Potentiometric titration, acid-base precipitation

Potentiometric titration is actually a form of the multiple known subtraction method. The main advantage of titration procedures, similar to multiple addition techniques in general, is the improved precision, especially at high determinand concentrations. ISEs are suitable for end-point indication in all combination titrations (acid-base, precipitation, complexometric), provided that either the titrand or the titrant is sensed by an ISE. If both the titrant and the titrand are electro-inactive, an electrometric indicator must be added (for example Fe ion can be titrated with EDTA using the fluoride ISE when a small amount of fluoride is added to the sample solution [126]). 

In fact, any type of titration can be carried out potentiometrically provided that an indicator electrode is applied whose potential changes markedly at the equivalence point. As the potential is a selective property of both reactants (titrand and titrant), notwithstanding an appreciable influence by the titration medium [aqueous or non-aqueous, with or without an ISA (ionic strength adjuster) or pH buffer, etc.] on that property, potentiometric titration is far more important than conductometric titration. Moreover, the potentiometric method has greater applicability because it is used not only for acid-base, precipitation, complex-formation and displacement titrations, but also for redox titrations. 

In the practice of potentiometric titration there are two aspects to be dealt with first the shape of the titration curve, i.e., its qualitative aspect, and second the titration end-point, i.e., its quantitative aspect. In relation to these aspects, an answer should also be given to the questions of analogy and/or mutual differences between the potentiometric curves of the acid-base, precipitation, complex-formation and redox reactions during titration. Excellent guidance is given by the Nernst equation, while the acid-base titration may serve as a basic model. Further, for convenience we start from the following fairly approximate assumptions (1) as titrations usually take place in dilute (0.1 M) solutions we use ion concentrations in the Nernst equation, etc., instead of ion activities and (2) during titration the volume of the reaction solution is considered to remain constant. 

Titrimetric methods with potentiometric end point location can be applied when an electrode with the needed selectivity is not available. The precision and accuracy of potentiometric titrations are superior comparing it with the properties of direct potentiometry. However, the concentration range where potentiometric titration can be used effectively is narrower. A solution with analyte concentration below 1 mM seldom is determined by potentiometric titrations. Potentiometric end point location is most often employed in the case of acid-, base-, precipitate-, redox-, or complexometric titrations. 

Depending on the availability of a suitable electrode, a potentiometric method may be used for acid/base, precipitation or complexometric titrations. A glass electrode together with a reference electrode can be used for acid/base titrations especially when the test solution is coloured or when no suitable indicator is available. When an electrode is selective to one of the ions to be precipitated, the end-point of a precipitation titration is located potentiometrically. 

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. 

This experiment describes the use of coated graphite electrodes for the potentiometric monitoring of precipitation, acid-base, complexation, and redox titrations. 

The holistic thermodynamic approach based on material (charge, concentration and electron) balances is a firm and valuable tool for a choice of the best a priori conditions of chemical analyses performed in electrolytic systems. Such an approach has been already presented in a series of papers issued in recent years, see [1-4] and references cited therein. In this communication, the approach will be exemplified with electrolytic systems, with special emphasis put on the complex systems where all particular types (acid-base, redox, complexation and precipitation) of chemical equilibria occur in parallel and/or sequentially. All attainable physicochemical knowledge can be involved in calculations and none simplifying assumptions are needed. All analytical prescriptions can be followed. The approach enables all possible (from thermodynamic viewpoint) reactions to be included and all effects resulting from activation barrier(s) and incomplete set of equilibrium data presumed can be tested. The problems involved are presented on some examples of analytical systems considered lately, concerning potentiometric titrations in complex titrand + titrant systems. All calculations were done with use of iterative computer programs MATLAB and DELPHI. 

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. 

Titrations can be carried out in cases in which the solubility relations are such that potentiometric or visual indicator methods are unsatisfactory for example, when the reaction product is markedly soluble (precipitation titration) or appreciably hydrolysed (acid-base titration). This is because the readings near the equivalence point have no special significance in amperometric titrations. Readings are recorded in regions where there is excess of titrant, or of reagent, at which points the solubility or hydrolysis is suppressed by the Mass Action effect the point of intersection of these lines gives the equivalence point. 

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. 

Acid-base, redox, precipitation and chelometric titrations are usually dealt with in textbooks on analytical chemistry. The titration curves in these titrations can be obtained potentiometrically by use of appropriate indicator electrodes, i.e. a pH-glass electrode or pH-ISFET for acid-base titrations, a platinum electrode for redox titrations, a silver electrode or ISEs for precipitation titrations, and ISEs for... 

Preparation of 9-methyl-3-[(2-methyl-l-H-imidazol-l-yl)methyl]-l,2,3,9-tetrahydro-4H-carbazol-4-one hydrochloride dihydrate The process above described is followed, except that after cooling down the reaction mixture to room temperature after boiling, 20 ml of 37% aqueous hydrochloric acid are added thereto. Then, the precipitate is filtered off, washed with isopropanol and dried to obtain 2.40 g (65.6%) of the title salt, m.p. 178°-180°C. The active agent content of the product was found to be 100.3% based on potentiometric titration with sodium hydroxide solution. The theoretical water content is 9.85% (calculated for C18H19N30HCl2H20).The water content measured is 10.03%. 

