Weak acids potentiometric titration

Directions are provided in this experiment for determining the dissociation constant for a weak acid. Potentiometric titration data are analyzed by a modified Gran plot. The experiment is carried out at a variety of ionic strengths and the thermodynamic dissociation constant determined by extrapolating to zero ionic strength. 

The most obvious sensor for an acid-base titration is a pH electrode.For example, Table 9.5 lists values for the pH and volume of titrant obtained during the titration of a weak acid with NaOH. The resulting titration curve, which is called a potentiometric titration curve, is shown in Figure 9.13a. The simplest method for finding the end point is to visually locate the inflection point of the titration curve. This is also the least accurate method, particularly if the titration curve s slope at the equivalence point is small. 

Partanen, J. I. Karki, M. H. Determination of the Thermodynamic Dissociation Constant of a Weak Acid by Potentiometric Acid-Base Titration, /. Chem. Educ. 1994,... 

Potentiometric titration curves are used to determine the molecular weight and fQ or for weak acid or weak base analytes. The analysis is accomplished using a nonlinear least squares fit to the potentiometric curve. The appropriate master equation can be provided, or its derivation can be left as a challenge. 

The free maleic acid content in maleic anhydride is determined by direct potentiometric titration (166). The procedure involves the use of a tertiary amine, A/-ethylpipetidine [766-09-6J, as a titrant. A tertiary amine is chosen as a titrant since it is nonreactive with anhydrides (166,167). The titration is conducted in an anhydrous solvent system. Only one of the carboxyhc acid groups is titrated by this procedure. The second hydrogen s dissociation constant is too weak to titrate (166). This test method is not only used to determine the latent acid content in refined maleic acid, but also as a measure of the sample exposure to moisture during shipping. 

Potentiometric titration curves The procedure involves the addition of a salt of a weak acid to the resin and the determination of the pH of the equilibrated solution. Table 9 shows the pK values of the OH groups and dissociation constants of the studied resin. The first ionization occurs at a pH slightly higher than that of sul-... 

For the primary stage (phosphoric) V) acid as a monoprotic acid), methyl orange, bromocresol green, or Congo red may be used as indicators. The secondary stage of phosphoric) V) acid is very weak (see acid Ka = 1 x 10 7 in Fig. 10.4) and the only suitable simple indicator is thymolphthalein (see Section 10.14) with phenolphthalein the error may be several per cent. A mixed indicator composed of phenolphthalein (3 parts) and 1-naphtholphthalein (1 part) is very satisfactory for the determination of the end point of phosphoric(V) acid as a diprotic acid (see Section 10.9). The experimental neutralisation curve of 50 mL of 0.1M phosphoric(V) acid with 0.1M potassium hydroxide, determined by potentiometric titration, is shown in Fig. 10.6. 

It may be noted that very weak acids, such as boric acid and phenol, which cannot be titrated potentiometrically in aqueous solution, can be titrated conductimetrically with relative ease. Mixtures of certain acids can be titrated more accurately by conductimetric than by potentiometric (pH) methods. Thus mixtures of hydrochloric acid (or any other strong acid) and acetic (ethanoic) acid (or any other weak acid of comparable strength) can be titrated with a weak base (e.g. aqueous ammonia) or with a strong base (e.g. sodium hydroxide) reasonably satisfactory end points are obtained. 

Abstract Titration of weak bases in non-aqueous solvents can provide valuable information about these weak bases. Some primary amines 1-aminobutane, 1-aminopropane, 2-aminoheptane, aminocyclohexane, 3-amino-l-phenylbutane were titrated with hydrochloric acid in toluene solvent. All the primary amines gave very well-shaped potentiometric titration curves. The same titrations were done with hydrochloric acid in methanol solvent to show the effect of amphiprotic solvent in the titrations with hydrochloric acid. 

The potentiometric titration curves of gels, which relate the pH of the exterior solution to the degree of ionization of the gel, resemble the titration curves of monofunctional acids or bases. However, the dissociation constants differ, often by two orders of magnitude, from the expected value for the functional group, and the slope of the curves is not the usual one. Addition of neutral salt changes the picture markedly and brings the curves closer to expectation. In the case of weak or medium... 

