pH Scale
The pH scale is a logarithmic measure of the hydrogen ion concentration [H⁺] in a solution, ranging from 0 to 14 at 25 °C, where values below 7 indicate acidic conditions, 7 is neutral, and above 7 is basic or alkaline. Introduced by Danish chemist Søren Peder Lauritz Sørensen in 1909, the scale compresses a trillion-fold range of H⁺ concentrations into a convenient 0–14 range. pH measurement is critical in agriculture, biology, medicine, food science, and environmental monitoring.
Key Formula
pH = -log₁₀[H⁺]
LaTeX: \text{pH} = -\log_{10}[\text{H}^+]
| Symbol | Meaning | Unit |
|---|---|---|
| pH | Power of hydrogen (logarithmic acidity measure) | dimensionless |
| [H⁺] | Molar concentration of hydrogen ions | mol/L |
| log₁₀ | Base-10 logarithm | none |
Worked Example
Problem
Orange juice has a hydrogen ion concentration of 2.5 × 10⁻⁴ mol/L. Calculate its pH and classify it as acidic, neutral, or basic.
Solution
Step 1: Apply the formula: pH = -log[H⁺] Step 2: pH = -log(2.5 × 10⁻⁴) Step 3: pH = -(log 2.5 + log 10⁻⁴) = -(0.398 + (-4)) = -(-3.602) = 3.60 Step 4: pH 3.60 < 7, so the orange juice is acidic.
Answer
pH = 3.60; acidic
pH Values of Common Substances
| Substance | pH Value | Classification | H⁺ Concentration (mol/L) |
|---|---|---|---|
| Battery acid | 0–1 | Strongly acidic | ~1 |
| Gastric juice | 1.5–3.5 | Strongly acidic | ~0.03–3 × 10⁻² |
| Orange juice | 3.5–4.5 | Weakly acidic | ~3 × 10⁻⁴ |
| Pure water | 7.0 | Neutral | 1 × 10⁻⁷ |
| Blood plasma | 7.35–7.45 | Weakly basic | ~4 × 10⁻⁸ |
| Bleach | 12–13 | Strongly basic | ~0.01–0.1 |
Interactive Tools
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Related Terms
pOH
pOH is the negative base-10 logarithm of the hydroxide ion concentration [OH⁻] in a solution, serving as a measure of the basicity of an aqueous solution at a given temperature. At 25 °C, pH and pOH are complementary: they always sum to 14, making pOH a convenient way to express basic conditions. pOH is particularly useful in analytical chemistry when working with alkaline solutions, as it directly reflects the concentration of OH⁻ ions produced by bases.
Water Ionization Constant (Kw)
The water ionization constant (Kw) is the equilibrium constant for the self-ionisation of water, defined as the product of the molar concentrations of hydrogen ions and hydroxide ions in pure water at a given temperature. At 25 °C, Kw = 1.0 × 10⁻¹⁴ mol²/L², which establishes the fundamental link between pH and pOH. Kw increases with temperature (water ionises more at higher temperatures), so the neutral pH shifts below 7 at elevated temperatures.
Acid (Chemistry)
An acid is a substance that donates protons (hydrogen ions, H⁺) to another substance or accepts electron pairs, resulting in a sour taste, ability to turn blue litmus red, and a pH below 7 in aqueous solution. Acids play a fundamental role in chemical reactions, biological processes, and industrial applications ranging from digestion to manufacturing. Common examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and acetic acid (CH₃COOH) found in vinegar.
The "p" in pH stands for German "Potenz" (power/potency) and the "H" for the element hydrogen. The notation was introduced by Danish biochemist Søren Peder Lauritz Sørensen in 1909 to simplify hydrogen ion concentration measurements in enzyme research.