Lesson 7:
Acids and Bases



 

pH

Introduction

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. 

pH scale



Although this elementary understanding of the pH scale is enough for many water and wastewater treatment processes, you will need to know more in order to fully understand the chemistry of acids and bases.  pH is actually a measure of the concentration of hydrogen ions in the solution.  Since acids donate hydrogen ions to a solution, they tend to make the pH lower.  Bases, by accepting hydrogen ions, make the pH higher.  A neutral pH of 7, which is the pH of distilled water, contains the same number of hydrogen ions as hydroxide ions. 

The math used to calculate a solution's pH is relatively complicated, and we will not present it here.  Instead, you just need to remember that the lower the pH value, the higher the concentration of hydrogen ions.  For example, stomach acid with a pH of 1 has a much higher concentration of hydrogen ions than tomatoes do with a pH of 4.  Baking soda with a pH of 8 has more hydrogen ions than household ammonia with a pH of 11. 

You shouldn't find it surprising that pH is a measure of the hydrogen ion concentration of a solution, since we have already explained that acids and bases change that concentration.  What you may not realize is that the pH scale only covers a small range of acidity and alkalinity.  In fact, the pH scale is meant to mimic nature by covering the acidity and alkalinity values which might be found in natural waters.  Strong acids and bases, like those discussed on the last page, have pH values at the far ends of the scale or even off the scale.  Concentrated hydrochloric acid has a pH of 0, and one drop of 33% hydrochloric acid in a liter of distilled water can lower the pH from neutral to about 3.  We will discuss ways to measure the concentration of strong acids and bases in a later section.



Neutralization

A neutral pH of 7 may mean that you are dealing with distilled water containing no acids and bases.  In this case, the amount of hydrogen ions and hydroxide ions will be equivalent because they will both be due to the ionization of water.  However, a neutral pH can also be achieved in a solution containing acids and bases as long as the acids and bases have neutralized each other, meaning that the acids have donated as many hydrogen ions as have been accepted by the bases. 

Neutralization reactions occur whenever acids and bases are placed in proximity.  An acid combines with a base to create water and a salt, as shown below:

HCl + NaOH yields H2O + NaCl


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. 


 


Buffers

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
Salt
Acetic acid
Sodium acetate
Phosphoric acid
Potassium phosphate
Oxalic acid
Lithium oxalate
Carbonic acid
Sodium carbonate
Ammonium hydroxide
Ammonium nitrate

 

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