7.6: The pH and pOH Scales
- Page ID
- 221512
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 a logarithmic function of [H+] (or [H3O+])
pH = −log[H+] or pH = -log [H3O+]
pH 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.
Since in solution, [H+] =[H3O+], we can used both expression indistinctly. For simplicity, we will use [H+].
Example \(\PageIndex{1}\):
Label each solution as acidic, basic, or neutral based only on the stated pH.
- milk of magnesia, pH = 10.5
- pure water, pH = 7
- wine, pH = 3.0
Solution
- With a pH greater than 7, milk of magnesia is basic. (Milk of magnesia is largely Mg(OH)2.)
- Pure water, with a pH of 7, is neutral.
- With a pH of less than 7, wine is acidic.
Exercise \(\PageIndex{1}\)
Identify each substance as acidic, basic, or neutral based only on the stated pH.
- human blood, pH = 7.4
- household ammonia, pH = 11.0
- cherries, pH = 3.6
Answers
- basic
- basic
- acidic
Table \(\PageIndex{1}\) gives the typical pH values of some common substances. Note that several food items are on the list, and most of them are acidic.
Substance | pH |
---|---|
stomach acid | 1.7 |
lemon juice | 2.2 |
vinegar | 2.9 |
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:
[H+] = 10−pH
The pOH scale
In the same way that we used [H+] to define pH, alternatively, we could use [OH−] to define pOH:
pH = −log [OH−]
and
[OH−] = 10−pOH
The pOH scale also ranges from 0 to 14 values, and it can be used to describe the acidity or basicity of a solution. However, pOH scale is not commonly used and we will always use pH to quickly determine whether a given aqueous solution is acidic, basic, or neutral.
Key Takeaways
- pH is a logarithmic function of [H+].
- [H+] can be calculated directly from pH.
- pOH is related to pH and can be easily calculated from pH.