How is acid rain measured

However, we are becoming aware of an additional concern: many of our historic buildings and monuments are located in the areas of highest acidity. In Europe, where buildings are much older and pollution levels have been ten times greater than in the United States, there is a growing awareness that pollution and acid rain are accelerating the deterioration of buildings and monuments. Stone weathers deteriorates as part of the normal geologic cycle through natural chemical, physical, and biological processes when it is exposed to the environment.

This weathering process, over hundreds of millions of years, turned the Appalachian Mountains from towering peaks as high as the Rockies to the rounded knobs we see today. Our concern is that air pollution, particularly in urban areas, may be accelerating the normal, natural rate of stone deterioration, so that we may prematurely lose buildings and sculptures of historic or cultural value. This religious medieval sculpture, made of sandstone, has been degraded by the acidification of air and rains.

Many buildings and monuments are made of stone, and many buildings use stone for decorative trim. Granite is now the most widely used stone for buildings, monuments, and bridges.

Limestone is the second most used building stone. It was widely used before Portland cement became available in the early 19th century because of its uniform color and texture and because it could be easily carved. Sandstone from local sources was commonly used in the Northeastern United States, especially before Nationwide, marble is used much less often than the other stone types, but it has been used for many buildings and monuments of historical significance.

Because of their composition, some stones are more likely to be damaged by acidic deposition than others. Granite is primarily composed of silicate minerals, like feldspar and quartz, which are resistant to acid attack. Sandstone is also primarily composed of silica and is thus resistant. A few sandstones are less resistant because they contain a carbonate cement that dissolves readily in weak acid. Limestone and marble are primarily composed of the mineral calcite calcium carbonate , which dissolves readily in weak acid; in fact, this characteristic is often used to identify the mineral calcite.

Acid precipitation affects stone primarily in two ways: dissolution and alteration. When sulfurous, sulfuric, and nitric acids in polluted air react with the calcite in marble and limestone, the calcite dissolves.

In exposed areas of buildings and statues, we see roughened surfaces, removal of material, and loss of carved details. Stone surface material may be lost all over or only in spots that are more reactive. You might expect that sheltered areas of stone buildings and monuments would not be affected by acid precipitation.

However, sheltered areas on limestone and marble buildings and monuments show blackened crusts that have spalled peeled off in some places, revealing crumbling stone beneath.

This black crust is primarily composed of gypsum, a mineral that forms from the reaction between calcite, water, and sulfuric acid. Gypsum is soluble in water ; although it can form anywhere on carbonate stone surfaces that are exposed to sulfur dioxide gas SO 2 , it is usually washed away. It remains only on protected surfaces that are not directly washed by the rain.

Gypsum is white, but the crystals form networks that trap particles of dirt and pollutants, so the crust looks black. Eventually the black crusts blister and spall off, revealing crumbling stone. When rainwater is too acidic, it can cause problems ranging from killing freshwater fish and damaging crops, to eroding buildings and monuments.

Write a balanced chemical equation for the dissociation of nitric acid in water. The gaseous oxides found in the atmosphere, including CO 2 and NO are nonmetal oxides. What would happen to the pH of rainwater if the atmosphere contained metal oxides instead?

Briefly, explain your answer. What causes such a dramatic increase in the acidity of rain relative to pure water? The answer lies within the concentrations of nitric oxide and sulfur dioxide in polluted air. As shown in Table II and Figure 1, the concentrations of these oxides are much higher than in clean air. Humans cause many combustion processes that dramatically increase the concentrations of acid-producing oxides in the atmosphere. Thus, a large increase in the concentration of NO and SO 2 significantly affects the pH of rainwater, even though both gases are present at much lower concentration than CO 2.

About one-fourth of the acidity of rain is accounted for by nitric acid HNO 3. In addition to the natural processes that form small amounts of nitric acid in rainwater, high-temperature air combustion, such as occurs in car engines and power plants, produces large amounts of NO gas. This gas then forms nitric acid via Equations 4 and 5. Thus, a process that occurs naturally at levels tolerable by the environment can harm the environment when human activity causes the process e.

Most is accounted for by the presence of sulfuric acid H 2 SO 4 in rainwater. Although sulfuric acid may be produced naturally in small quantities from biological decay and volcanic activity Figure 1 , it is produced almost entirely by human activity, especially the combustion of sulfur-containing fossil fuels in power plants. When these fossil fuels are burned, the sulfur contained in them reacts with oxygen from the air to form sulfur dioxide SO 2.

The effects of burning fossil fuels can be dramatic: in contrast to the unpolluted atmospheric SO 2 concentration of 0 to 0. Sulfur dioxide, like the oxides of carbon and nitrogen, reacts with water to form sulfuric acid Equation 6.

At sea level and 25 o C, one mole of air fills a volume of Compute the mole fraction i. Typical acid rain has a pH value of 4. A decrease in pH values from 5. There are many high-tech devices that are used to measure pH in laboratories. There are several ways to test the pH of something, the simplest is by using litmus paper. Litmus paper is a special kind of paper that will change color depending on what kind of substance you put it in.

With all of this said, one easy way to test the acidity of rain is to collect some in a clean plastic container. This could be done by leaving a container outside next time it rains. You have to be careful to place the bucket at least 2 meters above the ground to avoid contamination from dust which could alter the pH of the collect rain.

After the rain has stopped, you could dip some litmus paper in the glass of rain and by comparing the color change in the paper to a corresponding chart, it would tell you the pH of the rain!

Now if you find all of this as interesting as I do, you could get more accurate results by using a pH meter, although these cost more money. To know if a sample of rain is "acid rain" we need to measure the concentration of hydrogen ions in the sample. Hydrogen ions are produced when any acidic substance is dissolved in water. The solutions of some substances, like carbon dioxide in the air, are weakly acidic.

When they are dissolved in water there is not a large increase in the concentration of hydrogen ions. Other substances give strong acids when dissolved and they cause a large increase the concentration of hydrogen ions.

Examples of these substances are sulfur dioxide and various types of nitrogen oxide. These are the main generators of acid rain. They come mostly from the emissions of energy plants that burn fossil fuels to produce electricity. Back to your question, how do we measure the concentration of hydrogen ions?

In a quick way, by using one of the many substances that change color depending on the concentration of hydrogen ions.