pH measures the concentration of hydrogen ions because acidity is fundamentally determined by how many ( \text{H}^+ ) ions are present in a solution. The more hydrogen ions there are, the more acidic the solution becomes. The pH scale provides a convenient way to express this concentration using a logarithmic scale, since the actual values of ([ \text{H}^+ ]) are typically extremely small. Instead of writing numbers like ( 1 \times 10^{-5} , \text{mol dm}^{-3} ), chemists use pH, defined as:
[
\text{pH} = -\log[ \text{H}^+ ]
]
This means that small changes in hydrogen ion concentration translate into noticeable numerical changes on the pH scale.
Hydrogen ions play a central role in all acid–base reactions. When an acid donates a proton, it increases the concentration of ( \text{H}^+ ) in solution, lowering the pH. When a base accepts a proton or produces hydroxide ions, it reduces ([ \text{H}^+ ]), raising the pH. Because acidity and basicity directly reflect proton activity, pH provides the most direct measure of a solution’s acid strength.
The logarithmic nature of the scale also helps manage the wide range of acidities observed in chemistry. For example, moving from pH 3 to pH 2 does not mean the solution becomes “a little” more acidic — it becomes ten times more acidic, because hydrogen ion concentration increases by a factor of 10. This logarithmic compression makes the scale practical for laboratory, biological and industrial applications.
Another advantage of pH is that it reflects the underlying equilibrium behavior of water. Water autoionizes slightly:
[
2H_2O \rightleftharpoons H_3O^+ + OH^-
]
The equilibrium constant for this reaction (Kw) allows us to relate ([ \text{H}^+ ]) and ([ \text{OH}^- ]) using:
[
pH + pOH = 14
]
This balance explains why pH is so valuable — it connects acidity, basicity and the equilibrium properties of water in a single number.
Ultimately, pH measures hydrogen ion concentration because proton activity defines acidity, and the logarithmic scale provides a simple, intuitive way to express the strength of acidic and basic solutions.
Frequently Asked Questions
Why is pH logarithmic instead of linear?
Because hydrogen ion concentrations span many powers of ten, and a linear scale would be impractical.
Does pH measure free hydrogen ions or hydronium ions?
Technically hydronium ions, but ([ \text{H}^+ ]) is used as a simplified representation.
Can pH be negative?
Yes. Extremely strong acids can produce ([ \text{H}^+ ]) greater than 1 mol dm⁻³, resulting in negative pH values.
RevisionDojo Call to Action
Need pH, pOH, Ka, and acid–base equilibrium explained clearly? RevisionDojo turns acid–base chemistry into intuitive, step-by-step reasoning so you can master every IB question with confidence.
