By Claire Gillespie
Updated Aug 30, 2022
Dilution is a routine laboratory technique that science students use to achieve precise concentrations. It also happens in everyday life—adding water to coffee or wine to taste. Importantly, dilution changes the pH of a solution, either making it less acidic or less alkaline, depending on the starting material.
When you dilute an acidic solution, its pH moves toward 7 (more neutral). Diluting a basic solution pushes its pH toward 7 from the alkaline side. Calculate the effect by converting the original pH to hydrogen‑ion concentration, adjusting that concentration for the new volume, and converting back to pH.
Adding solvent—usually water—to a solution increases the proportion of the solvent relative to the solute. The amount of solute stays the same, but the volume grows, so every molar concentration decreases by the ratio of the new volume to the original volume.
The pH scale, ranging from 0 to 14, quantifies acidity or alkalinity. A pH of 7 is neutral. Below 7, the solution is acidic; above 7, it is basic. The scale is logarithmic: each whole number change represents a ten‑fold difference in hydrogen‑ion concentration. For instance, pH 3 is ten times more acidic than pH 4 and a hundred times more acidic than pH 5.
pH is defined as pH = –log[H⁺], where [H⁺] is the molarity of hydrogen ions. Rearranging gives [H⁺] = 10–pH.
Acids (e.g., lemon juice, battery acid, coffee) contain H⁺ ions. Adding water reduces [H⁺], raising the pH toward 7. Because the added water is neutral, the pH of an acidic solution can never rise above 7.
Bases (e.g., ammonia, bleach, baking soda) contain OH⁻ ions. Water dilutes OH⁻, which lowers the pH toward 7. The pH of a basic solution can never fall below 7 when diluted with neutral water.
1. Convert the initial pH to hydrogen‑ion concentration. For a pH 6 solution, [H⁺] = 10–6 M.
2. Determine the dilution factor. If the final volume is four times the original, the concentration is divided by four.
3. Calculate the new [H⁺]. Using the example above: [H⁺]new = (10–6 M) ÷ 4 = 2.5 × 10–7 M.
4. Convert back to pH. pHnew = –log(2.5 × 10–7) ≈ 6.60.
Thus, a four‑fold dilution of a pH 6 acid raises the pH to about 6.6, moving it closer to neutrality.
