Acid-base titrations types

In the overview to this chapter we noted that the experimentally determined end point should coincide with the titration s equivalence point. For an acid-base titration, the equivalence point is characterized by a pH level that is a function of the acid-base strengths and concentrations of the analyte and titrant. The pH at the end point, however, may or may not correspond to the pH at the equivalence point. To understand the relationship between end points and equivalence points we must know how the pH changes during a titration. In this section we will learn how to construct titration curves for several important types of acid-base titrations. Our... 

In acid-base titrations the end point is generally detected by a pH-sensitive indicator. In the EDTA titration a metal ion-sensitive indicator (abbreviated, to metal indicator or metal-ion indicator) is often employed to detect changes of pM. Such indicators (which contain types of chelate groupings and generally possess resonance systems typical of dyestuffs) form complexes with specific metal ions, which differ in colour from the free indicator and produce a sudden colour change at the equivalence point. The end point of the titration can also be evaluated by other methods including potentiometric, amperometric, and spectrophotometric techniques. 

The indicator electrode employed in a potentiometric titration will, of course, be dependent upon the type of reaction which is under investigation. Thus, for an acid-base titration, the indicator electrode is usually a glass electrode (Section 15.6) for a precipitation titration (halide with silver nitrate, or silver with chloride) a silver electrode will be used, and for a redox titration [e.g. iron(II) with dichromate] a plain platinum wire is used as the redox electrode. 

Because synthetic products are isolated as the barium or, more frequently, the calcium salt of leucovorin, common acid-base titrations are not reported. If this type of titration or one in which the cation is exchanged were feasible, the results would require careful interpretation because impurities containing the glutamic acid moiety would respond similarly to leucovorin when the carboxyl groups are being analyzed. 

A common laboratory application of acid-base reactions is titration. A titration is a laboratory procedure in which we use a solution of known concentration to determine some information (such as concentration and mass) about an unknown substance. A titration may involve any type of reaction—acid-base, redox, and so on. In this section, we will only consider acid-base titrations. The... 

Titrations are veiy powerful techniques that contain two very different kinds of information and thus serve two different purposes (a) titrations are used for quantitative analytical applications, e.g. the determination of the concentration of an acid by an acid-base titration or the determination of a metal ion by a complexometric titration (b) titrations serve also as a method for the determination of equilibrium constants, e.g. the determination of the strength of the interaction between a metal ion and a ligand. Naturally, both objectives can be combined and the analysis of one titration can deliver both types of information. 

The pH window is very wide in solvents that are weak both in acidity and basicity. The widths of the pH window are well over 30 in such solvents, compared to about 14 in water (Table 6.6). The usefulness of these expanded pH regions is discussed in Section 3.2.2. In particular, potentiometric acid-base titrations in such solvents are highly useful in practical chemical analyses as well as physicochemical studies [22]. Acid-base titrations in lion-aqueous solvents were popular until the 1980s, but now most have been replaced by chromatographic methods. However, the pH-ISFETs are promising to realize simple, rapid and miniature-scale acid-base titrations in lion-aqueous solvents. For example, by use of an Si3N4-type pH-ISFET, we can get an almost complete titration curve in less than 20 s in a solution containing several different acids [17d]. 

The unimodality constraint allows the presence of only one maximum per profile (see Figure 11.7) [42, 55, 60], This condition is fulfilled by many peak-shaped concentration profiles, like chromatograms or some types of reaction profiles, and by some instrumental signals, like certain voltammetric responses. It is important to note that this constraint does not only apply to peaks, but to profiles that have a constant maximum (plateau) or a decreasing tendency. This is the case for many monotonic reaction profiles that show only the decay or the emergence of a compound [47, 48, 51, 61], such as the most protonated and deprotonated species in an acid-base titration, respectively. 

As it was for acid-base titrations, the concept of relative precision (Section 3-7) is useful for comparing the steepness of titration curves in the immediate vicinity of the end point. For the formation of a precipitate of symmetrical charge type (m = n), with activity coefficients assumed to be unity. 

Obviously, natural waters are not closed systems. The idealized model discussed so far is still useful because a natural water sample in the laboratory, for example, during acid-base titration, or waters in groundwater systems or in water supply distribution systems often behave, in first approximation, as in a closed system. Figure 4.2 illustrates metaphoric models for various types cf open and closed systems. 

We find two types of titration errors in acid/base titrations. The first is a determinate error that occurs when the pH at which the indicator changes color differs from the pH at the equivalence point. This type of error can usually be minimized by choosing the indicator carefully or by making a blank correction. 

