Thermodynamic sensors

Thus, the measurement of exchange rates of backbone amide hydrogen serves as a precise thermodynamic sensor of the local environment. 

Thermodynamic equilibrium in the sample has to be ensured. That requires that the sample is isothermal, otherwise the heat flux cannot be attributed to the single temperature indicated at the sensor. Furtheron, the sample has to be in reaction equilibrium, so there should be no subcooling of the sample. 

In the past few years, a large number of experimental and theoretical studies have focused on metal oxide surfaces with the aim of gaining insight into their catalytic, photocatalytic, and gas-sensing activity [68]. Owing to its thermodynamic stability and relatively easy preparation, the rutile Ti02(l 10) surface has evolved into one of the key models for metal oxide surfaces. For example, it has been extensively used in the research of biocompatible materials, gas sensors, and photocatalysts [69]. 

Optical methods are a perfect tool to characterize interaction processes between a sensitive chemical or bio polymer layer and analytes1. Time-resolved measurements of this interaction process provide kinetic and thermodynamic data. These types of sensors allow the monitoring of production processes, quantification of analytes in mixtures and many applications in the area of diagnostics, biomolecular interaction processes, DNAhybridization studies and evenprotein/protein interactions2,3. 

Electrochemical sensors, however, currently share one key advantage an excitation signal may be imposed that can trigger a sensing reaction, and the energy required for an otherwise thermodynamically unfavorable extraction and/or binding process can be... 

Particular cases are potassium selective potentiometric sensors based on cobalt [41] and nickel [38, 42] hexacyanoferrates. As mentioned, these hexacyanoferrates possess quite satisfactory redox activity with sodium as counter-cation [18]. According to the two possible mechanisms of such redox activity (either sodium ions penetrate the lattice or charge compensation occurs due to entrapment of anions) there is no thermodynamic background for selectivity of these sensors. In these cases electroactive films seem to operate as smart materials similar to conductive polymers in electronic noses. 

In conclusion, the unique properties of Prussian blue and other transition metal hexa-cyanoferrates, which are advantageous over existing materials concerning their analytical applications, should be mentioned. First, metal hexacyanoferrates provide the possibility to develop amperometric sensors for non-electroactive cations. In contrast to common smart materials , the sensitivity and selectivity of metal hexacyanoferrates to such ions is provided by thermodynamic background non-electroactive cations are entrapped in the films for charge compensation upon redox reactions. 

For a superior introduction to this difficult topic, try Peter Rock s now classic book, Chemical Thermodynamics, Oxford University Press, Oxford, 1983. The treatment in Temperature Measurement (second edition), by Ludwik Michalski, Joseph McGhee, Krystyna Eckersdorf and Jacek Kucharski, Wiley, New York, 2001, is aimed at engineers manufacturing temperature-measuring machines, such as electrical and optical sensors, but some of its introductory material might help. 

In the early part of this century, many types of solid electrolyte had already been reported. High conductivity was found in a number of metal halides. One of the first applications of solid electrolytes was to measure the thermodynamic properties of solid compounds at high temperatures. Katayama (1908) and Kiukkola and Wagner (1957) made extensive measurements of free enthalpy changes of chemical reactions at higher temperatures. Similar potentiometric measurements of solid electrolyte cells are still made in the context of electrochemical sensors which are one of the most important technical applications for solid electrolytes. 

The amount of water in the reaction mixture can be quantified in different ways. The most common way is to nse the water concentration (in mol/1 or % by volume). However, the water concentration does not give much information on the key parameter enzyme hydration. In order to have a parameter which is better correlated with enzyme hydration, researchers have started to nse the water activity to quantify the amount of water in non-conventional reaction media (Hailing, 1984 Bell et al, 1995). For a detailed description of the term activity (thermodynamic activity), please look in a textbook in physical chemistiy. Activities are often very nselul when studying chemical equilibria and chemical reactions of all kinds, but since they are often difficult to measure they are not used as mnch as concentrations. Normally, the water activity is defined so that it is 1.0 in pure water and 0.0 in a completely dry system. Thus, dilute aqueous solutions have water activities close to 1 while non-conventional media are found in the whole range of water activities between 0 and 1. There is a good correlation between the water activity and enzyme hydration and thns enzyme activity. An advantage with the activity parameter is that the activity of a component is the same in all phases at eqnihbrium. The water activity is most conveniently measnred in the gas phase with a special sensor. The water activity in a liqnid phase can thns be measured in the gas phase above the liquid after equilibration. 

