Rain, unpolluted

Unpolluted rain water has a pH of about 5.5. Acid rain has been shown to have a pH as low as 3.0. Calculate the [H+] ratio of acid rain to unpolluted rain. 

Acid rain. Natural (unpolluted) precipitation is naturally acidic with a pH often in the range of 5 to 6 caused by carbonic acid from dissolved carbon dioxide and sulfurous and sulfuric acids from natural emissions of SO and H2S. Human activity can reduce the pH very significantly down to the range 2 to 4 in extreme cases, mainly caused by emissions of oxides of sulfur. Because atmospheric pollution and clouds travel over long distances, acid rain is not a local problem. The problem may manifest itself a long way from the source. Problems associated with acid rain include ... 

As regards the pollutants monitoring, from the measurements available so far it could be concluded that acid rain is coming to be a major problem in Asia. In many industrially developed and new developed countries such as Japan, China, Taiwan, South Korea, Thailand etc., values of pH <5 are encountered at many sites, and they represent more than 50% of monitored rain events on a regional scale. In some developing countries of South-East Asia (Myanmar, Laos, Cambodia) most rainwater pH measurements tend to be around 5.6, the pH of natural rainwater, and the acid rain precipitation is mainly due to localized industrial pollution. There is some evidence that pH values below 5 at unpolluted sites may be due to the contribution of weak organic acids, such as formic and acetic acids (Radojevic, 1998). 

Acid Hydrolysis. The water that enters soil as rain or snow is in equilibrium with CO2 in the atmosphere, which dissolves to form carbonic acid. Unpolluted rainwater has a pH of approximately 5.7, whereas water in soil pores may be exposed to air containing a higher partial pressure of CO2 than the free atmosphere, and hence soil water may be more acidic (see Section 5.4). It is the attack on soil minerals by this weak carbonic acid that is the major chemical weathering process in most soils. For example, acid hydrolysis of calcium carbonate yields calcium and bicarbonate ions ... 

The atmosphere is a major source of soil acidity. Even in unpolluted environments rainwater is slightly acidic, having a pH of about 5.7 due to the dissolution of atmospheric CO2 to form the weak carbonic acid (see Worked example 5.4). The CO2 concentration in the partially enclosed soil pore system can be significantly higher (typically up to about 10 times) than in the free atmosphere due to respiration of soil microorganisms and plant roots. This results in a lower pH. In areas affected by industrial pollution, sulfur dioxide and nitrogen oxides dissolve in rainwater to produce sulfuric and nitric acids (acid rain), which are both strong acids and cause even more acidity. 

If you collect rainwater in a relatively unpolluted area, you will discover that the rainwater is essentially a nonconductor of electricity. A small concentration of carbonic acid from the carbon dioxide in the air added to the rainwater causes the rain water to be a weak conductor. Pure rainwater conducts almost as poorly as distilled water. However, most of the water we use comes from wells, lakes, or rivers. This water has been in contact with soil and rocks, which contain ionic compounds that dissolve in the water. Consequently, tap water conducts electricity. The conduction is not high, but the water can conduct enough current to stop a person s heart. So, for example, a person should not use an electrical appliance when in the bathtub or shower. 

Consequently, an intensive uptake of sulfur occurs in hydrosphere, lithosphere, and biosphere. This is the main reason that the content of gaseous sulfur species in the atmosphere is rather small and even in polluted air does not exceed 2-3 ppmv. In unpolluted atmosphere the concentration of most S compounds is at ppbv levels, despite the intense sulfuroutgassing from the Earth s interior. The atmospheric content of the major gaseous S species, either SO2 or H2S, is highly variable and is influenced by both natural and anthropogenic factors. The role of anthropogenic sulfur emission in acid rain chemistry will be discussed in Chapter 10. The influence of natural parameters, microbiological activity in particular, is described in Box 7. 

