Instrumentation potentiometry

Techniques, such as spectroscopy (Chapter 10), potentiometry (Chapter 11), and voltammetry (Chapter 11), in which the signal is proportional to the relative amount of analyte in a sample are called concentration techniques. Since most concentration techniques rely on measuring an optical or electrical signal, they also are known as instrumental techniques. For a concentration technique, the relationship between the signal and the analyte is a theoretical function that depends on experimental conditions and the instrumentation used to measure the signal. For this reason the value of k in equation 3.2 must be determined experimentally. 

In the present chapter consideration is given to various types of indicator and reference electrodes, to the procedures and instrumentation for measuring cell e.m.f., to some selected examples of determinations carried out by direct potentiometry, and to some typical examples of potentiometric titrations. 

The most common potentiometry involves an instrument called a pH-Staf , in which a glass (pH) electrode follows reactions that either consume or produce protons. Since pH... 

Some of the typical parameters or properties utilized for NIR detection are potentiometry,(5) absorbance,(52 54) refractometry/18,19) or fluorescence spectros-copy.(55) Of these, has proven to be the most valuable detection method in fiber optic applications/2,56) In standard spectroscopic techniques, the detection limits of a method are greatly determined by the instrument and by the chemical method used for the analysis. However, in OFCD research the detection limits are governed by a series of other variables including the dye, the matrix, and the instrument. By optimizing these variables, low detection limits can be obtained with this technique. 

The main advantages that should gradually promote potentiometry with ISEs to routine use in clinical laboratories are simplicity of instrumentation, the possibility of decreasing the sample volume (especially important in pediatry), and the possibility of avoiding tedious centrifugation necessary for preparation of plasma and serum (significant under intensive care conditions). On the other hand, work with ISEs requires experience and skill... 

Other methods of instrumental analysis include polarography, potentiometry, emission spectroscopy, x-ray diffraction, and x-ray fluorescence. 

The most common analytical methods used were gas chromatography, HPLC, AA spectrophotometry, polarography, colorimetry, and potentiometry with ion-selective electrodes. In this study GC/MS and other more expensive instrumentation were avoided. If sorbent tubes could not be used for gaseous substances, then the less desirable miniature bubblers or impingers were considered. Although these devices are inconvenient they were often used because no better alternatives were available. Bags were used in a few cases where the analyte could not be retained on a sorbent because of volatility and a small tendency to sorb. Filters were used for particulates. Combinations of collection devices were used if we felt that both particulates and vapor might be present in the analyte. 

Numerous methods have been published for the determination of trace amounts of tellurium (33—42). Instrumental analytical methods (qv) used to determine trace amounts of tellurium include atomic absorption spectrometry, flame, graphite furnace, and hydride generation inductively coupled argon plasma optical emission spectrometry inductively coupled plasma mass spectrometry neutron activation analysis and spectrophotometry (see Mass SPECTROMETRY Spectroscopy, OPTICAL). Other instrumental methods include polarography, potentiometry, emission spectroscopy, x-ray diffraction, and x-ray fluorescence. 

S. L. Truman, Potentiometry pH and ion-selective electrode, in W. E. Galen, Analytical Instrument Handbook, Marcel Dekker, New York, 1997. 

Laboratory investigations play an essential role in medicine. Laboratory results are taken into consideration in about two thirds of all medical decisions in medical systems of industrialized countries today. The vast majority of clinical chemistry analyses are based on few analytical principles including photometry, ligand binding assays and potentiometry. For these standard methods complete automation has been achieved and multi-channel, random access analyzers realize several hundred analyses per instrument and hour on a very high level of user-friendliness. Consequently, clinical chemistry is very cost efficient today typically clinical chemistry analyses contribute less than 5 % of all costs of tertiary care hospitals. 

When voltammetry measurements are made in nonaqueous solvents, the problems of an adequate reference electrode are compounded. Until the 1960s the most common reference electrode was the mercury pool, because of its convenience rather than because of its reliability. With the advent of sophisticated electronic voltammetric instrumentation, more reliable reference electrodes have been possible, especially if a three-electrode system is used. Thus, variation of the potential of the counter electrode is not a problem if a second non-current-canying reference electrode is used to monitor the potential of the sensing electrode. If three-eleetrode instrumentation is used, any of the conventional reference electrodes common to potentiometry may be used satisfactorily. Our own preference is a silver chloride electrode connected to the sample solution by an appropriate noninterfering salt bridge. The one problem with this system is that it introduces a junction potential between the two solvent systems that may be quite large. However, such a reference system is reproducible and should ensure that two groups of workers can obtain the same results. 

