Chapter 16, Problem 48RQ

Using the list of indicators in Table 16.7, choose the appropriate indicators for the titrations depicted in Figures 16.8 and 16.9.

TABLE 16.7 Common Acid-Base Indicators

Indicator	Approximate pH Range over which the Color Changes	Color Change (lower to higher pH)
Methyl green	0.2-1.8	Yellow to blue
Thymol blue	1.2-2.8	Yellow to blue
Methyl orange	3.2-4.4	Red to yellow
Ethyl red	4.0-5.8	Colorless to red
Methyl purple	4.8-5.4	Purple to green
Bromocresol purple	5.2-6.8	Yellow to purple
Bromothymol blue	6.0-7.6	Yellow to blue
Phenol red	6.4-8.2	Yellow to red/violet
Litmus	4.7-8.3	Red to blue
Cresol red	7.0-8.8	Yellow to red
Thymol blue	8.0-9.6	Yellow to blue
Phenolphthalein	8.2-10.0	Colorless to pink
Thymolphthalein	9.4-10.6	Colorless to blue
Alizarin yellow R	10.1-12.0	Yellow to red
Clayton yellow	12.2-13.2	Yellow to amber

Before the titration begins. The solution at this point is simply a solution of the weak base, ${\text{NH}}_{\text{3}}$. Since the only solute is a base, we must use K_b to calculate the pH.

Between the start and the equivalence point. As we add HCl to the ${\text{NH}}_{\text{3}}$, the neutralization reaction produces ${\text{NH}}_{\text{4}}{}^{\text{+}}$, so the solution contains both ${\text{NH}}_{\text{3}}$ and ${\text{NH}}_{\text{4}}{}^{\text{+}}$ (it is a buffer solution). We do a limiting reactant calculation to determine the moles of ammonia left and the moles of ammonium ions produced by the addition of HCl. We use the equilibrium law to perform the calculation as we did with the weak acid.

At the equivalence point. Here all the ${\text{NH}}_{\text{3}}$ has reacted and the solution contains the salt ${\text{NH}}_{\text{4}}\text{Cl}$. We calculate the concentration of ammonium ions and then use the techniques we developed to determine the pH of the conjugate acid of a weak base.

After the equivalence point. The ${\text{H}}^{\text{+}}$ introduced by further addition of HCl has nothing with which to react, so it causes the solution to become more and more acidic. The concentration of ${\text{H}}^{\text{+}}$ calculated from the excess of HCl is used to calculate the pH.

Figure 16.9 illustrates the titration curve for this system.