Chapters Potentiometric titrators

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. 

C. Potentiometric methods. This is a procedure which depends upon measurement of the e.m.f. between a reference electrode and an indicator (redox) electrode at suitable intervals during the titration, i.e. a potentiometric titration is carried out. The procedure is discussed fully in Chapter 15 let it suffice at this stage to point out that the procedure is applicable not only to those cases where suitable indicators are available, but also to those cases, e.g. coloured or very dilute solutions, where the indicator method is inapplicable, or of limited accuracy. 

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. 

Potentiometric titrations are readily automated by using a motor-driven syringe or an automatic burette coupled to a chart recorder or digital printout system. This is described in more detail in Chapter 12. A micro-processor-controlled titrator is discussed in Chapter 13. 

It is possible to monitor the course of a titration using potentiometric measurements. The pH electrode, for example, is appropriate for monitoring an acid-base titration and determining an end point in lieu of an indicator, as in Experiment 10 in Chapter 5. The procedure has been called a potentiometric titration and the experimental setup is shown in Figure 14.11. The end point occurs when the measured pH undergoes a sharp change—when all the acid or base in the titration vessel is reacted. The same... 

The electron formed as a product of equation (2.5) will usually be received (or collected ) by an electrode. It is quite common to see the electrode described as a sink of electrons. We need to note, though, that there are two classes of electron-transfer reaction we could have considered. We say that a reaction is heterogeneous when the electroactive material is in solution and is electro-modified at an electrode which exists as a separate phase (it is usually a solid). Conversely, if the electron-transfer reaction occurs between two species, both of which are in solution, as occurs during a potentiometric titration (see Chapter 4), then we say that the electron-transfer reaction is homogeneous. It is not possible to measure the current during a homogeneous reaction since no electrode is involved. The vast majority of examples studied here will, by necessity, involve a heterogeneous electron transfer, usually at a solid electrode. 

It is possible, however, to measure the potential al an electrode immersed in a homogeneous system, for example measuring an emf during a potentiometric titration - see Chapter 4. 

Note that all cations are initially in solution and will certainly be solvated to some extent. In addition, notice that the symbol for the electron is again subscripted to show that charge comes from an electrode rather than from a homogeneous electron-transfer reaction in solution (cf. the potentiometric titrations we discussed in the previous chapter). 

The procedure for potentiometric titration is presented in Chapter 1.6. In this titration, a standard acid titrant is added to a measured volume of sample aliquot in small increments of 0.5 mL or less, that would cause a change in pH of 0.2 unit or less per increment. The solution is stirred after each addition and the pH is recorded when a constant reading is obtained. A titration curve is constructed, plotting pH vs. cumulative volume titrant added. The volume of titrant required to produce the specific pH is read from the titration curve. 

The solubility of metal-hydroxide precipitates in water varies depending on ionic strength and number of pairs and/or complexes (Chapter 2). A practical approach to determining the pH of minimum metal-hydroxide solubility, in simple or complex solutions, is potentiometric titration, as demonstrated in Figure 12.3. The data show that potentiometric titration of a solution with a given heavy metal is represented by a sigmoidal plot. The long pH plateau represents pH values at which metals precipitate the equivalence point, or titration end point, indicates the pH at the lowest metal-... 

Fig. 5.35. (A) Degree of ionisation (a as a function of mobile phase pHapp for the solutes BA and D,L-PA and for PBA. a was calculated from the corresponding potentiometric titration data (see Fig. 2.14, in Chapter 2). (B) Product of the degree of ionisation of the solute (a B) and the polymer PBA (b a) (x 100) versus mobile phase pHapp (solid line). Overlayed are the experimental data from Fig. 5.34 (dashed line). From Sellergren and Shea [129].

Since the second term in both the numerator and the denominator are identical, an increase in a s will obviously lead to a decrease in a. In support of this explanation, the non-specific binding increases above pH pp = 6. This is clearly seen in the plot of k versus pHapp for BA on the L-PA MIP (Fig. 5.34) and in the parallel increase in a for the latter (Fig. 5.35). This can also be seen from the plot of the estimated separation factor of BA ( ba) versus pHapp (Fig. 5.34), where a is highest at low pH values (below pATa(BA)) and decreases over a large pH interval. Furthermore, the potentiometric titrations showed that the LPA MIP had a lower average pAfa than a non-imprinted blank polymer (see Chapter 2). This strongly suggests that the carboxylic acid groups of the selective sites are more acidic than those of the non-selective sites. 

Some means of detecting the end-point of the reaction between the standard and test solutions, e.g. a chemical indicator or, in the case of potentiometric titrations, a pH electrode (see Chapter 34). Some reactions exhibit a colour change at the end-point without the addition of an indicator. 

Automatic titrators for carrying out potentiometric titrations are available from several manufacturers. The operator of the instrument simply adds the sample to the titration vessel and pushes a button to initiate the titration. The instrument adds titrant, records the potential versus volume data, and analyzes the data to determine the concentration of the unknown solution. A photograph of such a device is shown on the opening page of Chapter 14. 

