pH Calculator
Calculate pH, pOH, hydrogen ion concentration [H+], and hydroxide ion concentration [OH-] from any one of these values. Based on the water autoionization constant Kw = 10^-14 at 25°C.
The pH scale measures how acidic or basic a solution is. It is a logarithmic scale based on the concentration of hydrogen ions.
Key Relationships (at 25°C):
- pH = -log₁₀[H⁺]
- pOH = -log₁₀[OH⁻]
- pH + pOH = 14
- [H⁺] × [OH⁻] = 10⁻¹⁴ (Kw, the water autoionization constant)
The pH Scale:
- pH 0-6.9: Acidic (more H⁺ than OH⁻)
- pH 7.0: Neutral (equal H⁺ and OH⁻)
- pH 7.1-14: Basic/Alkaline (more OH⁻ than H⁺)
Common pH Values:
- Battery acid: pH ~0
- Lemon juice: pH ~2
- Vinegar: pH ~2.5
- Coffee: pH ~5
- Pure water: pH 7
- Baking soda: pH ~9
- Ammonia: pH ~11
- Bleach: pH ~13
Important: These calculations assume 25°C (298.15 K). At other temperatures, Kw changes, and pH + pOH will not equal exactly 14.