Reactivity 3.1.9 — The pOH Scale

The pOH scale provides a logarithmic measure of the hydroxide ion concentration, [OH⁻], in aqueous solutions. It is complementary to the pH scale and offers a convenient way to describe the basicity of a solution.

Definition of pOH:

• The pOH of a solution is defined as the negative base-10 logarithm of the hydroxide ion concentration:
• \( \text{pOH} = -\log_{10} [\text{OH}^-] \)
• Conversely, if pOH is known, the [OH⁻] can be calculated as:
• \( [\text{OH}^-] = 10^{-\text{pOH}} \)

Key Characteristics of the pOH Scale:

• The pOH scale typically ranges from 0 to 14, similar to the pH scale.
• Low pOH values (close to 0) correspond to high hydroxide ion concentrations — these solutions are strongly basic.
• High pOH values (close to 14) indicate very low hydroxide ion concentrations — these solutions are acidic or near neutral.
• A pOH of 7 corresponds to a neutral solution at 25°C, where [OH⁻] = \( 1.0 \times 10^{-7} \, \text{mol dm}^{-3} \).

Relationship with Ion Concentration:

• The pOH value is directly related to the concentration of hydroxide ions, which increase as the solution becomes more basic.
• Because it is logarithmic, a change of 1 unit in pOH represents a 10-fold change in [OH⁻].

Why Use the pOH Scale?

• In strongly basic solutions, the concentration of [OH⁻] is high, and pH values may approach 14. Using pOH offers a more intuitive way to express how basic a solution is.
• It complements the pH scale and simplifies calculations in aqueous acid–base equilibria, especially when [OH⁻] is the directly measured or given quantity.

Interconverting [H⁺], [OH⁻], pH, and pOH  

Acid–base calculations often involve converting between concentration of hydrogen ions \([ \text{H}^+ ]\), hydroxide ions \([ \text{OH}^- ]\), pH, and pOH. These values are mathematically related, and using logarithmic functions allows us to switch between them easily.

Key Relationships:

• \( \text{pH} = -\log_{10} [\text{H}^+] \)

Converts hydrogen ion concentration to the pH scale. A higher \([ \text{H}^+ ]\) means a lower pH (more acidic).

• \( \text{pOH} = -\log_{10} [\text{OH}^-] \)

Converts hydroxide ion concentration to the pOH scale. A higher \([ \text{OH}^- ]\) means a lower pOH (more basic).

• \( [\text{H}^+] = 10^{-\text{pH}} \)

Rearranged form of the first equation to find hydrogen ion concentration from pH.

• \( [\text{OH}^-] = 10^{-\text{pOH}} \)

Rearranged form of the second equation to find hydroxide ion concentration from pOH.

• \( \text{pH} + \text{pOH} = 14 \) (valid only at 25°C)

Used to find either pH or pOH if one is known. Only accurate at 25°C, as \( K_w \) is temperature-dependent.

• \( [\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14} \) (from \( K_w \))

This equation, derived from the ionic product of water, allows interconversion of \([ \text{H}^+ ]\) and \([ \text{OH}^- ]\).

Step-by-Step Conversion Guide:

Important Notes:

• The value of pOH = 7 corresponds to neutrality only at 25°C, because it is based on the ion product of water, \( K_w = 1.0 \times 10^{-14} \).
• At different temperatures, the value of \( K_w \) changes, which affects the neutrality point on both the pH and pOH scales.
• Use at least 3 significant figures for all calculations unless stated otherwise.
• Always check the final units and ensure concentrations are in mol dm⁻³ before using logarithmic functions.