Titration, potentiometric differential method

Potentiometric titration and measurement of conductivity are the classic methods for determining pKa experimentally. With a differential method that involves comparison of meter readings for two substances under carefully controlled conditions, it is possible to obtain accurate differences in pKa. 

A non-aqueous differentiating titration procedure (39) involves passage of the artificial sweeteners mixture through a cationic exchange resin, Dowex 50-X8, to separate and convert saccharin to its acid form. The solution obtained is titrated potentiometrically in methyl isobutyl ketone with 0.1 N sodium methoxide. The method has been applied in presence of cyclamate and benzyl alcohol. 

Conductometric titrations offer several advantages compared with potentiometric titration methods, such as better precision and better differentiation of the basic components in polymers, but they are more laborious. ASTM D 4928-96 is an established KF method for the determination of water in crude oils. 

A novel capillary electrophoresis method using solutions of non-crosslinked PDADMAC is reported to be effective in the separation of biomolecules [211]. Soil studies conducted with PDADMAC report the minimization of run-off and erosion of selected types of soils [212]. In similar studies, PDADMAC has found to be a good soil conditioner [213]. The use of PDADMAC for the simultaneous determination of inorganic ions and chelates in the kinetic differentiation-mode capillary electrophoresis is reported by Krokhin [214]. Protein multilayer assemblies have been reported with the alternate adsorption of oppositely charged polyions including PDADMAC. Temperature-sensitive flocculants have been prepared based on n-isopropylacrylamide and DADMAC copolymers [215]. A potentiometric titration method for the determination of anionic polyelectrolytes has been developed with the use of PDADMAC, a marker ion and a plastic membrane. The end-point is detected as a sharp potential change due to the rapid decrease in the concentration of the marker due to its association with PDADMAC [216]. 

Potentiometric titration — A - titration method based on the measurement of the potential of a suitable -> indicator electrode as a function of - titrant volume. Usually the -> cell voltage or a - p-function associated with the concentration of the - analyte is plotted as a function of the volume of -> titrant added [i]. See also -> differential potentiometric titration, and - retarded electrode. 

Differential Titration.—The object of potentiometric titration is to determine the point at which AEjAv is a maximum, and this can be achieved directly, without the use of graphical methods, by utilizing the principle of differential titration. If to two identical solutions, e.g., of sodium chloride, are added v and v + 0.1 cc. respectively of titrant, e.g., silver nitrate, the difference of potential between similar electrodes placed in the two solutions gives a direct measure of AEjAVy where Av is 0.1 cc., at the point in the titration corresponding to the addition of t + 0.05 cc. of silver nitrate. The e.m.f. of the cell made up of these two electrodes will thus be a maximum at the end-point. 

By differentiating the titration curve twice and then equating the second derivative to zero, it can be shown that for a symmetrical titration curve ( i = the point of maximum slope theoretically coincides with the equivalence point. This conclusion is the basis for potentiometric end-point detection methods. On the other hand, if 2> the titration curve is asymmetrical in the vicinity of the equivalence point, and there is a small titration error if the end point is taken as the inflection point In practice the error from this source is usually insignificant compared with such errors as inexact stoichiometry, slowness of titration reaction, and slowness of attainment of electrode equilibria. 

Computerized method provides convenient and accurate determination of the ionization constant in aqueous solution and of the apparent ionization constant in the presence of octanol. From these parameters, partition coefficients and apparent partition coefficients are easily calculated and agree with data reported using the shaker technique or HPLC. The curve-fitting method has been applied to the differential titration technique in which the solvent curve is subtracted from the solution curve before calculations are begun. This method has been applied to the potentiometric titration of aqueous solutions of the salts of bases with very low solubility in water. 

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. 

The discussion of potentiometric titrations will deal first with acidimetry, by the direct and differential procedures, after which methods depending upon precipitations and oxidation-reduction reactions will be considered. 

Nicholson proposed a differential potentiometric tltrator involving two indicator electrodes for the automatic control of processes in industrial plants [35]. As can be seen from Fig. 7.20, the sample and reagent streams are split and led to two half-cells via capillary tubes adjusted to provide slightly different titrated fractions. The potential difference (AE) between the two indicator electrodes Is transmitted to a control and detection system (D) which regulates the flow of titrant in an automatic fashion by means of valve V, thereby maintaining the preselected AE between the two ends of the cell. The speed of titrant addition, reflected by the flow meter (M), is a measure of the sample composition. An evaluation of the instrument carried out by the titration of dichromate with iron(II) revealed that the conditions to be used must be carefully selected. Thus, stable electrode responses are only obtained in the zone where Fe(II) prevails, and not in that where dichromate prevails over the former as the process determining the potential obtained in such a zone is irreversible. This method therefore has limited application in the control of slow reactions. 

Grunwald s differential titration method can also give results of high precision provided the slope of the titration curve at the equivalence point is not too large. The results of Ritchie and Heffley s study of the ionisation of some picolinium ions by this method are given in Appendix 3.5.3. A direct, rather than differential, potentiometric titration had to be used for derivatives of bicyclo[2.2.2]octane-l-carboxylic acid these results are given in Appendix 3.5.4. 

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