pH Calculator
Strong & weak acids/bases, buffer solutions and Henderson–Hasselbalch — step-by-step pH calculations.
Select input type
E.g. 7.4
pH in Everyday Life
0
Battery acid
2
Stomach acid
2.5
Lemon juice
3.5
Soft drink
4.5
Tomato
5
Coffee
6
Milk
7
Pure water
7.4
Blood
8
Seawater
9
Baking soda
11
Ammonia
12.5
Lime
14
NaOH
Key Formulas
Frequently Asked Questions
What is pH and how is it defined?
pH is the negative logarithm of hydrogen ion activity: pH = -log₁₀(aH⁺). IUPAC defines it using activity. For education and dilute solutions, activity ≈ concentration is commonly used.
How do strong and weak acid pH calculations differ?
Strong acids dissociate completely, so [H⁺] = C × ν and pH = -log[H⁺]. Weak acids partially ionize; an ICE table with Ka gives a quadratic equilibrium equation.
When should I use the Henderson–Hasselbalch equation?
For buffer solutions — weak acid + conjugate base or weak base + conjugate acid: pH = pKa + log([A⁻]/[HA]). It works well when the ratio is 0.1–10 and concentration > 10⁻³ M.
Why is 25°C assumed?
Water's ionization constant Kw depends on temperature; at 25°C Kw = 10⁻¹⁴ (pKw = 14). At 37°C Kw ≈ 2.4×10⁻¹⁴. This calculator fixes 25°C.
Why is a different formula needed for very dilute solutions?
When C < 10⁻⁶ M, autoionization of water cannot be ignored. A quadratic correction prevents wrong results such as pH = 8 for 10⁻⁸ M HCl: [H⁺] = ½(C + √(C² + 4Kw)).
How are pKa and buffer capacity related?
An acid-base buffer is most effective within ±1 pH unit of pKa. In this range the system best resists added strong acid or base.
What does Ka × Kb = Kw mean?
At 25°C for a conjugate pair, Ka × Kb = Kw = 10⁻¹⁴, so pKa + pKb = 14. This links acid Ka to base Kb (or vice versa).
How should I calculate polyprotic acids?
Acids like H₃PO₄ have a separate Ka for each step. This tool uses a monoprotic approach first; when Ka₁ >> Ka₂, each step can be treated separately.
🧪 Acid-Base Simulations
Reinforce pH calculations with visual experiments — interactive chemistry simulations