The pH of rainwater

Exercise 5.2 suggested that the absorption of CO2 in rainwater to form a weak solution of carbonic acid would give natural rainwater a pH of about 5.6. We can now consider this exercise quantitatively by using some of the concepts just introduced. 

Let us now calculate the concentration of H30 (aq), HjCOjlaq), HC03(aq), OH (aq), and COf Caq) when CO2 from the atmosphere dissolves in otherwise pure rainwater, given that the solubility of CO2 in water is 1.0x 10 M at 25°C and 1 atm. Since we have five unknowns we need five equations to solve this problem, and so far we have only two equations, namely (5.39) and (5.40). The other three equations are provided by the ion-product constant for water 

The solution of Eqs. (5.39) through (5.43) is simplified if we make some approximations. Since K, the contributions to [H30 (aq)] from Reaction (5.38) is negligible compared to that from Reaction (5.37). Also, since the only source of C03 (aq) is from Reaction (5.38), [C03 (aq)] will be small compared to [H2C03(aq)j and [HC03(aq)j. Finally, since OH (aq) derives only from the dissociation of water, and an acid has been added to the water, we can assume that [H30+(aq)] [OH (aq)]. Hence, from Eq. (5.43), [H30 (aq)] — [HCO (aq)], and Eqs. (5.39) and (5.42) become 

Therefore, rainwater (or any other water) that is exposed only to atmospheric CO2 at 25 C and 1 atm will have a pH of about 5.7. 

Of course, neither rainwater nor any other water is generally exposed to CO2 alone. Even in regions of the globe well removed from sources of anthropogenic pollution, rainwater is exposed to natural SO2 gas and sulfate particles in the air, which can decrease the pH of the rain to below 5.7. In the absence of bases, such as NH3 (the main atmosphere gaseous base) and CaC03 (from soil dusts), which are often in low concentrations in natural air, the pH of rainwater can vary from about 4.5 to 5.6 (with an average value of about 5) due solely to variability in the sulfur content of the air. 

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The pH of rainwater in equUibrium with atmospheric CO2 is 5.6, a value frequendy cited as the natural background pH. However, in the presence of other naturaUy occurring species such as SO2,, NH, organic acids, sea salt, and alkaline cmstal dust, the natural values of unpoUuted rainwater vary... 

The pH of rainwater is normally about 6 but can be reduced significantly by absorption of acidic exhaust gases from power stations, industrial combustion or other processes, and vehicles. Acids may also enter the waterways as a component of industrial effluent. In addition to the direct adverse effects on aquatic systems (Table 16.12) low pH can result in the leaching of toxic metals from land, etc. 

It is often taken for granted that the oxygen content of the air is nearly constant at ca. 20% of the atmospheric volume, that most of the liquid water on the planet is aerobic (i.e. contains O2), and that most water has pH values relatively close to neutral" (close to 7). However, these circumstances are not mere coincidences but are in fact consequences of the interaction of key global biogeochemical cycles. For instance, the pH of rainwater is often determined by the relative amounts of ammonia and sulfuric acid cycled through the atmosphere, a clear example of interaction between the nitrogen and sulfur cycles. 

Because of this reaction, the pH of rainwater tends to be about 5.6. 

Sulfuric acid is a stronger acid than sulfurous [pAa(l) < 0, p7fa(2) = 1.99 at 25 °C and infinite dilution] rain as acidic as pH 2.1 has been recorded at Hubbard Brook, New Hampshire, and the pH of water droplets in clouds can be as low as 1.5 (for comparison, the pH of rainwater saturated with atmospheric CO2 is about 5.6 at 15 °C). Acid rain destroys building materials (especially marble), kills fish and vegetation, accelerates metallic corrosion (Sections 16.5 and 16.7), and can be directly harmful to humans (e.g., it causes the alligator skin condition reported in Cubatao, Brazil). Sulfate rain is not completely without redeeming features, as many soils (e.g., in southern Alberta, Canada) are sulfur-deficient. On balance, however, its acidity is unacceptable, and sulfur oxide emissions must be controlled at the source. Several control measures are possible ... 

