Although some sulfur dioxide in air is oxidized by gas-phase reactions, most of it is converted to sulfuric acid after it has dissolved in tiny sus- pended water droplets present in clouds, mists, etc. The uncatalyzed oxida- tion of dissolved aqueous SO2 by dissolved O2 proceeds at very slow rate unless a catalyst such as Fe3 or the ions of other transition metals are also present in the droplets.
The most important oxidizing agents in the airborne droplets, though present only in tiny concentrations, are not molecular oxygen but the dis- solved atmospheric gases ozone and hydrogen peroxide, H2O2. The con- centration of these two pollutants is much greater in air masses undergoing photochemical smog than in clean air.
In general, the concentration of a dissolved gas in the liquid phase can be determined by considering the equilibrium between its two forms. Thus for hydrogen peroxide, we have
H2O2 (g) !1 H2O2(aq)
The useful form of the equilibrium constant for such processes is the Henry’s law constant, KH, which is equal to the concentration of the dissolved species divided by the partial pressure of the gas. For the above reaction, we have
KH __ [H2O2] P H2O2
If the concentration is expressed as a molarity, and the unit of pressure is atmospheres, then from experimental data
KH 7.4 104 M atm1
Using this information, we can determine the molarity of H2O2 in a raindrop for typical clean-air conditions of 0.1 ppb, i.e., equivalent to 0.1 109 atm.
[H2O2] KH P H2O2
7.4 104 M atm1 0.1 109 atm 7.4 106 M
Although a concentration of 7.4 M seems tiny by comparison with values routinely encountered in laboratories, it is sufficient to oxidize dissolved SO2 at an appreciable rate. The hydrogen peroxide concentration in smoggy air is an order of magnitude or more larger than in clean air, so its concentration in water droplets rises accordingly, as does the rate of oxidation of sulfur dioxide.
The calculation of the solubility of SO2 in raindrops is more compli- cated, since in the aqueous phase it exists as sulfurous acid, H2SO3:
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For unknown reasons, hydrogen peroxide levels in suspended aerosol droplets in the air at several California locations are orders of magnitude larger even than predicted by Henry’s law.
Improving Air Quality: Sulfur-Based Emissions 117
SO2(g) H2O(aq) !1 H2SO3(aq)
The Henry’s law expression does not include the concentration of the solvent, water:
KH __ [H2SO3] P SO2
Since KH 1.0 M atm1 for SO2, and since its concentration in a typical sample of air is about 0.1 ppm, i.e., equivalent to 0.1 106 atm, the equi- librium concentration of sulfurous acid is
[H2SO3] KH P SO2
1.0 M atm1 0.1 106 atm.
1.0 107 M
This value of about 107 M for the equilibrium concentration of H2SO3 is deceptive since it by no means represents all the sulfur dioxide that dis- solves in a water droplet (see Figure 3-18). Sulfurous acid is a weak acid whose ionization to the bisulfite ion, HSO3, must also be considered in calculating the solubility of sulfur dioxide:
H2SO3!1 H HSO3
The acid dissociation (or ionization) constant Ka for H2SO3 is equal to 1.7 102, where Ka is related to concentrations by the expression
Ka [H___ ] [HSO3] [H2SO3]
The concentrations in such expressions are equilibrium values. Since the equilib- rium molarity of H2SO3 is determined in the raindrop by its interchange with SO2 in air, we can substitute that known value into the Ka expression:
Ka [H___ ] [HSO3] 1.0 107
Rearranging the equation to solve for the ion concentrations, which from stoichiom- etry are equal in value, we obtain
[HSO3]2 1.7 102 M 1.0 107 M and hence
[HSO3] 4 105 M 0!
Water Droplet
O3
H2O2 H2SO3
HSO4–
HSO3– + H+
SO2
FIGURE 3-18 Dissolution of atmospheric gases SO2, O3, and H2O2 into a water droplet and their subsequent reactions.
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Thus, the equilibrium ratio of bisulfite ion to sulfurous acid in water is about 400 : 1. Consequently, the total dissolved sulfur dioxide is about 4 105 M, rather than just the 1 107 M that represents the contribution from the un-ionized acid.
Since the concentration of hydrogen ion produced by the reaction is also 4 105 M, the pH of such raindrops is 4.4. Rain does not become much more acidic than this if no strong acids are dissolved in the droplets.
PROBLEM 3-12
Bisulfite ion can act as a weak acid and ionize further:
HSO3!1 H SO32
Given that Ka for HSO3 is 1.2 107, calculate the concentration of SO32 that is present in the raindrops of pH 4.4 discussed above. [Hint: The concen- trations of bisulfite and hydrogen ion will be very close to their values estab-
lished previously.] ●
PROBLEM 3-13
Calculate the pH of rainwater in equilibrium with SO2 in a polluted air mass for which the sulfur dioxide concentration is 1.0 ppm. [Hint: Recall the rela- tionship between partial pressure and ppm concentration discussed earlier in
the chapter.] ●
PROBLEM 3-14
Calculate the concentration of SO2 that must be reached in polluted air if the dissolved gas is to produce a pH of 4.0 in raindrops without any oxidation
of the sulfur. ●
PROBLEM 3-15
(a) Confirm by calculation that the pH of CO2-saturated water at 25°C is 5.6, given that the CO2 concentration in air is 390 ppm. For carbon dioxide, the Henry’s law constant KH 3.4 102 M atm1 at 25°C. The Ka for carbonic acid, H2CO3, is 4.5 107 at that temperature. (b) Recalculate the pH for a carbon dioxide concentration of 560 ppm, i.e., double that of
the preindustrial age. ●
Particulates in Air Pollution
The black smoke released into the air by a diesel truck is often the most obvi- ous form of pollution that we routinely encounter. The smoke is composed largely of particulate matter. Particulates are tiny solid or liquid
Most oxidation of dissolved sulfur dioxide occurs via bisulfite ion, as indicated in Figure 3-18.
PROBLEM 3-12
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PROBLEM 3-13
PROBLEM 3-14
PROBLEM 3-15
Review Questions 13–17 are based on the material in the preceding sections.
Particulates in Air Pollution 119
particles—other than those of pure water—that are temporarily suspended in air and that are usually individually invisible to the naked eye. Collec- tively, however, such particles often form a haze that restricts visibility.
Indeed, on many summer days the sky over North American and European cities is milky white rather than blue. More importantly, breathing air that contains particulates is known to be hazardous to human health. Ozone and particulate matter have the greatest negative effects on human health of all air pollutants in most parts of the world, as discussed in Chapter 4. In the material that follows, we investigate the wide range of sizes of the suspended particles and their origins.