Solutions of a Weak Acid or Base

We have seen how to estimate the pH of a solution of a weak acid or base (Chapter 10), but suppose that a salt of the acid or base is also present. How does that salt affect the pH of the solution Suppose we have a dilute hydrochloric acid solution and add to it appreciable concentrations of the conjugate base, the Cl- ion, as sodium chloride. Because the acid is strong, its conjugate base is extremely weak and so has no measurable effect on pH. The pH of 0.10 M HCl(aq) is about 1.0, even after 0.10 mol NaCl has been added to a liter of the solution. Now suppose instead that the solution contains acetic acid to which sodium acetate has been added (the acetate ion, CH jC()2, is the conjugate base of CH COOH). Because the conjugate base of a weak acid is a base, we can predict that adding acetate ions (as sodium acetate) to a solution of acetic acid will increase the pH of the solution. Similarly, suppose we have a solution of ammonia and add ammonium chloride to it. The... 

A solution of a weak acid or base with its salt. 

The pH of a solution of a weak acid (or base) and its salt is given by the Henderson-... 

Buffer solution Solution of a weak acid (or base) and its conjugate base (or acid) that is resistant to changes in pH works by reacting with any added acid or base to restore the equilibrium of the weak acid (or base) and neutralize the added acid or base. 

Outline the procedure for the exact treatment of acid-base equilibrium and use it to find the pH of a very dilute solution of a weak acid or base (Section 15.8, Problems 69-70). 

Before the titrant is added, the analyte is a solution of a weak acid or base in water. Determine the pH of the solution by following the procedures in Chapter 10 in the text. 

Chemically a buffer solution is defined as one that resists a change in pH on addition of acid or alkali. Buffer solutions contain compoimds that react with both acid or base so that the H+ ion concentration in the solution remains constant. The buffer solutions thus have reserve acidity or reserve alkalinity . Buffer solutions usually consist of a mixtures of solutions of a weak acid or base and its salt, for example, acetic acid and sodium acetate. The pH of the buffer is governed by salt-acid ratio and the ionisation constant of the acid and is given by Henderson... 

The chapter ends with a reassessment and rigorous derivation of the pH of a solution of a weak acid or base where no approximations are made. This section can be ignored on a first reading, and only studied when a full assimilation of the earlier material has been achieved. 

Buffer solution. A solution of a weak acid or base plus its conjugate it resists change in its pH even on addition of strong acid or base. 

We have indicated how to determine the various kinetic constants appearing in the expression for specific acid and base catalysis. Let us now consider how to evaluate the various contributions to the rate constant in the case of general acid-base catalysis. For reactions of this type in a solution of a weak acid or base and its corresponding salt, the possible catalysts indicated by equation (7.3.3) are the hydro-nium ion, the hydroxide ion, the undissociated weak acid (or base), and the conjugate base (or acid), In the case of acetic acid the general acid would be the neutral CHjCOOH species and the conjugate base would be the acetate ion (CH3COO"). In this case the apparent rate constant can be written as... 

Solutions of a Weak Acid or Base with Another Solute... 

In the preceding sections, we looked at solutions that contained either a weak acid, a weak base, or a salt of a weak acid or base. In the remaining sections of this chapter, we will look at the effect of adding another solute to a solution of a weak acid or base. The solutes we will look at are those that significantly affect acid or base ionization— that is, strong acids and bases, and salts that contain an ion that is produced in the acid or base ionization. These solutes affect the equihbrium through the common-ion effect. 

A large number of other reactions are also possible, e.g., hydration of the various substances, or (if the solute is a salt of a weak acid or base) hydrolysis, but in all cases the concentration of each molecular species is defined by the total amount of solute in a given mass of solution, and the ionisation proceeds as if all the other reactions did not occur at all (cf. 158). 

We have already seen how to estimate the pH of the initial analyte when only weak acid or weak base is present (point A in Fig. 11.6, for instance), as well as the pH at the stoichiometric point (point S). Between these two points lie points corresponding to a mixed solution of some weak acid (or base) and some salt. We can therefore use the techniques described in Toolbox 11.2 and Example 11.6 to account for the shape of the curve. 

The above examples assume that the strong base KOH is completely dissociated in solution and that the concentration of OH ions was thus equal to that of the KOH. This assumption is valid for dilute solutions of strong bases or acids but not for weak bases or acids. Since weak electrolytes dissociate only slightly in solution, we must use the dissociation constant to calculate the concentration of [H" ] (or [OH ]) produced by a given molarity of a weak acid (or base) before calculating total [H" ] (or total [OH ]) and subsequendy pH. 

The dissociation constant of a weak acid or base is altered by ionic strength, too. Ionic strength I is a measure for total content of ions within a solution and depends on concentration as well as on charge of ions ... 

Buffer solutions comprise a weak acid or base and its salt (conjugate acid or base), and provide a means of resisting or modulating changes in pH of solutions, in order to keep the environment constant to allow optimal conditions for biochemical reactions. 

An aqueous solution of a weak acid or a weak base contains two substances that react with each other to some extent—the acid or base and the water. When another substance is added to this solution, it can affect the original pair of reactants without necessarily reacting directly with either. It may, as predicted by LeChatelier s principle, suppress the reaction of the original two reactants. For example, if sodium acetate, NaC2H302, is added to an aqueous solution of acetic acid, HC2H3O2, it does not react directly with the acid or with the water. Instead,... 

An indication of the buffer capacity of any acid-base system can thus be obtained directly from the pH-neutralization curve if the curve is flat, d(pH)/d6 is obviously small and the buffer capacity, which is the reciprocal of this slope, is large. An examination of curves Ia and Ib, Fig. 106, shows that a relatively concentrated solution of strong acid or base is a buffer in regions of low or high pH, respectively. A solution of a weak acid or a weak base alone is not a good buffer, but when an appreciable amount of salt is present, i.e., towards the middle of the individual neutralization curves Ha, III a, II b or IIIb, the buffer capacity of the system is very marked. As the equivalence-point is approached the pH changes rapidly and so the buffer capacity of the salt solution is small. If the acid or base is very weak, or if both are moderately weak, the slope... 

A solution containing a weak acid or base and one of its salts -> Example a solution of hydrofluoric acid and sodium fluoride... 

We are often interested in the changes in composition that occur while a solution of a weak acid or a weak base is being titrated. These changes can be visualized by plotting the relative equilibrium concentration uq of the weak acid as well as the relative equilibrium concentration of the conjugate base aj as functions of the pH of the solution. 

Based on the results of this illustration, if we are interested only in computing the pH of a solution containing a weak acid (or base), it may be possible to neglect the ion activity coefficients. However, if our interest is in the extent of dissociation of the weak acid (or base), the activity coefficients of the ions should be included. What happens in the calculation here (and in other examples later in this chapter) is that there is some cancellation between the effect of the ion nonideality on the calculation of the equilibrium and on the calculation of the pH. This is especially true for a 1 1 acid (that is, an acid that on ionization produces a cation of charge +1 and an anion of charge -I). 

The procedures for finding the pFI of a solution containing a weak acid or base plus a common ion are very similar to the procedures, which we covered in Chapter 14, for solutions containing the acids or bases alone. For example, in the case of a weak acid, the only important difference is that the initial concentration of the anion is not zero in a solution that also contains the salt NaA. Example 15.1 illustrates a typical example using the same general approach we developed in Chapter 14. 

The extraction of a weak acid or base into an organic phase is a function of the pH of the aqueous solution, the pfCa of the analyte, and the partition coefficient describing the distribution of the analyte between the particular solvent and water. Under ideal conditions, the fraction extracted, F, is given by... 

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