Definition and Determination of pH

Specification Reference Physical Chemistry, Acids and bases 3.1.12.2

Quick Notes

pH is a measure of hydrogen ion concentration in a solution, using a log scale: pH = −log₁₀[H⁺].
To find [H⁺] from pH: [H⁺] = 10^−pH.
pH scale ranges from 0 to 14 where (at 298 K):
- Acidic solutions: pH < 7 ( [H⁺] > [OH⁻] )
- Neutral solutions: pH = 7 ( [H⁺] = [OH⁻] )
- Alkaline solutions: pH > 7 ( [H⁺] < [OH⁻] )
pH can be determined experimentally using:
- pH meters (accurate measurement).
- Universal indicator (colour comparison).

Full Notes

Definition of pH

The concentration of H⁺ ions in solution can cover a very wide range. To make things easier (and avoid using very big or very small numbers), pH is used to describe H⁺ ion concentration.

pH is a logarithmic scale

The pH equation is:

AQA A-Level Chemistry equation showing pH equals minus log base ten of hydrogen ion concentration.

This is also the same as

AQA A-Level Chemistry relationship showing hydrogen ion concentration equals ten to the power of negative pH.

Since pH is logarithmic:

A pH decrease of 1 means [H⁺] increases by a factor of 10.
A pH increase of 1 means [H⁺] decreases by a factor of 10.

Worked Example

Calculation: If the concentration of H⁺ ions in a solution is 1.00 × 10⁻³ mol dm⁻³, find the pH.

Use pH = −log(1.00 × 10⁻³).
pH = 3.00.

Worked Example - Calculating [H⁺] from pH

Calculation: If pH = 4.50, find [H⁺].

[H⁺] = 10^−4.5.
[H⁺] = 3.16 × 10⁻⁵ mol dm⁻³.

The pH Scale

The pH scale ranges from 0 to 14.

Low pH = Higher [H⁺] than [OH^-] = Acidic solution
High pH = Lower [H⁺] than than [OH^-] = Alkaline solution

Determining pH Experimentally

1. Using a pH Meter

AQA A-Level Chemistry diagram showing rinsing a pH probe with distilled water and placing it in a sample connected to a pH meter.

A pH probe is placed in the solution, and the pH is displayed digitally. More accurate than indicators.

Steps:

Calibrate the pH meter using buffer solutions of known pH.
Rinse the probe with distilled water to avoid contamination.
Place the probe in the solution and record the pH.

2. Using a Universal Indicator

AQA A-Level Chemistry diagram showing universal indicator paper dipped in a solution with a colour chart from red (1) to purple (14).

A colour-changing paper or solution that gives an approximate pH value. Less accurate than a pH meter.

The solution changes colour based on pH:

Red (pH 0–3) → Strong acid
Yellow (pH 4–6) → Weak acid
Green (pH 7) → Neutral
Blue (pH 8–11) → Weak base
Purple (pH 12–14) → Strong base

Summary

Concept	Key points
Definition of pH	pH is a measure of hydrogen ion concentration in a solution, using a log scale.
Equations	pH = −log₁₀[H⁺] and [H⁺] = 10^−pH
pH scale (298 K)	Acidic: pH < 7 ( [H⁺] > [OH⁻] ) ; Neutral: pH = 7 ( [H⁺] = [OH⁻] ) ; Alkaline: pH > 7 ( [H⁺] < [OH⁻] )
Experimental determination	pH meters (accurate measurement). Universal indicator (colour comparison).