pH
Definition and meaning of pH in chemistry.
pH is a measure of the acidity or alkalinity of a solution, defined as the negative logarithm (base 10) of the hydrogen ion concentration [H+]. The pH scale typically ranges from 0 to 14 at 25°C, where values below 7 indicate acidic solutions, 7 indicates neutral solutions, and values above 7 indicate basic or alkaline solutions.
In more detail
The logarithmic pH scale is used because hydrogen ion concentrations in aqueous solutions span an enormous range, from about 1 (100) to 10-14 M; expressing this as simple numbers from 0 to 14 is far more practical than using exponents. At 25°C, pure water has [H+] = 1.0 × 10-7 M and a pH of 7.0. The related scale pOH (based on hydroxide ions) follows the relationship pH + pOH = 14 at 25°C, allowing calculations of either acidity or alkalinity from a single measurement.
Key facts
| Definition | pH = -log[H+] |
|---|---|
| Scale | 0–14 (at 25°C in aqueous solutions) |
| Neutral point | pH 7 (at 25°C) |
| Field | Analytical Chemistry |
A 0.1 M solution of hydrochloric acid (HCl) has a pH of 1, pure water at 25°C has a pH of 7.0, and a 0.1 M solution of sodium hydroxide (NaOH) has a pH of 13.
Frequently asked questions
Why use a logarithmic scale instead of measuring hydrogen ion concentration directly?
Hydrogen ion concentrations vary over many orders of magnitude (1 to 10-14 M). A logarithmic scale compresses this enormous range into manageable numbers from 0 to 14, making it easy to compare solutions.
Does pH change with temperature?
Yes. The neutral pH depends on water's ionization constant (Kw), which increases at higher temperatures. The pH scale is defined at 25°C (where neutral pH equals 7), but at 100°C, neutral pH is approximately 6.1 because Kw is larger at higher temperatures.