Calcium carbonate has intrinsically low solubility in water, Ksp = 6 x 10 (mol/L) at 25°C. However, because the carbonate anion is basic, CaCOs becomes increasingly soluble as the pH drops. It is known that even unpolluted rain water has pH 5.6 and the rainwater is slightly acidic. The reaction may be summarized as... 

Unpolluted rain is not harmful. However, many industrial and power plants burn coal and oU. The smoke produced may contain large quantities of sulfur oxides, suspended particles, and nitrogen oxides. Automobiles also contribute to the problem by emitting similar oxides. These chemicals react with water in the air to form acids, such as sulfuric acid. These acids reach the surface of Earth in fog, rain, snow, and dew. Acid rain can have a disastrous effect when it reaches bodies of water and waterways. But if a lake has a high limestone content, it is able to somewhat neutralize the acid. 

We should stress here that even completely unpolluted rainwater does not have a pH of 7.0, because it is not pure distilled deionizing water and it contains equilibrium amounts of atmospheric gases, namely, carbon dioxide at an average present concentration of 365 ppbv. Acid rain is generally defined as having pH lower than 5.6. Low acidic rainwater has a pH between 5.6-5.0, acidic rainwater, 5.0-4.5, strong acidic rainwater, 4.5-3.5, and extremely acidic, < 3.5. 

PROBABLE FATE photolysis-, no data for rate of photolysis in aquatic environment oxidation-, in aquatic systems not expected to be important fate, photooxidation in troposphere is probably the predominant fate hydrolysis expected to be slow, neutral aqueous hydrolysis half-life 25 °C >50 years, first-order hydrolysis half-life 37 years pH 7 volatUiz/ttion primary transport process, volatilization from soil will occur biological processes NA evaporation from water 25 °C of 1 ppm solution is 50% after 21 min and 90% after 102 min release to water primarily through evaporation (half-life days to weeks) rate of evaporation half-life from water 21 min photodegrades slowly by reaction with hydroxyl radicals, half-life 24-50 days in polluted atmosphere to a few days in unpolluted atmospheres will be removed in rain... 

Scientists discover acid rain in North America. pH measurements show polluted rain to be 100 times more acidic than unpolluted rain. 

Carbon dioxide in the atmosphere dissolves in raindrops to produce carbonic acid (H2CO3), causing the pH of clean, unpolluted rain to range fiom about 5.2 to 5.6. What are the ranges of [H ] and [OH ] in the raindrops ... 

Atmospheric CO2 levels have risen by nearly 20% over the past 40 years from 315 ppm to 380 ppm. (a) Given that the average pH of clean, unpolluted rain today is 5.4, determine the pH of unpolluted rain 40 years ago. Assmne that carbonic acid (H2CO3) formed by the reaction of CO2 and water is the only factor influencing pH. 

Precipitation incident to a plant canopy can be relatively well characterized chemically. Table I shows the composition of "unpolluted rain, based on analytical data for each of the constituents in precipitation and assuming equilibrium with atmospheric concentrations of carbon dioxide.In this system, the solution pH would be expected to be approximately pH 5.65. 

Unpolluted air Concept of what the air would be like if there were no anthropogenic sources. This is no more than a concept, because the act of measuring air requires the presence of people and their equipment even at remote locations at sea, the poles, deserts, or mountains, where the air can be best described as dilute polluted air (see Tables I, II, and III). The suspended particulate matter concentration of unpolluted air is quite variable depending on whether it has been recently loaded by natural events such as volcanic emp-tions or recently cleansed by rain- or snowfall. 

Traditionally rain with a pH of less than 5.6 has been considered acidic. However, a number of studies show evidence of unpolluted rainfall having a pH in the range 4.5 to 7.4. 

Acid rain is defined as any atmospheric precipitation that is more acidic than usual. The increase in acidity might be from natural or industrial sources. Rain acidity varies throughout the world and across the United States. The pH of rain is generally lower in the eastern United States and higher in the West. Unpolluted rain has a pH of 5.6, and so is slightly acidic. This acidity results from the dissolution of carbon dioxide in the water producing carbonic acid ... 