In potentiometric titration a voltage is obtained from an electrode that is sensitive to an ionic species such as H-jO+, i.e., the pH of the solution in this case. We will consider the titration of the mixture of a strong acid (HC1) and a weak acid (CJ+jCOOH) with NaOH (ref. 10). As 2 ml volumes of the base are given to the acidic solution, the pH increases and when one of the acids is neutralized the pH changes very rapidly by a small addition of NaOH. We want to find these maximum points of the first derivative of the titration curve. In the following main program the DATA lines contain 32 data pairs, each consisting of the volume of the added NaOH in ml and the measured pH. 

The pH window is very wide in solvents that are weak both in acidity and basicity. The widths of the pH window are well over 30 in such solvents, compared to about 14 in water (Table 6.6). The usefulness of these expanded pH regions is discussed in Section 3.2.2. In particular, potentiometric acid-base titrations in such solvents are highly useful in practical chemical analyses as well as physicochemical studies [22]. Acid-base titrations in lion-aqueous solvents were popular until the 1980s, but now most have been replaced by chromatographic methods. However, the pH-ISFETs are promising to realize simple, rapid and miniature-scale acid-base titrations in lion-aqueous solvents. For example, by use of an Si3N4-type pH-ISFET, we can get an almost complete titration curve in less than 20 s in a solution containing several different acids [17d]. 

A. Kraft, Determination of the pAfa of Multiprotic, Weak Acids by Analyzing Potentiometric Acid-Base Titration Data with Difference Plots, ... 

The determination of partition caefcients by the potentiometric method is part of the function of the Sirus PCA101 (Avdeef, 1993). Typically, a preacidiLed solution of a weak acid is alkalimetrically titrated to some appropriately high pH the partition solvent such as octanol is then added, and the dual-solvent mixture is acidimetrically titrated back to the starting pH. Analysis of the two titration curves will yield two fKgS pl< and pol, where pdQ, is the apparent constant derived from... 

Gerakis, A. M., M. A. Koupparis, and C. E. Efstathiou. 1993. Micellar acid-base potentiometric titrations of weak acidic and/or insoluble drugfe.Pharm. Biomed. Anall1 33-41. 

Rhodes et al (31) determined a mixture containing ace-tylsalicylic acid, acetaminophen and salicylamide, potentiometrically by non-aqueous titration. The tit-rant was 0.1 N tetrabutylammonium hydroxide in benzene/ methanol mixture and the titration solvent was dimethyl-formamide. The difference in the pKa values for these weak acidic drugs (aspirin pKa 3-5, acetaminophen pKa 8.31) was sufficient to permit useful differentiation. 

An estimation of the amounts of weak acid was made from direct titration on the samples (method D). Most oil samples have TAN of 1 to 2 units (mg KOH g 1 oil) however, samples 8 and 9 have an acidity index of around 4.5 units. Heat of neutralization of acidic products ranges from -3.7 to -20.4 kJ mol"1 and indicates the presence of weak acids in engine oil samples. The precision of calorimetric titration is comparable with conductometric and potentiometric methods. The advantages of calorimetric and conductometric techniques are rapidity, good reproducibility and avoidance of poisoning of detectors by organic substances. 

For multistep complexation reactions and for ligands that are themselves weak acids, extremely involved calculations are necessary for the evaluation of the equilibrium expression from the individual species involved in the competing equilibria. These normally have to be solved by a graphical method or by computer techniques.26,27 Discussion of these calculations at this point is beyond the scope of this book. However, those who are interested will find adequate discussions in the many books on coordination chemistry, chelate chemistry, and the study and evaluation of the stability constants of complex ions.20,21,28-30 The general approach is the same as outlined here namely, that a titration curve is performed in which the concentration or activity of the substituent species is monitored by potentiometric measurement. 

Figure 1.8. Potentiometric titration plot of a weak monoprotic acid with a pKa of 4.8 (after Segel, 1976, with permission).

Classical potentiometric titrations have a major weakness—their inability to characterize weak acids with a pKa less than 3 and greater than 10. This is because water in those pH regions is capable of acting as weak acid or weak base, respectively (Fig. 1.9). In such cases colorimetric techniques are more suitable. 

The intramolecular dipolar interaction in solution should decrease with increasing size of the substituents in the order pyridiniimi > N (CH3)2 > N (C2H5)2 [34]. The potentiometric titration curves of polybetaines resemble those of the titration of weak acids and weak bases. Measurements of the... 

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