Acid-base titration selectivity

The utility of acid-base titrimetry improved when NaOH was first introduced as a strong base titrant in 1846. In addition, progress in synthesizing organic dyes led to the development of many new indicators. Phenolphthalein was first synthesized by Bayer in 1871 and used as a visual indicator for acid-base titrations in 1877. Other indicators, such as methyl orange, soon followed. Despite the increasing availability of indicators, the absence of a theory of acid-base reactivity made selecting a proper indicator difficult. 

Properties of Selected Indicators, Mixed Indicators, and Screened Indicators for Acid-Base Titrations... 

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

If the oxidation or reduction of H2O is carried out externally using the generator cell shown in Figure 11.25, then H3O+ or OH can be dispensed selectively into a solution containing a basic or acidic analyte. The resulting reaction is identical to that in an acid-base titration. Coulometric acid-base titrations have been used for... 

Given three add-base indicators—methyl orange (end point at pH 4), bromthymol blue (end point at pH 7), and phendphthalein (end point at pH 9)—which would you select for the following acid-base titrations ... 

Conductometric titrations. Van Meurs and Dahmen25-30,31 showed that these titrations are theoretically of great value in understanding the ionics in non-aqueous solutions (see pp. 250-251) in practice they are of limited application compared with the more selective potentiometric titrations, as a consequence of the low mobilities and the mutually less different equivalent conductivities of the ions in the media concerned. The latter statement is illustrated by Table 4.7108, giving the equivalent conductivities at infinite dilution at 25° C of the H ion and of the other ions (see also Table 2.2 for aqueous solutions). However, in practice conductometric titrations can still be useful, e.g., (i) when a Lewis acid-base titration does not foresee a well defined potential jump at an indicator electrode, or (ii) when precipitations on the indicator electrode hamper its potentiometric functioning. 

To select an indicator for an acid-base titration it is necessary to know the pH of the end point before using equation (5.5) or standard indicator tables. The end point pH may be calculated using equations (3.27), (3.29) or (3.30). Alternatively, an experimentally determined titration curve may be used (see next section). As an example, consider the titration of acetic acid (0.1 mol dm 3), a weak acid, with sodium hydroxide (0.1 mol dm-3), a strong base. At the end point, a solution of sodium acetate (0.05 mol dm 3) is obtained. Equation (3.28) then yields... 

The strength of nitric acid can be determined by acid-base titration against a standard solution of a strong base such as NaOH using a color indicator, or by potentiometric titration using a pH meter. Nitrate ion, NO3 in its aqueous solution, may be measured with a nitrate ion-selective electrode or by ion chromatography following appropriate dilution. 

An important question is whether we can use any indicator electrode. A redox electrode, i.e. inert in the range of potential where measurements are being done, is a possibility, especially for redox titrations. In other cases, the use of electrodes selective to the ion being titrated is better, such as pH electrodes in acid-base titrations. The method of analysis of the data obtained is, naturally, the same in all cases and independent of electrode material. 

The acid-base behavior of proteins can reveal some important properties with respect to both their composition (selectivity) and their concentration (sensitivity). The most direct way to exploit these acid-base properties is to make use of acid-base titration, Titrant should be added somehow and the resulting change in pH should be measured. Since the ion-sensitive field-effect transistor (ISFET) is suitable for fast (and local) pH detection, an ISFET can be used for protein titration if the protein to be detected can be immobilized in a membrane, deposited on top of the device. Advantages are the small amount of protein necessary for the characterization owing to the small membrane volume, and the relatively short time needed to perform a full titration. 

Selection of a Visual Indicator for a Redox Titration Because of the relatively small number of indicators available and their pH dependence, selection is not as straightforward as in the case of acid-base titrations. For example, iron(ll) may be titrated with cerium(IV) or chromium(VI) (table 5.4), whilst equation (5.9) in conjunction with table 5.5... 

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. 

Explain how you would select an indicator for an acid-base titration. 

How would you select an indicator for a particular acid-base titration ... 

Titration The determination of assay values for reference standards, counter-ions, or impurities can often be independently determined via titration. While titration assays generally have less selectivity in comparison to chromatographic methods, the advantages of a broad spectrum of classical titration techniques that exist for organic functional groups is often overlooked. The methodologies include not only classical potentiometric acid/base titrations but also nonaqueous, redox, indirect, precipitation, and derivatization titrations.76-79... 

The values of PZC referred to as cip and pH" in Table 3.1 are obtained by acid-base titration. The experimental procedure is basically the same (although many variants have been described), and the substantial difference is in the method of selection of the zero point. Titration gives very accurate changes in uo from one pH value to another (or at least the part of due to proton adsorption/dissociation) without assumptions, but some assumptions are necessary to obtain the zero point. Existence of CIP does not prove the absence of specifically adsorbed ions. Lyklema [44] showed that in case of specific adsorption of metal cations the shift in the CIP to pH below the pristine PZC is more pronounced for metals having stronger affinity toward the surface (e.g. Pb > Ca) and the do at CIP is also more positive for more strongly bound cations. 

Skill 10.6 Applying the principles of acid-base titration, including the selection of indicators, and interpreting the results of acid-base titrations... 

Figure 19.5 shows the color changes and their pH ranges for some common acid-base indicators. Selecting an indicator requires that you know the approximate pH of the titration end point, which in turn requires that you know which ionic species are present. Because the indicator molecule is a weak acid, the ratio of the two forms is governed by the [H30" ] of the test solution ... 

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