Potentiometric titrations of weak acids

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. 

M. Satoh, J. Komiyama, A new model of counterion condensation in polyelectrolyte solutions, I. Applications to potentiometric titration of weak acids, Polym. J. 19 (1987) 1201-1208. 

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... 

In all potentiometric titrations, both slope and height of the pH variation determine the detectable limit of the amount and strength of functional groups. In differential curves, this is expressed by the height and sharpness of the peaks in proximity of the equivalence points. In particular, in the titration of weak acids or bases, such as the surface functional groups of carbon, at halfway to the inflection point the concentration is almost equal to the functional groups to be titrated. For example, for a base -C-OH ... 

Valik19 made differential potentiometric titrations of aspartic acid, one series of results being given in curve (c) of Fig. 6. The volume of solution of alkali necessary to titrate tlie second acid dissociation for which the ionization constant is 2.5 X 10"10 should be exactly equal to that for the first unless there is a difference between the potentiometric and stoichiometric end points. Within the rather large limit of error, this was found to be true, but the end point could not be located with accuracy due to the flatness of the curve, as shown above. Differences between the stoichiometric and potentiometric end points are predicted for titrations of weak acids or weak bases. Such a difference increases the weaker the acid or base, but the difficulty of locating the end point also increases. It may be safely concluded that within the accuracy to which the potentiometric end point of a titration can be established it is identical with the stoichiometric end point. 

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. 

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. 

The effects of meso substituents on the acidities of benzimidazoles have been examined by potentiometric titration of a series of 2-substituted benzimidazoles in acetonitrile. The 2-substituents appear to exert primarily inductive effects on the acidity centre, with the pAg dependent on glass electrodes are inaccurate at pH values of > 11 in water (these compounds are very weak acids). As acetonitrile has a self-protolysis constant, pAg, of 33.3 (cf. water, pAg 14) it becomes possible to study the acidic properties of such slightly dissociated compounds. In... 

It is possible to estimate, roughly at least, the values of the ionization constants of weak acids and bases by means of potentiometric titration methods. Since the ionization constant K of a weak acid HA is given by... 

As has been discussed in Sec. II.B, hydrophobicity of supporting cations is expected to enhance the apparent complexation of weak acidic polyelectrolytes, and it is of interest to study how the metal complexation equilibria are affected by the addition of hydrophobic supporting cations, such as tetra-alkyl ammonium ions. Representative plots obtained by a potentiometric titration study on Ca2+/PAA in the presence of excess TMA+C1 salt [43] are shown in Figure 15. By comparison of the log(Arca)app vs. a plots of the systems of Ca2+/PAA/Na+ with Ca2+/PAA/TMA+, it is apparent that Ca2+ complexation is highly enhanced by the addition of TMA+ at any salt con-... 

At equilibrium, during each step of the potentiometric titration of a weakly acidic (or weakly basic) gel (HA)V, in the presence of a simple electrolyte MX, the chemical potential fi of each diffusible component (HX, MX, and H20) is equal in both phases for example,... 

Figure 15.18 Schematic plot of the potentiometric titration of three weak acids with tetrabutylam-monium hydroxide. Note the equivalence points after the addition of 3.5, 8.5, and 13.0 mL of titrant.

The principal impurities are water and carbon dioxide which can be removed by distillation from potassium hydroxide pellets. Like ethylene-diamine and pyridine it has been used as a solvent for non-aqueous potentiometric titrations of very weak acids. Contact with skin leads to severe burns. The vapours cause irritation if inhaled. 

The ester cannot be determined by saponification because this slowly removes the sulphonate group as well as hydrolysing the ester group. It can be determined by acid hydrolysis followed by measurement of the a-sulphonated carboxylate salt produced, either by potentiometric titration of the weak acid or by two-phase titration with benzethonium chloride. Both procedures measure the a-sulphonated ester plus the unsulphonated ester. If the latter is present at a significant level, it can be determined (section 5.11.2) and corrected for. 

Mixtures of weak bases with WS amphoterics (carboxybetaines) can be analysed by the method described in section 7.2.4 [2]. The betaine is determined by potentiometric titration of an initially acid solution in methyl isobutyl ketone with alkali, and the weak base by potentiometric titration of an initially alkaline solution in 50% aqueous propan-2-ol with acid. This procedure distinguishes between tertiary amines and amine oxides (section 7.8). 

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,... 

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 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. 

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