The Titration of Weak Acids and Bases

In titrating a weak acid with a strong base) or a weak base with a strong acid) greater care is needed in the selection of an indicator. Let us consider the titration of 0.2 N acetic acid, a moderately weak acid with = L80 X 10 , with 0.2 N sodium hydroxide. When an amount of the alkali equivalent to that of the acid has been added, the resultant solution is the same as would be obtained by dissolving 0.1 mole of the 

Ratio of equivalents of base to acid in titration of a 0.2 N acid with a 0.2 N base, either acid or base being strong 

The salt NaCoH302 is completely dissociated into ions, Na and C2H302, when it is dissolved in water. The ion Na has no protons, and hence is not an acid. The acetate anion, C2H302. is, however, a base —it is the base conjugate to the acid HC2H3O2, and it can accept a proton from an acid, such as H O  

This reaction takes place to the extent determined by the value of its equilibrium constant  

We may say that an aqueous solution of the neutral salt sodium acetate is alkaline, with pH greater than 7, because it contains the base (proton acceptor) acetate ion. 

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The key to understanding the titrations of weak acids and bases is to be familiar with the species in solution and the dominant equilibrium at each point along the titration curve. Example reinforces these qualitative features. 

Very many problems in solution chemistry are solved with use of the acid and base equilibrium equations. The uses of these equations in discussing the titration of weak acids and bases, the hydrolysis of salts, and the properties of buffered solutions are illustrated in the following sections of this chapter. 

THE TITRATION OF WEAK ACIDS AND BASES. THE HYDROLYSIS OF SALTS... 

Weak Acids and Bases. The titration of weak acids and bases does not result in as sharp an endpoint as is obtained with strong acids and bases. 

This technique uses both direct and back titrations of weak acids and bases. Values of are obtained directly. In purely aqueous media, over the pH range 2-10, the titration of dilute (0.005 to 0.05 M) solutions of weak monovalent acids and bases with a glass electrode can lead to reliable thermodynamic pKs. Over this pH interval, the activity coefficients of the ionic species can be calculated by means of the Debye-Hiickel equation. Also, the activity coefficients of the neutral species remain essentially constant and... 

As we will soon discover, the pH is not 7.00 at the equivalence point in the titration of weak acids or bases. The pH is 7.00 only if the titrant and analyte are both strong. 

There are a few main types of titrations a strong acid titrated with a strong base (or a strong base titrated with a strong acid) a weak acid titrated with a strong base a weak base titrated with a strong acid and a polyprotic acid titrated with a strong base. Each one of these produces characteristic results and will need to be discussed separately. For the solutions of weak acids and bases, the process is complicated by the common-ion effect. 

Of considerable difference with the titrations of weak acids and weak bases are the buffering effects of the conjugate salts. The titration curves (see Figure 14.6) all contain a buffering region near the equivalence point where most of the solution consists of the conjugate base (for a weak acid titration a conjugate acid for a weak base titration). 

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 ... 

Textbooks of analytical chemistry should be consulted for further details concerning the ionization of weak acids and bases and the theory of indicators, buffer solutions, and acid-alkali titrations. 

In analytical chemistry, where non-aqueous solvents are used in the acidimetric and alkalimetric titration of weak acids and bases, solvents are classified as acidic and basic [Gy 70]. In the former group the proton-donating and in the latter group the proton-accepting property predominates. Accordingly, bases can be titrated in acidic solvents, which readily transfer their protons to the base dissolved in them, and acids can be titrated in basic solvents, which readily accept protons from the acid dissolved in them. 

CO2 changes the concentration of base over a period of time and reduces the sharpness of the end point in the titration of weak acids. Strong base attacks glass and should not be kept in a glass bottle or in a buret longer than neeessary. If base is kept in a tightly capped polyethylene bottle, it can be used for weeks with little change. 

However, weak acids do not yield many hydrogen ions and weak bases do not yield many hydroxide ions, and titration of weak acids and bases or their salts with strong acids and bases may equally well be regarded as the addition or removal of a proton ... 

Chapter 14, Acids and Bases, discusses acids and bases and their strengths, conjugate acid-base pairs, the dissociation of weak acids and bases and water, pH and pOH, and buffers. Acid-base titration uses the neutralization reactions between acids and bases to calculate quantities of acid in a sample. Section 14.9, Acid-Base Properties of Salt Solutions, has been deleted. Combining Ideas from Chapters 11,12,13, and 14 follows as an interchapter problem set. 

This approach can be used to sketch titration curves for other acid-base titrations including those involving polyprotic weak acids and bases or mixtures of weak acids and bases (Figure 9.8). Figure 9.8a, for example, shows the titration curve when titrating a diprotic weak acid, H2A, with a strong base. Since the analyte is... 

O Effect of pKa in the titration of weak acid with strong base. Use Equation 11-9 with a spreadsheet such as the one shown in Figure 11-11 to compute and plot the family of curves at the left side of Figure 11-3. For a strong acid, choose a large A"a, such as Ka = 102 or pKa = -2. 

Ethylenediamine (en), NH2C2H4NH2, a strongly basic substance, may be considered to represent solvents that are weakly acidic compared with water. Ethylenediamine is therefore useful as a solvent for the titration of weakly acidic substances. It is a leveling solvent for adds whose ionization constants are larger than about 10 in water thus acetic add and hydrochloric acid are leveled to about equal strength. The titrant base normally used in en is sodium ethanolamine. The autoprotolysis constant of en is 5 x 10" for the equilibrium... 

Titrations involving weak acids and bases are far more complex to model, paticularly when the acids or bases are polyprotic and volume corrections are made. For detailed discussion of both simple and complex acid/base modeling with volume corrections, the reader is referred to Butler (1964), Pankow (1991), and Stumm and Morgan (1996). 

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... 

The acidity or basicity of a solution is frequently an important factor in chemical reactions. The use of buffers of a given pH to maintain the solution pH at a desired level is very important. In addition, fundamental acid-base equihbria are important in understanding acid-base titrations and the effects of acids on chemical species and reactions, for example, the effects of complexation or precipitation. In Chapter 6, we described the fundamental concept of equilibrium constants. In this chapter, we consider in more detail various acid-base equilibrium calculations, including weak acids and bases, hydrolysis, of salts of weak acids and bases, buffers, polyprotic acids and their salts, and physiological buffers. Acid-base theories and the basic pH concept are reviewed first. 

Hydrolysis of salts of weak acids and bases. In aqueous solution the end-point in titrations of strong acids with strong bases, or vice versa. 

Weak acids and bases can be determined either alone or in the presence of strong acids and bases by titration between two pHs. It is immaterial whether they are present as free acid or base, or as a salt, or as a mixture of acid or base and salt. 

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