Acids and Bases
You should already be familiar with pH, which is the scale we use to measure the acidity or alkalinity of water. You will remember that the pH scale runs from 0 to 14, with numbers less than 7 being acidic and with numbers more than 7 being basic (also known as alkaline.) A pH of 7 means that the substance is neutral.
Titration, which we will introduce in a later lesson, is based on this neutralization reaction. You will also need to understand neutralization if you spill an acid or base in lab and want to clean it up safely. Neutralizing an acid with a base (or vice versa) can aid in cleanup, but you should also be aware that strong acids and bases can react explosively. Always use weak or low concentration acids or bases for neutralization reactions.
Neutralization occurs in nature as well. For example, organisms living in very acidic environments tend to excrete basic wastes which bring the environment back into equilibrium. Organisms living in basic environments excrete acids instead. This is one of the reasons that most natural waters, including septic tanks and wastewater treatment ponds which have been allowed to work for some time, tend to have a pH near 7. In addition, buffers (which we will explain in the next section) neutralize acids and bases both in natural waters and in the laboratory.
A buffer is a solution containing a weak acid and one of its salts or a weak base and one of its salts. This solution is able to neutralize acids and bases without allowing the pH of the solution to change greatly. In lab, buffers are used when the pH of a solution must remain stable.
Some examples of the pairs which make up buffer solutions are shown in the table below.
Acid or Base
In order for a buffer to "resist" the effect of adding strong acids or bases, it must have both an acidic and a basic component. However, you cannot mix any two acid/base combination together and get a buffer. If you mix HCl and NaOH, for example, you will simply neutralize the acid with the base and obtain a neutral salt, not a buffer. For a buffer to work, both the acid and the base component must be part of the same equilibrium system - that way, neutralizing one or the other component (by adding strong acid or base) will transform it into the other component, and maintain the buffer mixture. Therefore, a buffer must consist of a mixture of a weak conjugate acid-base pair.
Of course, a buffer will not continue to neutralize the solution indefinitely. Eventually, the acid or salt will be used up, and the pH of the solution will begin to change. The amount of acid or base which a buffer solution is able to neutralize is known as the buffer capacity.
Buffer solutions are not limited to the lab. In natural water systems, carbon dioxide from the air often enters the water, forming carbonic acid. A salt of carbonic acid, such as calcium carbonate (limestone), may become dissolved in the water from the surrounding rocks and soil. Thus, a natural buffer solution is formed.
Part 3: Normality