pH Calculator
pH is a logarithmic scale that measures the acidity or basicity of an aqueous solution, defined as the negative base-10 logarithm of the hydrogen ion concentration. This calculator converts H⁺ concentration in mol/L to pH and automatically computes the complementary pOH, giving you a complete picture of the solution's acid-base character.
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Formula
pH = −log₁₀[H⁺]
pH is the negative base-10 logarithm of the molar concentration of hydrogen ions [H⁺]. pOH equals 14 minus pH at 25°C, derived from the water dissociation constant (Kw = 10⁻¹⁴). A pH of 7 is neutral; below 7 is acidic and above 7 is basic. Each whole-number change in pH represents a tenfold change in [H⁺] because of the logarithmic scale.
How to use the pH Calculator
- 1
Enter your h⁺ concentration
Enter the molar concentration of hydrogen ions (e.g. 0.001 for 1×10⁻³ M)
- 2
Read your results instantly
Results update in real time as you type.
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The pH scale explained
The pH scale runs from 0 to 14 under normal aqueous conditions at 25°C, though values outside this range are possible for very concentrated acids and bases. A pH of 7 corresponds to pure water at 25°C, where [H⁺] = [OH⁻] = 10⁻⁷ mol/L. Values below 7 indicate excess hydrogen ions (acidic solutions), and values above 7 indicate excess hydroxide ions (basic or alkaline solutions). Because pH is a logarithmic scale, each unit decrease in pH represents a tenfold increase in [H⁺]. This means stomach acid at pH 1.5 is about 30 times more acidic than vinegar at pH 3, and roughly 3 million times more acidic than blood at pH 7.4.
Why pH matters in biology, medicine, and industry
pH is one of the most tightly regulated parameters in living systems. Human blood must stay between 7.35 and 7.45 — a range narrower than a single pH unit — because enzymes and proteins lose their shape and function outside this window. In agriculture, soil pH determines which nutrients are available to plants; most crops prefer a slightly acidic pH of 6.0–7.0. In water treatment, pH controls the effectiveness of disinfectants and the solubility of heavy metals. In food science, pH governs preservation, texture, and flavor. In pharmaceutical manufacturing, drug stability and bioavailability often depend critically on the pH of the formulation. Understanding and controlling pH is therefore fundamental across an extraordinary range of fields.
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Calculating pH from common acid concentrations
For strong acids that fully dissociate in water — such as HCl, HBr, and HNO₃ — the H⁺ concentration equals the molar concentration of the acid. A 0.01 M HCl solution has [H⁺] = 0.01 mol/L, so pH = −log₁₀(0.01) = 2. For weak acids that only partially dissociate, you must use the acid dissociation constant (Ka) to find [H⁺] before applying the pH formula. Weak acid calculations are more complex but follow the same final step: pH = −log₁₀[H⁺]. This calculator accepts the [H⁺] concentration directly, so you can use it for both strong and weak acid solutions as long as you calculate the equilibrium [H⁺] first.
Tips & Insights
Use scientific notation for very dilute solutions
Concentrations like 1×10⁻⁹ mol/L are easier to enter as 0.000000001. Double-check your number of zeros — one missing zero shifts the pH by a full unit.
pH and pOH always add to 14 at 25°C
At room temperature, pH + pOH = 14. If you know the pOH, you can find the pH simply by subtracting from 14, and vice versa. This relationship changes at other temperatures.
Logarithms make small concentration changes dramatic
A shift from pH 7 to pH 6 looks small on paper but represents a 10-fold increase in acidity. Always think in terms of the concentration ratios, not the pH numbers themselves.
Worked Examples
Vinegar (acetic acid solution)
pH = 3.00, pOH = 11.00 — a mildly acidic solution typical of dilute acetic acid.
Neutral water
pH = 7.00, pOH = 7.00 — pure water at 25°C is perfectly neutral.
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Frequently Asked Questions
What does a pH of 7 mean?
A pH of 7 at 25°C indicates a neutral solution where the concentrations of H⁺ and OH⁻ are equal (both 10⁻⁷ mol/L). Pure water has a pH of 7 under these conditions.
Can pH be negative?
Yes. Extremely concentrated strong acids can have negative pH values. For example, a 12 M HCl solution has a pH around −1.08. These conditions are rare outside industrial settings.
How is pH measured experimentally?
The most accurate method uses a pH meter with a glass electrode calibrated against buffer solutions of known pH. pH indicator strips provide a quick, less precise estimate by color change.
Does temperature affect pH?
Yes. The relationship pH + pOH = 14 holds only at 25°C. At higher temperatures, Kw increases, so neutral pH is slightly below 7. At 37°C (body temperature), neutral pH is approximately 6.81.
What is the difference between strong and weak acids in terms of pH?
Strong acids fully dissociate, so [H⁺] equals the acid concentration. Weak acids only partially dissociate, so [H⁺] is less than the acid concentration and must be found using the Ka equilibrium expression.
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