a) Acid and Bases

In a discussion of acid rain, it is first necessary to define acids and to understand the way scientists quantify them.  We can define an acid as a substance that contains hydronium ions (H₃O⁺) and a base as a substance that contains hydroxide (OH^-) ions.  Hydronium ions are often abbreviated as hydrogen ions (H⁺).  An acid produces hydronium ions in solution by donating a proton (H⁺) to water.  We can write a general reaction for this process using HA to represent the acid:

HA(aq)  +  H₂O(l)  ®  A^-(aq)  +  H₃O⁺(aq)

or, using hydrochloric acid as an example:    HCl + H₂O ====> H₃O⁺ + Cl^-

In contrast, a base is a substance that produces hydroxide ion (OH^-) by accepting a proton from water.  Using B to represent the base:

                      B(aq)  +  H₂O(l)  ®  BH⁺(aq)  +  OH^-(aq)

Acid and base are opposites, and solutions can be acidic or basic.  When the acids and bases in a solution exactly counterbalance each other, we say a solution is neutral.  This occurs when there are exactly the same number of hydronium and hydroxide ions in an aqueous solution.

Experimentally, we can monitor the process of ion dissociation in acids and bases two different ways: by pH measurement and by titration.

Figure 1. The pH scale

b) Monitoring acids using pH measurement

There is a second way to measure the acid present in a solution, by titration. A titration is a chemical reaction in which an acid sample to be measured is reacted with a base solution of exactly known concentration.  The titration is based on the neutralization reaction that acids and bases undergo.  To analyze an acid, we react it with a solution containing a known concentration of OH^-.  This solution is known as the titrant.  The titrant is added just until the point when all of the acid has been neutralized.  From a precise and accurate measurement of the volume of titrant necessary to neutralize the acid in solution, its concentration can be determined.  An indicator is added to the acid solution before titration begins.  The indicator changes color when all of the acid has been neutralized.  This is known as the endpoint of the titration.  At this point the following relationship should be true.

moles acid in sample = moles base added in titrant

Unlike pH measurement, which measures only the hydronium ions which are free in solution, a titration measures all of the hydrogen ions that can be removed from the acid by base in aqueous solution.

The differences between strong and weak acids are most notable when one compares the pH of solutions of strong and weak acids with the total acid concentration found by titration.  Since pH is a measure of only the fraction of the acid that has dissociated to hydronium ions, the pH of a weak acid will be higher (less acidic) than the pH of a comparable molar concentration of strong acid.  One way to identify the differences between strong and weak acids is to compute the ratio of free hydronium ions (calculated using pH measurements) to the total amount of acid available for titration with base.  Strong acids give a ratio of about 0.5 to 1.0 since all of the acid present has liberated hydrogen ions into the solution.  Weak acids give very small ratios, typically less than 0.10, since fewer hydrogen ions are produced.

Let us look at the chemistry of rain.  When water is exposed to the air, substances in the air dissolve in the water.  Carbon dioxide is always present in air.  It dissolves by a series of reactions to form carbonic acid , H₂CO₃ (the acid that makes soda fizzy).  The acid which is naturally present in rain is carbonic acid.

CO_2(g) + H₂O_(l) <===> H₂CO_3(aq) + H₂O<===> HCO₃^-_(aq)  +  H₃O⁺

These reactions are all chemical equilibria, which means they can proceed in both directions depending on the quantities of each chemical present and on the temperature and pressure.  Carbonic acid is a weak acid, even weaker than acetic acid.  Carbonic acid produces few hydrogen ions in water.  Only about 0.3% of this acid is dissociated, so it is about 10 times weaker than acetic acid.  Therefore, even large concentrations of carbonic acid would not produce rain of very low pH.  With the current levels of carbon dioxide in the atmosphere (about 360 ppm in 1997), water naturally has a pH of about 5.6, which corresponds to 2.3x10^-6 M/l hydrogen ions.  Thus, rain is naturally slightly acidic.  This slight acidity is a natural part of the transport of carbon in the environment.  Based on ice core data, we know that the concentration of carbon dioxide in the atmosphere has varied from a low of 280 ppm to the present high of 360 ppm over the past 100,000 years.  During this time, the natural pH of rain has remained between 5.7 and 5.6 and all Earth’s ecosystems are adapted to this value.

The acids that are present in rain due to the activities of people are mostly strong acids, such as sulfuric and nitric acids.  Even small amounts of these acids can have a very dramatic effect on the pH and the conductivity of rain.  These contribute the most to acid rain.

Acidity due to sulfur

Both oil and coal contain sulfur. In coal, the sulfur is primarily due to the presence of the mineral pyrite, FeS₂. When the coal is burned in the presence of oxygen, pyrite reacts according to the following equation:

4 FeS₂  + 11 O₂  <===> 2 Fe₂O₃  +  8 SO₂

Just as iron may be present in coal as a sulfide, mineral, pyrite, other metal ores are also metal sulfides.  The most notable one is NiS, the principle ore used in the production of nickel for stainless steel.  Nickel ores are “roasted”, heated in air, which releases the sulfur as sulfur dioxide and converts the ore to an oxide.

            2 NiS + 2 O₂ ====> 2 NiO + 2 SO₂

Sulfur is also present in all petroleum products due to organic compounds with a mercapto group (-SH) attached and in rings containing sulfur called thiophene.  These also produce SO₂ when burned.  Natural gas also contains varying amounts of sulfur in the form of H₂S, however, this is removed during processing since it is both toxic and terribly smelly.

 The actual amount of sulfur in fuels varies considerably from much less than 1% to as much as 4-5% depending on the source, but sulfur is always present.  It is the fossil remnant of the sulfur in the living organisms that were converted to fuel by geological processes.

 he presence of dust particles containing metal oxides or carbonates along with sulfur dioxide can be beneficial by reducing the acidity of the resulting acid precipitation.  Some of the sulfuric acid can be neutralized due to the presence of amines and calcium or magnesium carbonate dust in the air.  The total sulfate ion concentration in precipitation can, therefore, be greater than the sulfuric acid present due to reactions with dust from agricultural land, construction sites, and unpaved roads.

Acidity due to nitrogen oxides

The other source of acidic compounds in rain is also related to burning fuels.  All combustion processes that use air as a source of oxygen produce pollutants referred to as NO_x (read this as the three letters, "N" "O" "X").  This is a generic term that includes a number of nitrogen oxides such as NO, NO₂, and N₂O₄.  If you look across any city on a clear day and see a yellow-brown haze in the air, NO_x pollution is the cause.  The nitrogen oxides are produced through a complex series of reactions that begin with the formation of NO and NO₂, which happens whenever things are burned in air.

N₂  + O₂  + heat  <====>  2NO^.·[2]

This nitric oxide radical (2NO^.) can react in a number of ways.  It reacts with oxygen as follows:

2NO^.  +  O₂  <===>  2 NO₂

The bottom line to all of these reactions is that every nitrogen oxide emitted as a pollutant can ultimately lead to the release of a hydrogen ion in the rain.

In addition to their contribution to acid rain, nitrogen oxide pollutants also contribute to the production of urban smog and ozone.  We are living with a decision made in the 1970 Clean Air act to try to reduce urban smog by reducing hydrocarbon emissions.  The recipe for smog is simple.

NO_x + hydrocarbons + sunlight  ===>  smog + ozone