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pH Calculator

Ready to calculate
pH ↔ [H⁺].
Henderson-Hasselbalch.
Strong + Weak Acids.
100% Free.
No Data Stored.

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How it Works

01Enter Concentration

Provide [H⁺] or [OH⁻] in molar.

02Compute pH/pOH

pH = −log[H⁺] and pH + pOH = 14 at 25 °C.

03Buffer Mode

Henderson-Hasselbalch for buffer systems.

04Acidity Range

Classification from strong acid to strong base.

What is a pH Calculator?

The pH Calculator converts between hydrogen ion concentration [H⁺], hydroxide concentration [OH⁻], pH, and pOH at 25°C. Enter any one of the four values and the calculator returns the other three using the relationships pH = −log[H⁺], pOH = −log[OH⁻], pH + pOH = 14 (water autoionization at 25°C).


Designed for chemistry students, water-quality analysts, biochemists working with buffers, and anyone needing to translate between acid-base concentration units. Pair with the Henderson-Hasselbalch Calculator for buffer pH calculations.

How to Use the Calculator

Pick input type: [H⁺], [OH⁻], pH, or pOH.
Enter the value: concentrations in mol/L (M); pH/pOH unitless.
Calculate: Returns all four values plus acid/base/neutral classification.

The Math Behind It

Core relationships at 25°C:

  • pH = −log₁₀[H⁺]  →  [H⁺] = 10^(−pH)
  • pOH = −log₁₀[OH⁻]  →  [OH⁻] = 10^(−pOH)
  • pH + pOH = 14 (autoionization, K_w = 1.0 × 10⁻¹⁴ at 25°C)
  • [H⁺] × [OH⁻] = 10⁻¹⁴

Acidic: pH < 7. Neutral: pH = 7. Basic: pH > 7. Each pH unit = 10× concentration change. K_w varies with temperature (1.0 × 10⁻¹⁴ at 25°C; 1.5 × 10⁻¹⁴ at 30°C; neutral pH at 50°C is 6.63, not 7.0).

Real-World Example

Worked Example

Vinegar measured at pH 2.4:

  • [H⁺] = 10^(−2.4) = 4.0 × 10⁻³ M = 4 mM
  • pOH = 14 − 2.4 = 11.6
  • [OH⁻] = 10^(−11.6) = 2.5 × 10⁻¹² M
  • Classification: strongly acidic (4000× more H⁺ than neutral water)

Who Uses It

1
🧪 Chemistry Students: Verify problem-set conversions; check work.
2
💧 Water-Quality Analysts: Translate between pH meter readings and [H⁺] for kinetic models.
3
🧬 Biochemists: Quick conversions when designing buffers.
4
🌊 Aquarists / Pond Keepers: Understand fish-toxic ammonia/H⁺ ratios.
5
🏭 Process Chemists: Set acid/base dosing rates from target pH.
6
🧫 Microbiologists: Predict growth-stage pH shifts in fermentation.

Technical Reference

Common solution pH values:

  • Battery acid (~1 M H₂SO₄): 0.3
  • Stomach acid: 1.5–3.5
  • Lemon juice: 2.0–2.6
  • Vinegar: 2.4–3.4
  • Soda: 2.5
  • Black coffee: 4.5–5.0
  • Pure rain (CO₂ saturated): 5.6
  • Milk: 6.5
  • Pure water: 7.0
  • Blood: 7.35–7.45 (tightly buffered)
  • Seawater: 8.1
  • Baking soda solution: 8.5
  • Ammonia (household): 11–12
  • Bleach (1% NaClO): 12.5
  • 1 M NaOH: 14

Key Takeaways

pH is logarithmic — a 2-unit drop is a 100× concentration change. The simple identity pH + pOH = 14 holds only at 25°C; for high-temperature work, use the actual K_w. For buffer calculations and titration curves, use Henderson-Hasselbalch.

Frequently Asked Questions

Why is pH logarithmic?
H⁺ concentrations span ~14 orders of magnitude (1 M strong acid to 10⁻¹⁴ M neutralized water). Logarithms compress that range into the familiar 0–14 scale that's easier to read and graph.
Can pH go below 0 or above 14?
Yes, for very concentrated strong acids/bases. Concentrated H₂SO₄ has pH ≈ −1. 12 M HCl has pH ≈ −1.1. The 0–14 range is convenient, not absolute.
Why does pH + pOH = 14?
Water autoionizes: H₂O ⇌ H⁺ + OH⁻ with K_w = [H⁺][OH⁻] = 10⁻¹⁴ at 25°C. Take −log: −log[H⁺] − log[OH⁻] = 14, i.e., pH + pOH = 14.
How accurate are pH meters?
Lab-grade pH meters: ±0.01 units after 2-point calibration. Pocket meters: ±0.05–0.1. Test strips: ±0.5. Always calibrate with fresh buffer at the same temperature as your sample.
What about activity vs concentration?
Strictly, pH = −log(a_H⁺), where a_H⁺ is the activity of H⁺ — concentration adjusted by an activity coefficient. For dilute solutions (< 0.01 M), activity ≈ concentration. For seawater or concentrated electrolytes, the difference matters and pH electrodes measure activity directly.
How does temperature affect pH?
Two effects: (1) the meter electrode response shifts with T (most meters auto-compensate); (2) the water itself: K_w doubles from 25°C to 50°C, so neutral pH drops from 7.00 to 6.63. Pure water at 100°C has pH 6.14 — still neutral!

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Disclaimer

pH ↔ concentration conversions assume 25°C and ideal-solution behavior. For high ionic strength (seawater, concentrated electrolytes) or extreme temperatures, use activity coefficients and the actual K_w at that temperature.