ACIDS & BASES
An acid can be defined as a chemical that increases the concentration of hydrogen ions in
solution. Conversely, we can define a base as a chemical that reduces the concentration of
hydrogen ions in solution (and increases the concentration of hydroxide ions).
Acid-base chemistry is an important part of everyday life. The excess hydrogen ions in acids give
them interesting properties. Acids can react with metals and other materials. The strong acid
HCl is produced in your stomach to help digest food. In dilute concentrations, acids are
responsible for the sour taste of lemons, limes, vinegar and other substances. Bases are also
very reactive. The strong base NaOH is used in many household cleaning agents such as oven
cleaner and drain clog-remover. But how do we measure the concentration of an acid or base?
The acidity (or basicity) of a solution is measured using the pH scale. The pH scale corresponds
to the concentration of hydrogen ions in a solution. In fact, if you take the exponent of the H+
concentration and remove the negative sign, you have the pH of a solution. For example, in
pure water the concentration of hydrogen ions is 1 x 10-7 M. Thus, the pH of a solution of pure
water is 7. The pH scale ranges from 0 to 14, where 7 is considered neutral ([H+] = [OH-]), below
7 acidic and above 7 basic. The further from 7 you are on the pH scale, the more acidic or basic
the solution. For example, a solution with a pH = 1 has a hydrogen ion concentration of 1 x 10 -1
M (0.1 M). The table below further illustrates the relationship between hydrogen ion
concentration and pH
[H+]
pH
[OH-]
Example
1 X 100
0
1 X 10-14
HCl (4%)
-1
1
1 X 10
-13
1 X 10-2
2
1 X 10-12
3
1 X 10
-11
Vinegar
1 X 10
-10
Soda
1 X 10
-9
Rainwater (unpolluted)
-8
Milk
1 X 10
Stomach acid
Lemon juice
1 X 10
-3
1 X 10
-4
1 X 10
-5
1 X 10
-6
6
1 X 10
1 X 10-7
7
1 X 10-7
Pure water
8
1 X 10
-6
Egg whites
1 X 10
-5
Baking Soda
1 X 10
-4
Ammonia
-3
1 X 10
-8
1 X 10
-9
4
5
9
1 X 10
-10
1 X 10
-11
11
1 X 10
1 X 10-12
12
1 X 10-2
13
-1
1 X 10
-13
1 X 10
-14
10
14
1 X 10
1 X 10
0
Drano
NaOH (4%)
Definitions of acidic, basic, and neutral solutions based on [H +]
o
acidic: if [H+] is greater than 1 x 10-7 M
o
basic: if [H+] is less than1 x 10-7 M
o
neutral: if [H+] if equal to 1 x 10-7 M
Example 1: What is the [H+] of a sample of lake water with [OH-] of 1.0 x 10-9 M?
Is the lake acidic, basic, or neutral?
Solution: [H+] = 1 x 10-14 / 1 x 10-9 = 1 x 10-5 M
Therefore the lake is slightly acidic
Remember: the smaller the negative exponent, the larger the number is.
Therefore:
o
acid solutions should have exponents of [H+] from 0 to -6.
o
basic solutions will have exponents of [H+] from -8 on.
Example 2: What is the [H+] of human saliva if its [OH-] is 1 x 10-8 M? Is human
saliva acidic, basic, or neutral?
Solution: [H+] = 1.0 x 10-14 / 1 x 10-8 = 1 x 10-6 M
The saliva is slightly acidic.