Determined from spectrophotometric titration. b Determined from potentiometric titration. Description of acronyms of the synthetic analogs is given in Chapter III.2. For discussion of the constants see text. 

The interpretation of potentiometric titration data in absence of strongly adsorbing species in Chapter 3 already involves some model of adsorption the protons are chemisorbed at the surface and their charge is balanced by the excess of... 

An equally compelling piece of evidence that protein phosphorylation is controlled by the redox state ofplastoquinone pool was provided by Horton, Allen, Black and Bennett " using potentiometric titration of both fluorescence and LHC II phosphorylation, as shown in Fig. 7 (B). Details on the use of redox titration ofPS-II fluorescence-yield changes to monitor the redox-state changes of the plastoquinone pool in PS II will be discussed in Chapter 17. The data points for fluorescence as well as for phosphorylation, as measured by the level of radioactive P-labeled LHC II, fit the Nernst equation [see footnote on p. 92] with an (at pH 7.8) of 0 mV and an n-value of 2, reflecting a two-electron change for the electron carrier involved, presumably PQ. Thus crucial links are established between the redox state of plastoquinone, the activation of kinase for LHC II phosphorylation and changes in chlorophyll fluorescence. 

The same symbol is used for corrected (Equation 2.10) and uncorrected (Equation 2.12) Og, and the details of data handling are seldom reported in the experimental parts of scientific papers. A few publications report PZCs obtained by titration without clear explanations of whether they were obtained as CIPs or if titration was performed at only one ionic strength. Such results are indicated as Titration in the Method columns of the tables in Chapter 3. A few methods that give a PZC equivalent to that from the potentiometric titration method are described in Section 2.8.4. 

A few methods may produce PZCs equivalent to those obtained by potentiometric titration (with or without correction for acid or base associated with solid particles). There are a limited number of such methods, although some of them have been re-invented several times, and given different names. Only a few of these names are used in the tables in Chapter 3. The methods that produce results... 

The terms batch equilibration [653], pH drift method [654], addition method [552], solid addition method [655], powder addition method (cited in [656] after [654]), potentiometric titration [234] ( sic —in the present book, the term potentiometric titration is reserved for a different method, described in Section 2.5), and salt addition [573] ( sic —in the present book, the term salt addition is reserved for a different method, described later in this section) refer to the same method, which is now described. A series of solutions of different pHs is prepared and their pHs are recorded. Then, the powder is added and the final pH is recorded. The addition of a solid induces a shift in the pH in the direction of the PZC. The pH at which the addition of powder does not induce a pH shift is taken to be the PZC. Alternatively, the PZC is determined as the plateau in the pHfln, (pH ,.,., .j) curve. The method assumes that the powder is absolutely pure (free of acid, base, or any other surface-active substance), which is seldom the case. Even with very pure powders, the above method is not recommended for materials that have a PZC at a nearly neutral pH. Namely, the method requires accurate values of the initial pH, which is the pH of an unbuffered solution. The display of a pH meter in unbuffered solutions in the nearly neutral pH range is very unstable, and the readings are not particularly reliable. The problem with pH measurements of solutions is less significant at strongly acidic or strongly basic pHs (see Section 1.10.3). The above method (under different names) became quite popular, and the results are referred to as pH in the Method columns in the tables in Chapter 3. The experimental conditions in the above method (solid-to-liquid ratio, time of equilibration, and nature and concentration of electrolyte) can vary, but little attention has been paid to the possible effects of the experimental conditions on the apparent PZC. The plateau in the pH, , (pH, ,, ) curve for apatite shifted by 2 pH units as the solid-to-liquid ratio increased from 1 500 to 1 100 [653]. Thus, the apparent PZC is a function of the solid-to-liquid ratio. 

A potentiometric titration curve often has an inflection point at the PZC (Section 2.6.3). This property has been proposed as a method to determine the PZC [673]. The inflection point method gained some popularity after a publication by Zalac and Kallay [670]. Also, the differential potentiometric titration described in [674] is equivalent to the inflection point method. This method is not recommended by the present author as a standalone method to determine the PZC, but a few results obtained by the inflection point method, usually in combination with other methods, are reported in the tables in Chapter 3 (as Inflection in the Methods columns). In [675], the potentiometric titration curve of one sample had two inflections, and the inflection at the lower pH was assumed to be the PZC. The potentiometric titration curves of other samples had one inflection each. Reference [676] reports an inflection point in the titration curve of niobia at pH 8, which is far from the pHg reported in the literature. A few examples of charging curves without an inflection point or with multiple inflection points are discussed in Section 2.6.3. 

In Chapter 12, we mentioned measurement of the potential of a solution and described a platinum electrode whose potential was determined by the half-reaction of interest. This was a special case, and there are a number of electrodes available for measuring solution potentials. In this chapter, we hst the various types of electrodes that can be used for measuring solution potentials and how to select the proper one for measuring a given analyte. The apparatus for making potentiomet-ric measurements is described along with limitations and accuracies of potentio-metric measurements. The important glass pH electrode is described, as well as standard buffers required for its calibration. The various kinds of ion-selective electrodes are discussed. The use of electrodes in potentiometric titrations is described in Chapter 14. 

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