Definition recall: Acidity or alkalinity (basicity) of an aqueous solution is expressed by pH, defined in terms of hydrogen-ion concentration [H+]. Which expression is correct?

Introduction / Context:
pH is ubiquitous in chemical process control, water treatment, biochemical engineering, and environmental monitoring. A precise definition avoids significant control errors in neutralization and fermentation.

Given Data / Assumptions:

• [H+] denotes molar concentration (or, more rigorously, activity) of hydrogen ions in aqueous solution at the measurement temperature.
• Base-10 logarithm is implied in the conventional pH scale.

Concept / Approach:
The Sorensen definition is pH = −log10(a_H+). For ideal dilute solutions, activity a_H+ ≈ [H+], giving pH = −log10[H+]. Low [H+] (acidic) yields higher log magnitude but with a negative sign, resulting in small pH values; high [H+] reduces the pH further.

Step-by-Step Solution:
Start from the definition: pH = −log10(a_H+).Assume ideality: a_H+ ≈ [H+].Thus, pH = −log10[H+].

Verification / Alternative check:
At neutrality (25°C), [H+] ≈ 1.0 × 10^-7 M → pH = −log10(10^-7) = 7, which matches the standard neutral pH reference.

Why Other Options Are Wrong:
pH = log10[H+] lacks the minus sign and reverses acidity–basicity interpretation; 1/[H+] is dimensionally inconsistent; −ln[H+] uses natural log and would not align with the conventional scale.

Common Pitfalls:
Confusing log bases; ignoring activity corrections in high-ionic-strength solutions.

Final Answer:
pH = -log10[H+]