formalism.
2.1 The Membrane Model Basically, a membrane is a phase which separates two other phases. In this way, ion-selective electrode membranes are true membranes. These separate the sample (or the calibrator) solution from either the internal solution of the electrode, or the internal solid contact. The model to be considered is based on several assumptions: 1. The membrane comprises a flat parallel ionically conducting piece of matter placed in between two aqueous electrolyte solutions. Although the system is three-dimensional, any changes may happen only along one axis: the x-axis which is perpendicular to the membrane plane. Therefore, the system is effectively one-dimensional. 2. There are no gradients of temperature and pressure within the system. 3. The interfaces between the membrane and solutions are at electrochemical equilibrium, while the system as a whole is in a steady state.
K. N. Mikhelson, Ion-Selective Electrodes, Lecture Notes in Chemistry 81, DOI: 10.1007/978-3-642-36886-8_2, Ó Springer-Verlag Berlin Heidelberg 2013
11
12
2 The Basics of the ISEs
2.2 Boundary (Interfacial) Potential, the Nernst Equation 2.2.1 The Physical Nature of the Boundary Potential Electric potentials at the interface between two phases may arise due to (1) partitioning of electrolytes, or due to (2) adsorption of charged species, or (3) even in the total absence of individual charged species (ions)—just due to some regular orientation of dipole molecules at the interface. Potentials caused by effects (2) and (3) are only stable in electrolyte-free systems. Otherwise, only in the case (1) are the potentials stable and reproducible. Therefore, since in this book we discuss practically relevant issues, we will focus on the interfacial potential formed due to partitioning of electrolytes between the phases in contact. As example, we consider here two liquid phases. First, we will consider a very simple and highly idealized situation: how an electric potential arises at the interface between two initially neutral (non-charged) phases. We will start with a single phase comprising, for example, an aqueous electrolyte solution with uniform distribution of ions within the whole volume of the phase (no concentration gradient). Ions bear electric charge, and therefore, there is som
Data Loading...