Other examples of potentiometric titrations include acid-base titrations, in which an indicator electrode provides a response to hydronium ions, such as the glass electrode, quinhydione electrode, or antimony electrode. In precipitation and complexation titrations the indicator electrode should provide the response to the active species in the solution. Thus, during the titration of chloride ions by silver nitrate, a silver electrode is an effective indicator electrode. 

Any titration involves the progressive change of the activities (or concentrations) of the titrated and titrating species and, in principle, can be done potentiometrically. However, for an accurate determination it is necessary that there is a fairly rapid variation in equilibrium potential in the region of the equivalence point. Useful applications are redox, complexation, precipitation, acid-base titrations, etc. From the titration curve it is possible to calculate values of the formal potentials of the titrated and titrating species, as explained below. 

Methacrylic Acid Content in Polymer. One gram of methanol-precipitated, water-washed, dried polymer was dissolved in 100 ml. tetra-hydrofuran (THF) and titrated to a faint pink phenolphthalein end point with 0.055 n-benzyltrimethylammonium hydroxide in THF. The base was standardized by potentiometric titration against 0.01N acetic acid in methanol. The value for a non-acid containing polymer of the same series was used as a blank. All analyses were within 5% of the theoretical value. 

By analogy to pH titration curves of acids and bases, it is customary in precipitation titrations to plot the quantity pM (defined by either — log [M " ] or — log a m ) against titration volume. For certain metals that form reversible electrodes with their ions, the measured electrode potential is a linear function of the logarithm of ion activity, so the titration curve can be realized experimentally in a potentiometric titration. In any case, the curve gives a useful indication of the sharpness of an endpoint break. 

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 precipitation of AgX out of solution drives the equilibrium to the right. Similar to acid-base titrations, potentiometric titrations measure the volume of a solution of one reactant that is required to completely react with a measured amount of another reactant, or until the equivalence point (or stoichiometric point) is reached. At the equivalence point, the silver nitrate has reacted stoichiometrically with all of the ionic halide present in your sample solution. In our case, the numbers of moles of halide and silver ions are equal, equation (2.11). 

Codeine is a strong mono-acid base and its solutions turn litmus blue, phenolphthalein pink, and helianthin yellow [65] it is incompletely precipitated from its salts by ammonia. The neutral point of a codeine-hydrochloric acid titration is at pH 4-93, so methyl red can be used as indicator [66-67] potentiometric titrations can also be used [68-72],... 

With the potentiometric approach, determination of intrinsic solubility is based upon the measurement of the pH shift caused by compound precipitation during acid-base titration of ionizable compounds. Two commercial potentiometric methods currently available are pSol [30, 39] and Cheqsol [40-42], In the pSol method developed by Avdeef, a minimum of three titrations in the direction of dissolution are performed. Normal pH versus volume titration plots are reexpressed as Bjerrum plots, that is, average number of bound protons versus pH. The Bjermm plots enable the shift in compound pKa to be more readily observed and are used to determine intrinsic solubility (S0) via Equation 2.5 ... 

In the case of the gel, on the other hand, the back titration with an acid for the sample dispersion, which has already been titrated with a base until an equivalent point, should be possible, since precipitation does not need to be taken into consideration. In contrast to the polymer, however, there are several difficulties in the analysis of the titration data for example, how to estimate the real acid-base equilibrium within the gel phase from the pH measurements of the outer solution (see Sec. IV.C.2). So far, no study has dealt with the potentiometric titrations of polyelectrolyte gels. We neverthe-... 

Among them, volumetric methods are presumably the most widely used for water analysis. They are titrimetric techniques which involve a chemical reaction between a precise concentration of a reagent or titrant and an accurately known volume of sample. The most common types of reactions as used within this method are acid-base neutralization, oxidation-reduction, precipitation, and complexation. The use of an indicator which identifies the equivalence point is required to develop this kind of method. The modem laboratories usually employ automated endpoint titrators, which largely improve the efficiency and reliability of the determination. Moreover, spectrophotometric, potentiometric, or amperometric methods to determine the endpoint of the reaction can... 

Amperometric titrations have a number of advantages over other titri-metric methods determinations can usually be carried out in very dilute solutions (about 10 N or even, in certain cases, 10 N) and they are rapid since only a few measurements of current, before and after the end-point, at a constant applied voltage, need be made. It is often possible to carry out titrations in cases where potentiometric or visual-indicator methods are unsuitable such as in acid-base titrations in which the reaction product is hydrolysed or in precipitation titrations in which the precipitate has a significant solubility. There is, too, no interference from foreign electrolytes. 

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