The goal of this volume is to provide (1) an introduction to the basic principles of electrochemistry (Chapter 1), potentiometry (Chapter 2), voltammetry (Chapter 3), and electrochemical titrations (Chapter 4) (2) a practical, up-to-date summary of indicator electrodes (Chapter 5), electrochemical cells and instrumentation (Chapter 6), and solvents and electrolytes (Chapter 7) and (3) illustrative examples of molecular characterization (via electrochemical measurements) of hydronium ion, Br0nsted acids, and H2 (Chapter 8) dioxygen species (02, OJ/HOO-, HOOH) and H20/H0 (Chapter 9) metals, metal compounds, and metal complexes (Chapter 10) nonmetals (Chapter 11) carbon compounds (Chapter 12) and organometallic compounds and metallopor-phyrins (Chapter 13). The later chapters contain specific characterizations of representative molecules within a class, which we hope will reduce the barriers of unfamiliarity and encourage the reader to make use of electrochemistry for related chemical systems. 

Evans, A., Potentiometry and Ion Selective Electrodes, Wiley, Chichester, 1987. Greef, R., Peat, R., Petes, L. M., Pletcher, D. and Robinson, J., Instrumental Methods in Electrochemistry, Ellis Horwood (Wiley), Chichester, 1985. 

More than brief discussion of the numerous ways in which end points can be taken other than by visual methods is beyond our scope. For example, end-point techniques may involve photometry, potentiometry, amperometry, conductometry, and thermal methods. In principle, many physical properties can be used to follow the course of a titration in acid-base titrations, use of the pH meter is common. In terms of speed and cost, visual indicators are usually preferred to instrumental methods when they give adequate precision and accuracy for the purposes at hand. Selected instrumental methods may be used when a suitable indicator is not available, when higher accuracy under unfavorable equilibrium conditions is required, or for the routine analysis of large numbers of samples. 

Potentiometry. Potentiometric methods rely on the logarithmic relationship between measured potential and analyte concentration. The most common involves an instrument called a pH-Stat , in which a glass (pH) electrode follows reactions that either consume or produce protons. Since pH changes cause changes in enzyme activity, the pH is maintained at a constant value by the addition of acid or base. The rate of titrant addition is then proportional to the rate of the enzymatic reaction. Precise measurements using the pH-Stat require low buffer concentrations in the enzymatic assay mixture. 

The most convenient approach to obtain kinetic data is to monitor the progress of the reaction continuously by spectrophotometry, conductometry, potentiometry, or some other instrumental technique. With the advent of inexpensive computers, instrumental readings proportional to concentrations of reactants or products, or both, are often recorded directly as a function of time, stored in the computer s memory, and retrieved later for data processing. 

The first group, which is developed in this chapter, use ion selective electrodes (ISE). The principle of these chemical sensors is to create an electric cell in which the analyte behaves in such a way that the potential difference obtained relates to its concentration. Measurement of pH, probably the most common and best known electroanalytical method, is part of this group. Most of the measurements concern the determination of ions in aqueous solution, though particular electrodes with selective membranes also allow the determination of molecules. The sensitivity of these methods is very great for certain ions but matrix is sometime responsible for lack of reliability in these measurements. In such cases, complexometric or titrimetric methods must replace direct potentiometry. It remains however for potentiometry multiple applications in which the instruments range from low-cost pH meters to automatic titrators. 

Electrochemical instruments are normally based on the changes in electrical energy that occur when a chemical reaction takes place, for example ion-selective electrodes (potentiometry) and voltammetric techniques. These can be measured in different ways and can give various qualitative and quantitative information about the reactants or products. In the case of conductivity measurements, changes in ionic content are monitored and, although nonspecific, can give useful data. 

Potentiometry requires a reference electrode, a working electrode and a potentialmeasuring instrument, e.g. voltmeter, otherwise known as a potentiometer. The test solution must be in direct contact with the working electrode, which is sometimes referred to as the chemical sensor as it is sensing the output of a chemical reaction. The reference electrode can also be placed in the test solution or can be brought into contact with the test solution via a salt bridge. The measured potential can be related to the concentration of the species being measured and this approach is called direct potentiometry. 

In recent years, it has become possible to measure more selective properties on-line by incorporating instruments such as potentiometry (Figure 9.6), IR and GC into the process stream. These provide a dynamic rather than historic measure of what is going on with the process. The data are fed back to controllers this permits better regulation of the process... 

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