Acid-base titration sensitivity

Sensitivity For an acid-base titration we can write the following general analytical equation... 

In practice, however, any improvement in the sensitivity of an acid-base titration due to an increase in k is offset by a decrease in the precision of the equivalence point volume when the buret needs to be refilled. Consequently, standard analytical procedures for acid-base titrimetry are usually written to ensure that titrations require 60-100% of the buret s volume. 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving redox titrations are similar to those described earlier in the chapter for acid-base and complexometric titrimetric methods. As with acid-base titrations, redox titrations can be extended to the analysis of mixtures if there is a significant difference in the ease with which the analytes can be oxidized or reduced. Figure 9.40 shows an example of the titration curve for a mixture of Fe + and Sn +, using Ce + as the titrant. The titration of a mixture of analytes whose standard-state potentials or formal potentials differ by at least 200 mV will result in a separate equivalence point for each analyte. 

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 intensity and colour of the fluorescence of many substances depend upon the pH of the solution indeed, some substances are so sensitive to pH that they can be used as pH indicators. These are termed fluorescent or luminescent indicators. Those substances which fluoresce in ultraviolet light and change in colour or have their fluorescence quenched with change in pH can be used as fluorescent indicators in acid-base titrations. The merit of such indicators is that they can be employed in the titration of coloured (and sometimes of intensely coloured) solutions in which the colour changes of the usual indicators would... 

Historically, pH sensitive dyes have been extensively used as indicators in acid-base titrations and in simple spot test papers, even leading to a common phrase in our everyday language, when people or topics are described as having passed the litmus test . The use of complexometric titrations for metal ions was a later but widely... 

Emphasis was therefore put on analytical procedures able to determine many elements in parallel and/or requiring almost no previous separation. procedures preferred were X-ray fluorescence using a Am source and Si(Li)-detector, atomic absorption spectrophotometry, gamma spectrometry using tracer isotopes and Ge(Li)-detector and acid-base titrations with recording of the pH-volume derivative. Table 2 summarises the use of these methods for the different elements, and it also gives a rough indication of interferences, sensitivity and accuracy obtained. 

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. 

Acid-base titrations can be performed in special hollow glass electrode cups which are available in different sizes. The increased sensitivity of modern pH meters allows the use of rather thick-walled electrodes which have a long life, even in routine laboratories. Sometimes it is desirable to exclude atmospheric carbon dioxide which necessitates special apparatus. See, for example, Sisco et al. (SIO) and Berret (B3). Most potentiometric methods are developed for special determinations,... 

As we mentioned, for a titration to be successful there must be some indication of when it has finished. Chemical indicators are a common way of determining whether a titration is complete. In acid-base titrations, chemical indicators are typically small molecules whose colors change depending on the pH of the solution (or, in other words, on their protonation state). In other cases, the indicator may change color based on whether it is coordinated to another species in a solution. It is also possible to use a device like a pH-sensitive electrode to determine when the titration has ended this doesn t rely on you (or another person) being able to notice a visible change in the color of the solution. Such a device can be more accurate, since it doesn t rely on a qualitative interpretation of a color change. 

Another critical aspect that must be considered for selection of a suitable indicator for an acid-base titration in a given solvent is the quality of color change of the indicator in this solvent. The quality of color change is related to the sharpness of the chromatic transition and to the sensitivity of the color change. 

Surfactant-sensitive electrodes are commercially available, and they are also easily made in the laboratory. They can be used to detect the end-point in titrations of anionic and cationic surfactants with surfactants of opposite charge. They are used in exactly the same way as a glass electrode in acid-base titrations, or a silver-silver chloride electrode in titrations of chloride with silver nitrate. The main advantages of potentiometric as opposed to two-phase titration are ... 

Acid or base concentrations can be determined by performing an acid-base titration with coulometrically generated OH" or ions at an actuator in close proximity to the pH-sensitive gate of an ISFET. In figure I the basic elements of such a device are shown. The titrant can be generated either by the electrolysis of water at a noble metal actuator electrode [1] ... 

