Sensors conductometric

In Chapters 6 and 7, we discussed potentiometric and amperometric sensors, respectively. The third basic electrochemical parameter that can yield sensory information is the conductance of the electrochemical cell (Fig. 8.1). Conductance is the reciprocal of resistance. It is related to current and potential through the generalized form of Ohm s law (C.l). If the measurement is done with AC signal conductance (G) becomes frequency-dependent conductance G(co) and the resistance R becomes impedance Z( (o). 

The unit of conductance is Siemens (mho), which is the reciprocal of resistance R (ohm). Resistance is related to the resistivity (p) by the ratio of the length (L) and cross-section (A). 

On the other hand, conductivity (cr) or specific conductance (reciprocal of resistivity p) is a material property that is normalized with respect to area, potential gradient, and time. It is expressed as the ratio of current density j (A cm-2) and electric field E (V cm-1). 

Its unit is Siemens-1 cm-1. These terms are used throughout this section interchangeably. These sensors are popularly called chemiresistors, which clearly implies their function. They are simple to fabricate, but the interpretation of their responses and the mechanism of their operation are anything but simple. The fundamental equivalency between current and resistance, which is inherent in the generalized Ohm s law (8.1), sometimes blurs the line between amperometric and conductometric sensors. Nevertheless, it is important to remember that chemiresis- 

Janata, Principles of Chemical Sensors, DOI 10.1007/978-0-387-69931-8 8, Springer Science+Business Media, LLC 2009 

The AC resistance (impedance) of electrodes is related in a highly complex manner to processes at the electrode surface, but also to the resistance of the homogeneous bulk solution. Conductometry might be considered an electrochemical method. For the sake of clarity, however, one should distinguish between methods connected with electrochemical processes at the electrodes and, on the other hand, methods dealing with properties of a homogeneous bulk solution. The latter is not a question of chemistry but of physical behaviour like ion mobility. In what follows, conductometric sensors are considered simple resistance probes. 

One of the oldest instrumental methods for concentration determination is to measure electrolytic conductance. The measurement set-up is simple. It is sufficient to determine the resistance between two inert metalhc electrodes. The well-known Wheatstone bridge can be utilized as with any other resistance measurement (Fig. 5.1). The bridge is balanced, i.e. the variable resistor Rv is adjusted to obtain zero at the instrument included in the circuit. Commonly this is an oscilloscope, since AC amplitudes must be detected. If the scope displays zero, the following condition is valid  

Strictly speaking, the measuring result contains not only ohmic quantities, but also some undesired capacitive contribution. Certain traditional methods can 

Obviously, with conductance measurements only non-specific results can be obtained. Fortunately, the individual constants k[ do not differ too much, except for HaO and OH in aqueous solution. It is allowed, therefore, to estimate the total ionic concentration or the overall salinity of seawater from the results of conductance measurements. Conductance sensors are thus encountered with all seawater probes commonly used in oceanography. In seawater, k[ can be considered a universal constant since sodium chloride is present in huge excess compared to all other components. 

Conductance sensors are widely used as detectors in an important special variant of liquid chromatography. 

---

Sergeyeva TA, Piletsky SA, Brovko AA, Slinchenko EA, Sergeeva LM, El skaya AV. Selective recognition of atrazine by molecularly imprinted polymer membranes. Development of conductometric sensor for herbicides detection. Anal Chim Acta 1999 392 105-111. 

These chapters divide the discussion of electrochemical sensors by the mode of measurement. This chapter is an introduction to the general parameters and characteristics of electrochemical sensors. Chapter 6 focuses on potentiometric sensors, which measure voltage. Chapter 7 describes amperometric sensors, which measure current. Chapter 8 examines conductometric sensors, which measure conductivity. 

Here also lies the reason for making the area of the working electrode much smaller than that of the auxiliary electrode. Because the two electrodes are connected in series, the larger impedance of the two dominates the overall i - V response of the cell. Because we want all the information to originate only from the working electrode, we have to make its area smaller. This point is frequently neglected when, for convenience of fabrication, electrodes of equal area are used in some microfabricated amperometric and conductometric sensors. 

On the other hand, for very high frequencies, the electrolyte resistance Rs dominates. That is, by the way, the principal reason for using high-frequency excitation in conductometric sensors (Chapter 8) when we want to avoid polarization of the electrodes. 

Discussion of general operation of conductometric sensors can begin by analyzing Fig. 8.1. Once again we realize that the electrochemical cell is a complex arrangement of resistances and capacitances. The primary interaction between the sample and the sensor involves the selective layer the sensory information is... 

Frequency as an experimental variable offers additional design flexibility. This approach has several advantages. The most important one is the lack of polarization of the contacts. The second one is the fact that equivalent electrical circuit analysis can be used that aids in elucidation of the transduction mechanisms. Perhaps the most important distinguishing feature of this class of conductometric sensors is the fact that their impedance is measured in the direction normal to their surface. In fact, there may be no requirement on their DC conductivity and their response can be obtained from their capacitive behavior. In the following section, we examine so-called impedance sensors (or impedimetric sensors see Fig. 8.1b). 

The idea of separating the gas sample by a gas-permeable membrane from the actual internal sensing element is common to several types of electrochemical and some optical sensors. The potentiometric Severinghaus electrode and the amperometric oxygen Clark electrode have already been discussed. Actually, most types of sensors can be used in this configuration and the conductometric sensor is not an exception (Bruckenstein and Symanski, 1986). 

Chapter 10 deals with composite films synthesized by the physical vapor deposition method. These films consist of dielectric matrix containing metal or semiconductor (M/SC) nanoparticles. The film structure is considered and discussed in relation to the mechanism of their formation. Some models of nucleation and growth of M/SC nanoparticles in dielectric matrix are presented. The properties of films including dark and photo-induced conductivity, conductometric sensor properties, dielectric characteristics, and catalytic activity as well as their dependence on film structure are discussed. There is special focus on the physical and chemical effects caused by the interaction of M/SC nanoparticles with the environment and charge transfer between nanoparticles in the matrix. 

Physical and chemical properties of PVD-produced composite films with M/SC nanoparticles including dark- and photo-induced conductivity, conductometric sensoring properties, dielectric characteristics, and catalytic activity. 

Experimental data relating to the conductivity of composite films with M/SC nanoparticles are described by the classical percolation model in terms of tunnel processes. Chemisorption of chemical compounds on the surface of M/SC nanoparticles in films and the subsequent reactions with participation of chemisorbed molecules change the concentration of conducting electrons and/or barriers for their tunnel transfer between the nanoparticles with the result of strong influence on the film conductivity. Such films are used as conductometric sensors for detecting various substances in an atmosphere. 

Ion-exchange polymers, redox polymers and especially electroconducting polymers have been utilised extensively in CMEs including CWEs, ISFETs, amperometric and conductometric sensors, both as selective accumulators and as... 

---