Understanding pH
pH is the negative base-10 logarithm of the hydrogen ion concentration in a solution. It’s the universal measure of acidity: low pH means high H⁺ concentration (acidic), high pH means low H⁺ concentration (basic). The scale is logarithmic, so each unit change represents a tenfold change in acidity.
This calculator works both directions: enter a known H⁺ concentration to find pH, or enter a pH value to find the corresponding H⁺ and OH⁻ concentrations. The relationship pH + pOH = 14 holds at 25°C (standard temperature).
Common mistake: forgetting that pH is logarithmic. A solution at pH 3 isn’t “a little more acidic” than pH 4 — it’s ten times more acidic.
How to Calculate pH (step by step)
Step One: Identify the hydrogen ion concentration
Determine [H⁺] in mol/L from your data or measurement.
For this example: [H⁺] = 0.001 mol/L.
Step Two: Take the negative log base 10
pH = -log₁₀([H⁺])
For [H⁺] = 0.001 M: pH = -log₁₀(0.001) = -(-3) = 3.00.
Step Three: Calculate pOH (optional)
pOH = 14 - pH
For pH 3: pOH = 14 - 3 = 11.00.
Reverse calculation: pH to [H⁺]
[H⁺] = 10^(-pH)
For pH 7: [H⁺] = 10⁻⁷ = 0.0000001 mol/L.