In the example of Fig. 9.1, a potentiometric set-up of indicator and reference electrodes is used to indicate the equivalence point. For acid-base titrations, the pH-sensitive glass electrode makes up a useful indicator probe. 

Different sample pretreatment operations include dilution, membrane-extraction (gas diffusion, dialysis), liquid-phase extraction techniques (liquid/liquid extraction, liquid-phase microextraction, single-drop microextraction) and solid reactors and packed columns aiming to facilitate online chemical derivatization, chromatographic separation of target species, removal of interfering matrix compounds, enzymatic assays, or determination of trace levels of analyte via sorptive preconcentration procedures (Marshall et al., 2003 Economou, 2005 Miro and Hansen, 2006 Theodoridis et al., 2007 McKelvie, 2008 Ruzicka, 2014). In this context, BIA and the LOV configurations are particularly useful. Acid-base titrations can also be automated using simple SIA manifolds and potentiometric (van Staden et al., 2002) or photometric (Kozak et al., 2011) detection. Typically, a zone of the sample to be titrated is sandwiched between two zones of titrant by aspiration. In the case of photometric detection, an additional zone of a suitable pH-sensitive colored indicator is aspirated. The stacked zones are delivered to the detector and the width of the peaks is monitored and related to the pH of the solution. 

Alkyldimethylbetaine and free amine can be determined by potentiometric acid-base titration (12). For titration of the betaine, the sample is dissolved in a mixture of 10 1 methyl isobutyl ketone/isopropanol to which a little HCl has been added. Three breaks are observed on titration with ethanolic KOH, corresponding respectively to excess HCl, the betaine, and combined impurities amine, glycolic acid, and monochloracetic acid (see Fig. 1). Carbon dioxide must be excluded during the titration. Sodium chloride is insoluble in the solvent system and precipitates. The titration is very sensitive to water concentration (16)... 

Since the acid-base (precipitation) reaction takes place in non-aque-ous solution (isopropanol), a glass pH electrode could not be used to follow the titration. However, PANI is known to be pH sensitive as a result of the acid-base equilibrium between the emeraldine base (EB) and emeraldine salt (ES) forms of PANI [1-3]. Interestingly, the GC/ PANI electrode was found to give a reproducible response during the titrations despite the presence of the precipitate (trimeprazine tartrate) in the stirred solution. The same GC/PANI electrodes were used repeatedly for more than 2 months without any significant changes in the... 

Electrochemical endpoint detection methods provide a number of advantages over classical visual indicators. These methods can be used when visual methods of endpoint detection cannot be employed because of the presence of colored or clouded solutions and in the case of detection of several components in the same solution. They are more precise and accurate. In particular, such methods provide increased sensitivity and are often amenable to automation. Electrochemical methods of endpoint detection are applicable to most oxidation-reduction, acid-base, and precipitation titrations, and to many complex-ation titrations. The only necessary condition is that either the titrant or the species being titrated must give some type of electrochemical response that is indicative of the concentration of the species. 

Due, M., Gaboriaud, R, and Thomas, F., Sensitivity of the acid-base properties of clays to the methods of preparation and measurement. Evidence from continuous potentiometric titrations, J. Colloid Intetf. Sci., 289, 148, 2005. 

As mentioned in Section 3.7.1.2, there is a considerable scatter of solubility product values obtained in the molten KCl-NaCl eutectic using different methods of solubility determination. This disagreement in the solubility parameters may be explained by differences in the sizes of oxide particles whose solubility is to be determined. The difference in size causes the scatter of the solubility data according to the Ostwald-Freundlich equation and the employment of the isothermal saturation method, which implies the use of commercial powders (often pressed and sintered), leads to values which are considerably lower than those obtained by the potentiometric titration technique where the metal-oxides are formed in situ. Owing to this fact, the regularities connected with the effect of physico-chemical parameters of the oxides or the oxide cations should be derived only from solubility data obtained under the same or similar experimental conditions. However, this does not concern the dissociation constants of the oxides, since homogeneous acid-base equilibria are not sensitive to the properties of the solid phase of... 

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