Buffered solutions titrations with weak acids

Beyond the buffer region, when nearly all of the acetic acid has been consumed, the pH increases sharply with each added drop of hydroxide solution. The titration curve passes through an almost vertical region before leveling off again. Recall from Chapter 4 that the stoichiometric point of an acid titration (also called the equivalence point) is the point at which the number of moles of added base is exactly equal to the number of moles of acid present in the original solution. At the stoichiometric point of a weak acid titration, the conjugate base is a major species in solution, but the weak acid is not. 

The common-ion effect is an application of Le Chatelicr s principle to equilibrium systems of slightly soluble salts. A buffer is a solution that resists a change in pH if we add an acid or base. We can calculate the pH of a buffer using the Henderson-Hasselbalch equation. We use titrations to determine the concentration of an acid or base solution. We can represent solubility equilibria by the solubility product constant expression, Ksp. We can use the concepts associated with weak acids and bases to calculate the pH at any point during a titration. 

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). 

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. 

Solution of base B and its perchlorate At the points in a titration between the initial and equivalence points, it is not possible either to reason directly by analogy with titrations in water or to use buffer formulas derived for weak acids in aqueous solutions. The dissociation equilibria of the ion pair BH C104, the acid HCIO4, the base B, and the solvent HOAc all must be considered. 

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 titration curves for weak acids and weak bases, pH changes near the equivalence point are too small for color indicators to be used. The solution is buffered both before and after the equivalence point. Figure 19-6 shows the titration curve for 100.0 mL of 0.100 M CH3COOH solution titrated with 0.100 M aqueous NH3. The calculation of values on the curve in Figure 19-6 other than the initial pH and the pH at the equivalence point is beyond the scope of this text. 

Further we introduced buffer chemistry and saw how pH may be calculated in buffer solutions and on how the buffer equation is often used in practice. When one has a solution of a weak acid and its corresponding weak base, both in concentrations of the same magnitude, one has a buffer system and the buffer equation may be used to calculate pH. Lastly, we looked at titration and on pH curves exemplifyed through examples of titration of monovalent weak acid with strong base and titration of divalent acid likewise with strong base NaOH. In the end we saw how colour indicator work. 

Titration of weak acid with weak base. In such cases, titration curves of the type shown in Fig. 4.17 are obtained. Over the region AB, corresponding to the initial addition of weak base, the ionization of the weak acid is suppressed by the buffer action so that the conductance falls. As the salt is progressively formed, the number of ions in solution rises with consequent increase in conductance over BC. In the region CD, addition of the weakly ionized base to a solution of its salt causes the conductance to almost level off. 

Now consider the overall shape of the pH curve. The slow change in pH about halfway to the stoichiometric point indicates that the solution acts as a buffer in that region (see Fig. 11.3). At the halfwayr point of the titration, [HA] = [A ] and pH = pfCa. In fact, one way to prepare a buffer is to neutralize half the amount of weak acid present with strong base. The flatness of the curve near pH = pKa illustrates very clearly the ability of a buffer solution to stabilize the pH of the solution. Moreover, we can now see how to determine pKa plot the pH curve during a titration, identify the pH halfway to the stoichiometric point, and set pKa equal to that pH (Fig. 11.8). To obtain the pfCh of a weak base, we find pK3 in the same way but go on to use pKa -1- pfq, = pKw. The values recorded in Tables 10.1 and 10.2 were obtained in this way. 

Point A iies aiong the section of the titration curve known as the buffer region. Buffering action comes from the presence of a weak acid and its conjugate base as major species in solution. Moreover, Point A iies beyond the midpoint of the titration, which teiis us that more than half of the weak acid has been consumed. We represent this soiution with two moiecuies of H four ions of A, and four H2 O moiecuies ... 

In the process of a weak acid or weak base neutralization titration, a mixture of a conjugate acid-base pair exists in the reaction flask in the time period of the experiment leading up to the inflection point. For example, during the titration of acetic acid with sodium hydroxide, a mixture of acetic acid and acetate ion exists in the reaction flask prior to the inflection point. In that portion of the titration curve, the pH of the solution does not change appreciably, even upon the addition of more sodium hydroxide. Thus this solution is a buffer solution, as we defined it at the beginning of this section. 

You learned about acids and bases in your previous chemistry course. In this chapter, you will extend your knowledge to learn how the structure of a compound determines whether it is an acid or a base. You will use the equilibrium constant of the reaction of an acid or base with water to determine whether the acid or base is strong or weak. You will apply your understanding of dissociation and pH to investigate buffer solutions solutions that resist changes in pH. Finally, you will examine acid-base titrations that involve combinations of strong and weak acids and bases. 

An acid-base titration is a method that allows quantitative analysis of the concentration of an unknown acid or base solution. In an acid-base titration, the base will react with the weak acid and form a solution that contains the weak acid and its conjugate base until the acid is completely neutralized. The following equation is used frequently when trying to find the pH of buffer solutions. 

This region of the titration shows clearly the buffering action of a mixture of a weak acid with its conjugate base. At the half-equivalence point V = VJ2, [CH3COOH]o = [CH3COO ]o, which corresponds to an equimolar buffer at this point pH - pK. On either side of this point the pH rises relatively slowly as the NaOH solution is added. 

When a weak acid is titrated with a strong hase, the curve is quite different. The solution is buffered before the equivalence point. It is basic at the equivalence point because salts of weak acids and strong bases hydrolyze to give basic solutions. So, we can separate the calculations on this kind of titration into four distinct types, which correspond to four regions of the titration curves. 

We have seen earlier how calculations of pH in solutions with strong acid and strong base are relatively simple because strong acids and strong bases are completely dissociated. On the contrary, pH calculations in cases where the titrated acid is weak is not as simple. In order to be able to calculate the concentration of HsO ions after the addition of a given amount of strong base it is necessary to look at the weak acids dissociation equilibrium. Calculations of pH curves for titration of a weak acid with a strong base involve a series of buffer-related problems. 

Figure 19.9 Curve for a weak base-strong acid titration. Titrating 40.00 mL of 0.1000 M NH3 with a solution of 0.1000 M HCl leads to a curve whose shape is the same as that of the weak acid-strong base curve in Figure 19.8 but inverted. The midpoint of the buffer region occurs when [NH3] = [NH4 ] the pH at this point equals the pKa of NH4. Methyl red (photo) is a suitable Indicator here.

The equivalent for basic buffers is lo mix the we base and its sail with a strong acid e.g. ethylamine and ethylamine hydrochloride), or lo titrate the weak base solution vviti. strong acid. 

Aqueous buffer solutions are frequently made up by adding similar amounts of a weak acid and its salt with a strong base (e.g. ethanoic acid and sodium ethanoate) to water (Table 1.3). Alternatively, one may start with a solution of weak acid (say) and titrate to the desired pH by addition of base. 

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