What is the relationship between pH and pOH?

Short Answer

The relationship between pH and pOH is that both values always add up to 14 at 25°C. This means:
pH + pOH = 14.
If the pH of a solution increases, the pOH decreases, and if pH decreases, pOH increases. This relationship comes from the ionisation of water, where the product of hydrogen ions (H⁺) and hydroxide ions (OH⁻) is constant.

This link helps us calculate one value when the other is known. It also helps us understand whether a solution is acidic, basic, or neutral.

Detailed Explanation :

Relationship Between pH and pOH

The relationship between pH and pOH is one of the fundamental concepts in acid–base chemistry. Both pH and pOH are scales used to measure the acidity and basicity of a solution. They are mathematically connected through the ionisation of water. Understanding this relationship allows scientists, students, and industries to determine the nature of a solution and how it will behave in chemical reactions.

The connection comes from the fact that water always ionises to form hydrogen ions (H⁺) and hydroxide ions (OH⁻), and their concentrations are linked. Because of this, pH and pOH are also linked and form a complete picture of the acid–base balance of any aqueous solution.

The Water Ionisation Concept

Water undergoes slight ionisation:

H₂O ⇌ H⁺ + OH⁻

Although the ionisation is small, it is enough to create a balance. At 25°C:

[H⁺] × [OH⁻] = 1 × 10⁻¹⁴

This constant is known as Kw, the ionic product of water.

Because Kw is constant at 25°C, whenever [H⁺] increases, [OH⁻] must decrease, and vice versa. This inverse relationship forms the basis of the pH–pOH equation.

Mathematical Relationship: pH + pOH = 14

By applying logarithms to Kw, we get:

pH = –log [H⁺]
pOH = –log [OH⁻]

Adding the two equations:

pH + pOH = –log [H⁺] – log [OH⁻]
= –log ([H⁺] × [OH⁻])
= –log (1 × 10⁻¹⁴)
= 14

Therefore:

pH + pOH = 14 (at 25°C)

This formula is extremely useful because it helps calculate one value when the other is known.

How pH and pOH Change Together

The pH and pOH values always shift in opposite directions:

If pH decreases

  • The solution becomes more acidic
  • [H⁺] increases
  • pOH increases
  • [OH⁻] decreases

If pH increases

  • The solution becomes more basic
  • [H⁺] decreases
  • pOH decreases
  • [OH⁻] increases

This opposite movement helps maintain the constant relationship.

Examples Showing the Relationship

Example 1:

If pH = 5
Then pOH = 14 – 5 = 9

This solution is acidic because pH < 7.

Example 2:

If pOH = 3
Then pH = 14 – 3 = 11

This solution is basic because pH > 7.

Example 3:

If pH = 7
Then pOH = 7

This is neutral because hydrogen and hydroxide ions are equal.

Importance of the pH–pOH Relationship

This relationship is extremely useful in many areas:

  1. Chemical Calculations

Chemists can calculate unknown ion concentrations using only one value.

  1. Acid–Base Reactions

During neutralisation, changes in pH and pOH help determine the strength of acids and bases.

  1. Environmental Studies

Monitoring water bodies requires checking both acidity and basicity.

  1. Medicine and Biology

Human blood maintains a pH around 7.4. Any shift affects pOH and overall body function.

  1. Industry

Manufacturing substances like soaps, shampoos, fertilizers, and medicines depends on correct pH and pOH levels.

pH and pOH in Neutralisation

When an acid reacts with a base:

H⁺ + OH⁻ → H₂O

As hydrogen ions and hydroxide ions combine, both pH and pOH move toward 7, creating a neutral solution. The relationship helps determine when complete neutralisation has occurred.

Misunderstandings About pH and pOH

  1. pH and pOH do not represent the same thing.
    • pH → measures acidity
    • pOH → measures basicity
  2. pH + pOH = 14 only at 25°C.
    At higher or lower temperatures, Kw changes, so the sum may differ.
  3. High pH does not mean low pOH automatically without calculation.
    Always use the formula to confirm the values.
Conclusion

The relationship between pH and pOH is expressed by the equation pH + pOH = 14 at 25°C. This relationship comes from the constant ionic product of water. When pH increases, pOH decreases, and when pH decreases, pOH increases. Understanding this connection helps identify whether a solution is acidic, basic, or neutral and is essential in chemistry, environmental science, medicine, and many industrial processes.