Several important elements that occur in organic and biological systems are conveniently determined by methods that involve an acid/base titration as the final step. Generally, the elements susceptible to this type of analysis are nonmetallic and include carbon, nitrogen, chlorine, bromine, and fluorine, as well as a few other less common species. Pretreatment converts the element to an inorganic acid or base that is then titrated. A few examples follow. 

Koretsky et al. calculated site densities for particular low index faces of six oxides and six silicate minerals and average site densities for cleavage and growth faces, and showed that these methods lead to very different results. Full documentation of depth, length of broken bond and Brown bond strengths of broken bonds for particular types of sites is presented. The ranges of TVs calculated by Koretsky et al. [7] and in their literature data collection (tritium exchange, acid-base titrations, NMR, adsorption and desorption of water at various conditions, and saturation experiments with different adsorbates in solution) for six oxides are listed in Table 5.1. 

When the analyte is a base or an acid, the required titrant is a strong acid or strong base, respectively. This procedure is called an acid-base titration. An indicator very commonly used for acid-base titrations is phenolphthalein, which is colorless in an acidic solution and pink in a basic solution. Thus, when an acid is titrated with a base, the phenolphthalein remains colorless until after the acid is consumed and the first drop of excess base is added. In this case, the endpoint (the solution changes from colorless to pink) occurs approximately one drop of base beyond the stoichiometric point. This type of titration is illustrated in the three photos in Fig. 4.18. 

Stoichiometry provides the basis for a procedure called titration, which is used to determine the concentrations of acidic and basic solutions. Titration is a method for determining the concentration of a solution by reacting a known volume of that solution with a solution of known concentration. If you wish to find the concentration of an acid solution, you would titrate the acid solution with a solution of a base of known concentration. You could also titrate a base of unknown concentration with an acid of known concentration. How is an acid-base titration performed Figure 18.21 illustrates one type of setup for the titration procedure outlined on the following page. In this procedure a pH meter is used to monitor the change in the pH as the titration progresses. 

In terms of laboratory work, an important thing to note about the affordances of the laboratory equipment, especially scientific instruments, is that there are intended and unintended consequences of the way in which the instrument works that can either encourage or hinder cognition about scientific concepts (Malina Nakhleh, 2001 Miller Nakhleh, 2001). As an example, a chemical indicator, a pH meter, or a computer-interfaced pH probe can monitor an acid-base titration, but these instruments offer different levels of information to a student and therefore different types of learning might occur out of the same acid-base titration, depending on which instrument is used to monitor the titration. (Nakhleh Krajcik, 1993, 1994). 

Equation 4-4a represents a type of reaction called autoprotolysis which, as we will see later, is important in describing the utility of solvents as acid-base titration solvents. 

All of the four types of titrations have been implemented coulometrically (i.e., acid-base, precipitation, complexometric, and redox titrations). Acid-base titrations are achieved by generating protons or hydroxide ions from the solvent water by electrolysis (the hydrogen and oxygen evolution reactions). A list of possible methods is given in Table 1. 

The means of detecting the endpoint will be dictated by the type of reaction employed. Acid-base titrations are most easily followed using a glass pH electrode while redox reactions lend themselves to amperometric detection (only a small fraction of the species detected is consumed at the indicator electrode). Other options are ion-selective electrodes and conductometric detection. 

Many different types of indicator electrodes can be used as endpoint indicators in potentiometric titrations. For example, an acid-base titration can be performed with a glass electrode as endpoint detector instead of using colored indicators, or chloride ions can be titrated with silver(I) using a chloride-ion- or silver-ion-selective electrode. 

For many years, analytical chemistry relied on chemical reactions to identify and determine the components present in a sample. These types of classical methods, often called wet chanical methods, usually required that a part of the sample be taken and dissolved in a suitable solvent if necessary and the desired reaction carried out. The most important analytical fields based on this approach were volumetric and gravimetric analyses. Acid-base titrations, oxidation-reduction titrations, and gravimetric determinations, such as the determination of silver by precipitation as silver chloride, are all examples of wet chemical analyses. These types of analyses require a high degree of skill and attention to detail on the part of the analyst if accurate and precise results are to be obtained. They are also time consuming, and the demands of today s high-throughput pharmaceutical development labs, forensic labs, commercial environmental labs, and industrial quality control... 

The experiments were carried out in two-liter autoclaves of stainless steel (A.l.S.I. Type 316) and Inconel Alloy 600 fitted with internal sampling tubes and filters. Three different experimental procedures were used nonisothermal procedures, isothermal procedures with saturation approached from below, and experiments in which saturation was approached by evaporation at approximately isothermal conditions. Samples of solution were removed periodically from the autoclave and analyzed by a potentiometric acid-base titration. 

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