Various potentiometric indicator electrodes work as sensors for ion solvation. Metal and metal amalgam electrodes, in principle, respond in a thermodynamic way to the solvation energy of the relevant metal ions. Some ion-selective electrodes can also respond almost thermodynamically to the solvation energies of the ions to which they are sensitive. Thus, the main difficulty in the potentiometric study of ion solvation arises from having to compare the potentials in different solvents, even though there is no thermodynamic way of doing it. In order to overcome this difficulty, we have to employ a method based on an extra-thermodynamic assumption. For example, we can use (1) or (2) below ... 

A/iAg as a function of time with a single and spatially fixed sensor at , or one can determine D with several sensors as a function of the coordinate if at a given time [K.D. Becker, et al. (1983)]. An interesting result of such a determination of D is its dependence on non-stoichiometry. Since >Ag = DAg d (pAg/R T)/d In 3, and >Ag is constant in structurally or heavily Frenkel disordered material (<5 1), DAg(S) directly reflects the (normalized) thermodynamic factor, d(pAg/R T)/ In 3, as a function of composition, that is, the non-stoichiometry 3. From Section 2.3 we know that the thermodynamic factor of compounds is given as the derivative of a point defect titration curve in which nAg is plotted as a function of In 3. At S = 0, the thermodynamic factor has a maximum. For 0-Ag2S at T = 176 °C, one sees from the quoted diffusion measurements that at stoichiometric composition (3 = 0), the thermodynamic factor may be as large as to 102-103. 

Dew-point measurement is a primary method based on fundamental thermodynamics principles and as such does not require calibration. However, the instrument performance needs to be verified using salt standards and distilled water before sampling (see Support Protocol). To obtain accurate and reproducible water activity results with a dew-point instrument, temperature, sensor cleanliness, and sample preparation must be considered. Equipment should be used and maintained in accordance with the manufacturer s instruction manual and with good laboratory practice. If there are any concerns, the manufacturer of the instrument should be consulted. Guidelines common to dew-point instruments for proper water activity determinations are described in this protocol. The manufacturer s instructions should be referred to for specifics. 

Reversibility in the context of chemical sensing means that the response follows concentration changes, both up and down. It does not have the usual thermodynamic meaning, despite the fact that a decrease of free energy is always the driving force in all interactions. Thus, sensors can be either thermodynamically reversible, as with ion-selective electrodes, or thermodynamically irreversible, as with enzyme electrodes if, however, they respond to a step up or a step down in the concentration... 

The first law of thermodynamics tells us that any process in which the internal energy of the system changes is accompanied by absorption or by evolution of heat. The class of chemical sensors discussed in this chapter uses the heat generated by a steady-state chemical reaction as the source of analytical information. 

Despite the pervasive use of electrochemical sensors and the fundamental importance of electrochemistry as a division of physical and analytical chemistry, this field of study has not traditionally been a favorite of students. One reason for this could be the fact that most electrochemical and electroanalytical textbooks introduce electrochemistry by explaining first the thermodynamics of the electrochemical cell. That approach is bound to discourage all but the brave few. 

This interface is also known as the perm-selective interface (Fig. 6.1a). It is found in ion-selective sensors, such as ion-selective electrodes and ion-selective field-effect transistors. It is the site of the Nernst potential, which we now derive from the thermodynamic point of view. Because the zero-current axis in Fig. 5.1 represents the electrochemical cell at equilibrium, the partitioning of charged species between the two phases is described by the Gibbs equation (A.20), from which it follows that the electrochemical potential of the species i in the sample phase (S) and in the electrode phase (m) must be equal. 

It is important to note that the electrode potential is related to activity and not to concentration. This is because the partitioning equilibria are governed by the chemical (or electrochemical) potentials, which must be expressed in activities. The multiplier in front of the logarithmic term is known as the Nernst slope . At 25°C it has a value of 59.16mV/z/. Why did we switch from n to z when deriving the Nernst equation in thermodynamic terms Symbol n is typically used for the number of electrons, that is, for redox reactions, whereas symbol z describes the number of charges per ion. Symbol z is more appropriate when we talk about transfer of any charged species, especially ions across the interface, such as in ion-selective potentiometric sensors. For example, consider the redox reaction Fe3+ + e = Fe2+ at the Pt electrode. Here, the n = 1. However, if the ferric ion is transferred to the ion-selective membrane, z = 3 for the ferrous ion, z = 2. 

As we have seen already, there are two kinds of selectivity thermodynamic selectivity and kinetic selectivity (Chapter 2). Let us first consider how we could use thermodynamic selectivity for amperometric sensors. The electrode and the solution form a double-layer capacitor. The minimum energy of this capacitor occurs at... 

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