Titrations of weak acids

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. 

For the non-aqueous titration of weak acids a solvent such as an alcohol or an aprotic, solvent is used that does not compete strongly with the weak acid for proton donation. Typical titrants are lithium methoxide in methanol or tetrabutyl ammonium hydroxide in dimethylformamide. End-point detection may be carried out with thymol blue as an indicator or potentiomctrically (see p. 65). 

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... 

Titration of Weak Acids with Strong Bases... 

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. 

I B I 11-2 Titration of Weak Acid with Strong Base... 

C02 changes the concentration of strong base over a period of time and decreases the extent of reaction near the end point in the titration of weak acids. If solutions are kept in tightly capped polyethylene bottles, they can be used for about a week with little change. 

Titration of weak acid with strong base ... 

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. 

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. 

In acid-base titrations of weak acids or bases the expressions are also more complicated. Nevertheless, the introduction of concentration dependence as a function of the composition of the medium in the Nernst equations always leads to the correct result. 

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... 

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... 

Normal salts acid salts basic salts. Hydrogen-ion concentration. / H. Indicators. Ionization equilibria of weak acids and bases. Acid constant basic constant. Titration of weak acids and bases. Choice of suitable indicator. 

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... 

Determination of acids For accurate results in the titration of weak acids with an indicator, one should be chosen that shows a transition color in the alkaline range coinciding as nearly as possible with the pH at the equivalence point. For best results a comparison solution of the indicator in a solution of the salt of the weak acid should be used. If the salt is not available, a buffer solution of the same pH may be substituted. 

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 ... 

Capillary electrophoresis High-speed, high-resolution electrophoresis performed in capillary tubes or in microchips. Carbonate error A systematic error caused by absorption of carbon dioxide by standard solutions of base that will be used in the titration of weak acids. 

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