pH Calculator

Calculate pH from [H⁺] concentration, or find [H⁺] from pH. Convert between pH and pOH.

📖 What is pH?

pH stands for "potential of Hydrogen" and is a logarithmic scale used to specify the acidity or basicity (alkalinity) of an aqueous solution. The scale runs from 0 to 14, where 7 is neutral (pure water at 25°C), values below 7 are acidic, and values above 7 are basic or alkaline.

The pH scale was developed by Danish chemist Søren Peder Lauritz Sørensen in 1909. Because it is logarithmic, each unit change represents a 10-fold change in hydrogen ion concentration. This means pH 3 is 10 times more acidic than pH 4, and 100 times more acidic than pH 5 - making the scale very powerful for expressing the enormous range of concentrations found in chemistry.

pH is critical across many fields: biology (maintaining cellular pH is essential for enzyme function and life itself), medicine (blood pH must be maintained between 7.35 and 7.45 - even small deviations cause serious illness), food science (controlling pH affects taste, safety, and shelf life), water treatment, agriculture (soil pH determines nutrient availability), and industrial chemistry.

📐 Formula

pH = -log₁₀[H⁺]

[H⁺] = 10^(-pH)

pOH = -log₁₀[OH⁻]

pH + pOH = 14 (at 25°C, since Kw = 10⁻¹⁴)

Where [H⁺] is the molar concentration of hydrogen ions (mol/L).

📖 How to Use This Calculator

[H⁺] → pH: Enter the hydrogen ion concentration in mol/L (e.g. 0.001 for 10⁻³ M). Click Calculate.

pH → [H⁺]: Switch to the second tab, enter the pH value, and click Calculate.

Results show pH, pOH, [H⁺], [OH⁻], and the acidity classification with a visual scale indicator.

💡 Example Calculations

Example 1 - Lemon juice

[H⁺] = 0.005 mol/L (5 × 10⁻³ mol/L)

pH = -log₁₀(0.005) = -(-2.301) = pH 2.30

pOH = 14 - 2.30 = 11.70

Classification: Acidic

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Example 2 - Baking soda solution

pH = 8.3

[H⁺] = 10⁻⁸·³ = 5.01 × 10⁻⁹ mol/L

pOH = 14 - 8.3 = 5.70

[OH⁻] = 10⁻⁵·⁷ = 2 × 10⁻⁶ mol/L

Classification: Weakly Basic

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Frequently Asked Questions

What is pH?+

pH stands for 'power of Hydrogen'. It is a logarithmic scale from 0 to 14 that measures how acidic or basic (alkaline) a solution is. pH = -log₁₀[H+], where [H+] is the hydrogen ion concentration in mol/L. pH 7 is neutral; below 7 is acidic; above 7 is basic.

What is the difference between pH and pOH?+

pH measures hydrogen ion concentration; pOH measures hydroxide ion concentration. At 25°C, pH + pOH = 14 (the ion product of water, Kw = 10⁻¹⁴). If pH = 4, then pOH = 10. A low pOH means a high [OH-] concentration - a basic solution.

What are common substances and their pH?+

Battery acid: ~0, stomach acid: ~1.5, lemon juice: ~2, vinegar: ~3, coffee: ~5, milk: ~6.5, pure water: 7, blood: ~7.4, baking soda: ~8.3, sea water: ~8, bleach: ~12, lye (NaOH): ~13.

How does pH relate to acid strength?+

A strong acid (like HCl, H₂SO₄) fully dissociates in water, releasing all its H+ ions. A 0.1 M HCl solution has pH = 1. A weak acid (like acetic acid in vinegar) only partially dissociates, so a 0.1 M solution has a higher pH (around 2.87).

What is a buffer solution?+

A buffer solution resists changes in pH when small amounts of acid or base are added. It contains a weak acid and its conjugate base (or weak base and its conjugate acid). Blood uses a carbonate buffer to maintain pH between 7.35 and 7.45 - variations outside this range are life-threatening.

What pH is neutral, acidic, or basic?+

pH ranges from 0 to 14 (and beyond in concentrated solutions). pH 7 is neutral (pure water at 25 degrees Celsius). pH below 7 is acidic - the lower the number, the stronger the acid. pH above 7 is basic (alkaline) - the higher the number, the stronger the base. Common examples: gastric acid pH 1.5-3.5, coffee pH 5, blood pH 7.35-7.45 (slightly alkaline), baking soda pH 8.3, bleach pH 12-13.

What does a 1-unit change in pH mean?+

The pH scale is logarithmic (base 10). A change of 1 pH unit represents a 10-fold change in hydrogen ion concentration. A change of 2 pH units = 100-fold change. So pH 3 is 10 times more acidic than pH 4, and 100 times more acidic than pH 5. This explains why strong acid spills are so dangerous: even small volumes of pH 1 acid have a hydrogen ion concentration 1 million times higher than neutral water (pH 7).

How does temperature affect pH measurements?+

Temperature significantly affects pH readings. The neutral point of water is pH 7.0 only at 25 degrees C. At higher temperatures, the neutral pH decreases - at 37 degrees C (body temperature), neutral water has a pH of about 6.8. pH electrodes are also temperature-sensitive in their voltage output, which is why precision pH meters have automatic temperature compensation (ATC). For critical measurements, always record the temperature alongside pH and use ATC-enabled equipment. For biological systems like blood, the Henderson-Hasselbalch equation accounts for temperature effects on buffer systems.

Why does pH change with temperature?+

The pH of pure water at 25 degrees C is 7.0 (neutral). At 100 degrees C, pure water has pH approximately 6.14 - still neutral, but the neutral point shifts because Kw (water's ionization constant) changes with temperature. A blood sample at body temperature (37 degrees C) has neutral pH approximately 6.8. Always specify temperature when reporting precise pH values in scientific work.