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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.

    1. milk of magnesia, pH = 10.5
    2. pure water, pH = 7
    3. wine, pH = 3.0

    Solution

    1. With a pH greater than 7, milk of magnesia is basic. (Milk of magnesia is largely Mg(OH)2.)
    2. Pure water, with a pH of 7, is neutral.
    3. 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.

    1. human blood, pH = 7.4
    2. household ammonia, pH = 11.0
    3. cherries, pH = 3.6

    Answers

    1. basic
    2. basic
    3. 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.

    Table \(\PageIndex{1}\) Typical pH Values of Various Substances*
    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.