Acid rain threatens lakes and forests throughout the world. Monitoring the pH of rainwater is a critical component of programs to measure and reduce the production of acid rain. 

Panel (c) shows typical results for the pH of rainwater. The average of the 17 measurements is given by the horizontal line at pH 4.14 and the letters s, t, u, v, w, x, y, and z identify types of pH electrodes. Types s and w had relatively large systematic errors. The type s electrode was a combination electrode (Figure 15-9) containing a reference electrode liquid junction with an exceptionally large area. Electrode type w had a reference electrode filled with a gel. 

Carbonic acid, as its name implies, behaves as an acid and lowers the pH of water. The C02 in the atmosphere brings the pH of rainwater to about 5.6— noticeably below the neutral pH value of 7. Because of local fluctuations, the normal pH of rainwater varies between 5 and 7. This natural acidity of rainwater may accelerate the erosion of land and, under the right circumstances, can lead to the formation of underground caves, as was discussed in this chapter s introduction. 

Rainwater is naturally acidic since it dissolves carbon dioxide gas from the atmosphere as it falls. Natural rainwater has a pH of about 5.7. In recent years, especially in central Europe, the pH of rainwater has fallen to between pH 3 and pH 4.8. This increase... 

The hydrological cycle involves pH variations. In fact, most natural waters have a pH between 4 and 9 that is controlled mainly by the carbon dioxide-carbonate equilibrium. For instance, as calculated in example 6.4, the pH of rainwater is around 5.7 because of its equilibrium with carbon dioxide, but as it touches the Earth and comes in contact with decomposing organic materials, it may acidify even further. On the other hand, if water comes in contact with alkaline environments, minerals or sediments — mainly carbonates (which upon dissolution in water generate strongly basic media), its pH will rise. 

The measurement of the pH of rainwater at a certain site gave the following data for different samples ... 

For zinc and copper which are metals whose corrosion resistance may be ascribed to a protective layer of basic carbonates and basic sulphates the pH value of rain seems to be of significance. If the pH of rainwater falls to values close to 4 or even lower, as may be seen in a potential- pH diagram for copper in FIG. 7, this may lead to dissolution of the protective coatings. 

Fertilizer production The pH of rainwater is naturally slightly acidic, and some of this acidity is... 

The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. Calculate the H ion concentration of the rainwater. 

A glass of water initially at pH 7.0 is exposed to dry air at sea level at 20°C. Calculate the pH of the water when equilibrium is reached between atmospheric CO2 and CO2 dissolved in the water, given that Henry s law constant for CO2 at 20°C is 0.032 mol/L atm. Hint Assume no loss of water due to evaporation and use Table 17.1 to calculate the partial pressure of CO2. Your answer should correspond roughly to the pH of rainwater.)... 

Show how to calculate the pH of rainwater, assuming equilibrium with CO2 and the absence of other acidic or basic solutes. 

As we have discussed in Chapter 4, acid rains are of great environmental concern in the Southeast Asian sub-region and the pH of rainwater drops often below 4.5 in many countries of question. Over the last 10 years, a number of authors have suggested that forest fires in the tropics may have an acidifying effect on rainwater with potential consequences to tropical ecosystems (see, for instance, Levine J. S., Global Biomass Burning, 1991, in the list of further reading). 

Radoejvic M. and Tan K. S., 2000. Impacts of biomass burning and regional haze on the pH of rainwater in Brunei Darussalam. Atmospheric Environment, 34, 2739-2744. 

Acid deposition interacts with the canopy of forest ecosystems (Lindberg and Lovett 1992), and in this way it may become either more acidic (wash off of accumulated dry deposition) or less acidic (exchanging H" against basic cations). As an example, in the Polish Swietokrzyski National Park the pH of rainwater (5.1) decreased as throughfall to 4.9 in a beech stand, and to... 

The pH of rainwater in Pasadena from February 1976 to September 1977 varied from 2.78 to 5.33. [H ] tends to be log-normally rather than normally distributed(15,16). The geometric mean pH was 45 pM... 

Fig. 3. The pH of Rainwater Measured at Several Sites in New York State over a Period of Years (Source U.S. Geological Survey)...

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