A site in Pennsylvania receives a total annual deposition of 2.688 g/m of sulfate from fertilizer and acid rain. The ratio by mass of ammonium sulfate to ammonium bisulfate to sulfuric acid in the deposited material is 3.0/5.5/1.0. (a) How much acid, expressed as kg of sulfuric acid, is deposited over an area of 10. km 7 (b) How many pounds of CaC03 are needed to neutralize this acid (c) If 10. km is the area of an unpolluted lake 3 m deep and there is no loss of acid, what pH would the lake s water have at the end of the year (Assume constant volume and negligible runoff from the surrounding land.)... 

How acidic is acid rain We might expect pure, unpolluted rain to have a pH of 7.0, but it does not. Rain is naturally slightly acidic because of the presence of carbon dioxide in the atmosphere. Carbon dioxide combines with water to form carbonic acid, a weak acid ... 

Why is rain acidic even in the absence of pollutants How acidic is unpolluted rain ... 

Pure unpolluted rain Is acidic due to the presence of dissolved carbon dioxide which is in equilibrium with hydrogen and hydrogencarbonate ions. Acid rain is more acidic (lower pH) than carbonic acid due to the presence of sulfurous and sulfuric acids, which are stronger acids than carbonic acid. 

Free-radical chain reactions also occur during the chlorination of methane (Chapter 10) and of the methyl group of methylbenzene. Ozone depletion by chlorofluorocarbons (CFCs), acid rain formation and formation of photochemical smog (Chapter 25 on the accompanying website) also involve free-radical reactions. (Free-radical reactions are also operating in unpolluted atmospheres and play an important role in all chemical reactions that occur in the gas phase.) The combustion of hydrocarbons, such as petrol, also proceeds via a free-radical mechanism, which has important consequences for the smooth running and performance of combustion engines. Chain reactions may also have ions as intermediates, as opposed to free radicals. 

The reaction between CO2 and H2O explains why pure water gradually becomes acidic when it is exposed to air, which contains CO2. The pH of rainwater exposed only to unpolluted air is about 5.5. The reaction between SO3 and H2O is largely responsible for acid rain. 

As was noted at the beginning of the chapter, rain in unpolluted areas is slightly acidic because it contains carbonic acid (H2CO3). Carbonic acid is a weak, diprotic acid that ionizes to give and the hydrogen carbonate ion ... 

Even unpolluted rain can contain both acids and bases. For example, carbon dioxide naturally present in air dissolves in rain to form carbonic acid (H2CO3), the weak acid responsible for the fizz in soda pop. If there were no other substances in rain, CO2 would lower the acidity from neutral (pH 7.0) to weakly acidic (pH 5.6). Unpolluted rain also contains small amounts of acid sulfate and acid nitrate that are produced in the stratosphere. These acids would further lower the natural pH of rain to about 5.4. Several alkaline substances partly neutralize the acidity in rain Ammonia, which is naturally present in the air, dissolves in rain to form ammonium salts. Soil dust containing alkaline minerals such as limestone (calcium carbonate) and dolomite (magnesium carbonate) can dissolve in rain and raise its pH. Fly ash from coal combustion contains calcium and magnesium oxides, which also are alkaline. The pH of rain therefore reflects the competing influence of several different substances. 

The Earth s atmosphere contains about 0.03% by volume of CO2, and the equilibrium in Equation 10.2 means that rain falling through even unpolluted skies will be slightly acidic. CO2 is produced by respiration of plants and animals, but is used by plants in the photosynthesis of carbohydrates. The fossilization of plants in the Carboniferous Period (354-290 million years ago) was responsible for the production of coal. Marine organisms use forms of calcium carbonate (calcite and aragonite) in their exoskeletons. An example of the role of this biomineral in protection is shown in Figure 10.15. Some of these organisms live in coral reefs, but others swim about in the sea, and when they die their shells fall to the sea bed. 

---