8.7: The pH Scale
Learning Objectives
- Define pH.
- Determine the pH of acidic and basic solutions.
As we have seen, [H+] and [OH−] values can be markedly different from one aqueous solution to another. So chemists defined a new scale that succinctly indicates the concentrations of either of these two ions.
pH is commonly defined as a logarithmic function of [H+]:
[latex]\text{pH} = −\log [\ce{H^{+}}] \label{1}[/latex]
pH for aqueous solutions is usually (but not always) between 0 and 14. Knowing the dependence of pH on [H+], we can summarize as follows:
- If pH < 7, then the solution is acidic.
- If pH = 7, then the solution is neutral.
- If pH > 7, then the solution is basic.
This is known as the pH scale and is the range of values from 0 to 14 that describes the acidity or basicity of a solution. You can use pH to quickly determine whether a given aqueous solution is acidic, basic, or neutral.
Example 8.7.1
Label each solution as acidic, basic, or neutral based only on the stated pH.
a. milk of magnesia, pH = 10.5
b. pure water, pH = 7
c. wine, pH = 3.0
Solution
a. With a pH greater than 7, milk of magnesia is basic. (Milk of magnesia is largely Mg(OH)2.)
b. Pure water, with a pH of 7, is neutral.
c. With a pH of less than 7, wine is acidic.
Exercise 8.7.1
Table 8.7.1 gives the typical pH values of some common substances. Note that several food items are on the list, and most of them are acidic.
Table 8.7.1 Typical pH Values of Various Substances*
| Substance | pH |
| stomach acid | 1.7 |
| lemon juice | 2.2 |
| vinegar | 2.5 |
| soda | 3.0 |
| wine | 3.5 |
| coffee (black) | 5.0 |
| milk | 6.9 |
| pure water | 7.0 |
| blood | 7.4 |
| seawater | 8.5 |
| milk of magnesia | 10.5 |
| ammonia solution | 12.5 |
| 1.0 M NaOH | 14.0 |
| *Actual values may vary depending on conditions | |
pH is a logarithmic scale. A solution that has a pH of 1.0 has 10 times the [H+] as a solution with a pH of 2.0, which in turn has 10 times the [H+] as a solution with a pH of 3.0 and so forth.
Using the definition of pH, it is also possible to calculate [H+] (and [OH−]) from pH and vice versa. The general formula for determining [H+] from pH is as follows:
[latex]\ce{[H^{+}]} = 10^{−\text{pH}}[/latex]
Key Takeaways
-
pH is a logarithmic function of [H+].
- [H+] can be calculated directly from pH.
- [OH–] can be calculated from pH by first finding [H+], then using Kw.
