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.
Key Formula
Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ mol²/L² at 25 °C
LaTeX: K_w = [\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14} \text{ mol}^2/\text{L}^2 \text{ at } 25\,°\text{C}
| Symbol | Meaning | Unit |
|---|---|---|
| Kw | Water ionisation constant (ion product of water) | mol²/L² |
| [H⁺] | Molar concentration of hydrogen ions | mol/L |
| [OH⁻] | Molar concentration of hydroxide ions | mol/L |
Worked Example
Problem
At 25 °C, a solution has [H⁺] = 1.0 × 10⁻⁵ mol/L. Use Kw to find [OH⁻] and determine whether the solution is acidic, neutral, or basic.
Solution
Step 1: Use Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ Step 2: Rearrange: [OH⁻] = Kw / [H⁺] = (1.0 × 10⁻¹⁴) / (1.0 × 10⁻⁵) = 1.0 × 10⁻⁹ mol/L Step 3: Since [H⁺] > [OH⁻], the solution is acidic. Step 4: Verify: pH = -log(10⁻⁵) = 5, which is below 7 (acidic).
Answer
[OH⁻] = 1.0 × 10⁻⁹ mol/L; solution is acidic (pH = 5)
Kw Values at Different Temperatures
| Temperature (°C) | Kw (mol²/L²) | Neutral pH | pKw |
|---|---|---|---|
| 0 | 1.14 × 10⁻¹⁵ | 7.47 | 14.94 |
| 10 | 2.93 × 10⁻¹⁵ | 7.27 | 14.53 |
| 25 | 1.01 × 10⁻¹⁴ | 7.00 | 14.00 |
| 37 | 2.42 × 10⁻¹⁴ | 6.81 | 13.62 |
| 60 | 9.61 × 10⁻¹⁴ | 6.51 | 13.02 |
| 100 | 5.13 × 10⁻¹³ | 6.14 | 12.29 |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
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.
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.
Acid Dissociation Constant (Ka)
The acid dissociation constant (Ka) is the equilibrium constant for the ionisation of an acid in water, quantifying the extent to which an acid donates its proton to water at a given temperature. A large Ka (> 1) indicates a strong acid that dissociates almost completely, while a small Ka (< 10⁻³) indicates a weak acid with limited dissociation. Ka values are fundamental in predicting reaction directions, calculating pH of weak acid solutions, and designing buffer systems.
Kw derives from the German "Konstante" (constant) with subscript "w" for "Wasser" (German for water). The concept of the autoionisation of water and its equilibrium constant was developed in the context of Arrhenius and Brønsted-Lowry theories in the early 20th century.