The pH of a Solution

EXERCISE 16.6 A solution has a hydroxide-ion concentration of 1.0 X 10 Af at 25°C. Is the solution acidic, neutral, or basic  

CONCEPT CHECK 16.3 Rank the following solutions from most acidic to most basic (water molecules 

I Tbe Danish biochemist S.P.L. Sorensen devised the pH scaie whiie working on the brewing I of beer. 

You see that whether an aqueous solution is acidic, neutral, or basic depends on the hydronium-ion concentration. You can quantitatively describe the acidity by giving the hydronium-ion concentration. But because these concentration values may be very small, it is often more convenient to give the acidity in terms of pH, which is defined as the negative of the logarithm of the molar hydronium-ion concentration  

Note that the number of places after the decimal point in the pH equals the number of significant figures reported in the hydronium-ion concentration.  

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Chemical properties of deposited monolayers have been studied in various ways. The degree of ionization of a substituted coumarin film deposited on quartz was determined as a function of the pH of a solution in contact with the film, from which comparison with Gouy-Chapman theory (see Section V-2) could be made [151]. Several studies have been made of the UV-induced polymerization of monolayers (as well as of multilayers) of diacetylene amphiphiles (see Refs. 168, 169). Excitation energy transfer has been observed in a mixed monolayer of donor and acceptor molecules in stearic acid [170]. Electrical properties have been of interest, particularly the possibility that a suitably asymmetric film might be a unidirectional conductor, that is, a rectifier (see Refs. 171, 172). Optical properties of interest include the ability to make planar optical waveguides of thick LB films [173, 174]. 

This relationship is one form of the Henderson-Hasselbalch equation It is a useful relationship m chemistry and biochemistry One rarely needs to cal culate the pH of a solution—pH is more often mea sured than calculated It is much more common that one needs to know the degree of ionization of an acid at a particular pH and the Henderson-Hasselbalch equation gives that ratio... 

From this equation it can be seen that when [CH3C02 ] = [CH3CO2H] then the second term is log 1 = 0 and pH = pK This means that when the pH of a solution is equal to the pK of a weak acid the con centration of the acid and its conjugate base are equal This is a relationship worth remembering... 

A more challenging problem is to find the pH of a solution prepared from a polyprotic acid or one of its conjugate species. As an example, we will use the amino acid alanine whose structure and acid dissociation constants are shown in Figure 6.11. 

This relationship is known as the Henderson-Hasselbalch equation. Thus, the pH of a solution can be calculated, provided and the concentrations of the weak acid HA and its conjugate base A are known. Note particularly that when [HA] = [A ], pH = pAl,. For example, if equal volumes of 0.1 MHAc and 0.1 M sodium acetate are mixed, then... 

It should be noted that whereas a completely soluble hydroxide (e.g. NaOH) will give a solution of high pH in which the pH will increase with concentration of the hydroxide, the pH of a solution of a sparingly soluble hydroxide will depend upon the equilibrium constant for hydrolysis and the activity of metal ions. 

The pH of a solution can be measured by an instrument called a pH meter. A pH meter translates the H+ ion concentration of a solution into an electrical signal that is converted into either a digital display or a deflection on a meter that reads pH directly (Figure 13.4). Later, in Chapter 18, we will consider the principle on which the pH meter works. 

Ordinarily, successive values of for polyprotic acids decrease by a factor of at least 100 (Table 13.3). In that case, essentially all the H+ ions in the solution come from the first step. This makes it relatively easy to calculate the pH of a solution of a polyprotic acid. 

Electrode for e pH meter. The pH of a solution can be determined with the aid of a "glass electrode." The voltage between the glass electrode and the reference electrode is directly related to pH. The leads from the electrodes are connected to a pH meter. 

The pH will depend upon the ionic strength of the solution (which is, of course, related to the activity coefficient — see Section 2.5). Hence, when making a colour comparison for the determination of the pH of a solution, not only must the indicator concentration be the same in the two solutions but the ionic strength must also be equal or approximately equal. The equation incidentally provides an explanation of the so-called salt and solvent effects which are observed with indicators. The colour-change equilibrium at any particular ionic strength (constant activity-coefficient term) can be expressed by a condensed form of equation (4) ... 

The rest of this chapter is a variation on a theme introduced in Chapter 9 the use of equilibrium constants to calculate the equilibrium composition of solutions of acids, bases, and salts. We shall see how to predict the pH of solutions of weak acids and bases and how to calculate the extent of deprotonation of a weak acid and the extent of protonation of a weak base. We shall also see how to calculate the pH of a solution of a salt in which the cation or anion of the salt may itself be a weak acid or base. 

Our first task is to calculate the pH of a solution of a weak acid, such as acetic acid in water The initial concentration of the acid is its concentration as prepared, as if no acid molecules had donated any protons. For a strong acid HA, the H30+ concentration in solution is the same as the initial concentration of the strong acid, because all the HA molecules are deprotonated. However to find the H30 concentration in a solution of a weak acid HA, we have to take into account the equilibrium between the acid HA, its conjugate base A-, and water (Eq. 8). We can expect the pH to lie somewhere between 7, a value indicating no deprotonation, and the value that we would calculate for a strong acid, which undergoes complete deprotonation. The Technique, which is based on the use of an equilibrium table like those introduced in Chapter 9, is set out in Toolbox 10.1. 

To calculate the pH of a solution of a weak base, set up an equilibrium table to calculate pOH from the value ofKh and convert that pOH into pH by using pH + pOH = 14.00. 

We saw in Section J that a salt is produced by the neutralization of an acid by a base. However, if we measure the pH of a solution of a salt, we do not in general find the neutral value (pH = 7). For instance, if we neutralize 0.3 M CHjCOOH(aq) with 0.3 M NaOH(aq), the resulting solution of sodium acetate has pH = 9.0. How can this be The Bronsted-Lowry theory provides the explanation. According to this theory, an ion may be an acid or a base. The acetate ion, for instance, is a base, and the ammonium ion is an acid. The pH of a solution of a salt depends on the relative acidity and basicity of its ions. 

The anions of strong acids—which include Cl, Br, I, N03, and CI04 — are such weak bases that they have no significant effect on the pH of a solution. 

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