pH Dilution Calculator

pH dilution calculates how the pH of a solution changes when you add pure water. Dilution reduces the concentration of hydrogen ions (for acids) or hydroxide ions (for bases), moving the pH toward 7 (neutral).

For acids (pH < 7):

[H⁺] initial = 10^(−pH)

[H⁺] final = [H⁺] initial × (V_initial / V_total)

pH final = −log₁₀([H⁺] final)

For bases (pH > 7):

[OH⁻] initial = 10^(pH − 14)

[OH⁻] final = [OH⁻] initial × (V_initial / V_total)

pH final = 14 − (−log₁₀([OH⁻] final))

What each variable means:

• Initial pH: the pH of your solution before adding water.
• Initial Volume: how much solution you start with (in mL).
• Water Added: how much pure water you are adding (in mL).
• V_total: initial volume plus water added. The dilution factor is V_total / V_initial.

When to use this calculator: Use it in chemistry labs when preparing diluted solutions, in pool maintenance when adjusting water chemistry, or in educational settings to understand how dilution affects pH.

Practical example: You have 100 mL of an acid solution at pH 3. You add 900 mL of pure water (a 10× dilution). [H⁺] initial = 10⁻³ = 0.001 M. [H⁺] final = 0.001 × (100 / 1000) = 0.0001 M. pH final = −log(0.0001) = 4.0. The pH moved one unit toward neutral.

Key rules to remember:

• Each 10× dilution shifts pH approximately 1 unit toward neutral (7).
• You can never dilute past pH 7 with pure water: the solution approaches 7 but never crosses it.
• This formula assumes ideal dilution with pure (deionized) water at pH 7.

Common mistakes: Do not assume that diluting an acid 100× will change the pH by 2 full units — this is only approximately true. Very dilute solutions are also affected by the autoionization of water. This calculator works best for solutions with pH between 1 and 13.