pH Calculator
Calculate pH from H+ concentration, or find [H+] and [OH-] from pH.
How It Works
Enter either the pH value or the hydrogen ion concentration [H⁺] in mol/L. The calculator uses the logarithmic relationship between the two, then computes pOH and [OH⁻] using the water autoionization constant (Kw = 10⁻¹⁴ at 25°C). Changing either input field updates all four output values instantly.
Formulas
Common pH Reference Values
- Battery acid: pH ≈ 0 | Stomach acid: pH ≈ 1–2 | Lemon juice: pH ≈ 2–3
- Coffee: pH ≈ 5 | Milk: pH ≈ 6–7 | Pure water: pH = 7
- Blood: pH ≈ 7.4 | Seawater: pH ≈ 8 | Baking soda: pH ≈ 8.3
- Bleach: pH ≈ 12 | NaOH (1 M): pH = 14
Why pH Uses a Logarithmic Scale
The concentration of H⁺ ions in common solutions spans 14 orders of magnitude — from 1 mol/L in strong acids to 10⁻¹⁴ mol/L in strong bases. A logarithmic scale compresses this enormous range into a convenient 0–14 scale. Each one-unit change in pH represents a tenfold change in H⁺ concentration, so a solution at pH 3 is 10 times more acidic than one at pH 4 and 100 times more acidic than one at pH 5.
Strong vs. Weak Acids and pH
For strong acids (HCl, H₂SO₄, HNO₃), [H⁺] equals the molar concentration because they dissociate completely. For a 0.01 M HCl solution, pH = −log(0.01) = 2. Weak acids (acetic acid, citric acid) only partially dissociate; their pH is calculated using the acid dissociation constant Ka and the Henderson-Hasselbalch equation, which is more complex. This calculator is suitable for strong acids and bases or situations where [H⁺] is directly known.
Buffers and pH Stability
A buffer solution resists pH changes upon addition of small amounts of acid or base. Buffers consist of a weak acid and its conjugate base (or weak base and its conjugate acid) in roughly equal concentrations. Biological systems rely heavily on buffers — blood is maintained at pH 7.35–7.45 by the bicarbonate buffer system. Even a small shift outside this range (below 7.35 = acidosis; above 7.45 = alkalosis) is a medical emergency.
Frequently Asked Questions
What is pH?
pH is a logarithmic measure of hydrogen ion (H⁺) concentration in a solution. pH 7 is neutral (pure water at 25°C); below 7 is acidic; above 7 is basic (alkaline). The scale formally runs from 0 to 14, though concentrated strong acids can have negative pH values.
How do you calculate pH from concentration?
pH = −log₁₀[H⁺]. For [H⁺] = 0.001 M (10⁻³ mol/L): pH = −log(0.001) = 3. Enter that concentration in the calculator field and it will display pH, pOH, and both ion concentrations automatically.
What is pOH?
pOH = −log[OH⁻]. At 25°C, the relationship pH + pOH = 14 always holds (this comes from the water autoionization constant Kw = [H⁺][OH⁻] = 10⁻¹⁴). A solution with pH 4 has pOH 10 and [OH⁻] = 10⁻¹⁰ mol/L.
Does temperature affect pH?
Yes. The Kw of water increases with temperature, so the neutral pH shifts slightly — at 37°C (body temperature), neutral pH is about 6.8, not 7.0. For most lab calculations at room temperature (25°C), using pH 7 as neutral is accurate.
What is the difference between pH and acidity?
pH is a specific measurement of H⁺ concentration. "Acidity" can refer to pH but also to total acid content (titratable acidity), which matters in food science. A food can have low pH but low titratable acidity (or vice versa). For chemistry calculations, pH is the standard measure.