pH Calculator
Convert between hydrogen-ion concentration and pH, with pOH and hydroxide concentration, and see where a solution falls on the acidic–neutral–basic scale.
كيفية استخدام هذه الحاسبة
- 1Choose whether you know the hydrogen-ion concentration or the pH.
- 2Enter that value.
- 3Read the pH (or concentration), plus pOH and the hydroxide concentration.
- 4Check the classification to see whether the solution is acidic, neutral, or basic.
طريقة الحساب
pH and pOH
pH = −log₁₀[H⁺] [H⁺] = 10^(−pH) pH + pOH = 14 (at 25°C) [H⁺] × [OH⁻] = 10⁻¹⁴ (at 25°C)
pH measures how acidic or basic a solution is, defined as the negative base-ten logarithm of the hydrogen-ion concentration in moles per litre. The logarithm is what compresses an enormous range of concentrations — from about 1 mol/L in strong acids down to 10⁻¹⁴ in strong bases — onto the familiar 0-to-14 scale. Because it is logarithmic, each whole pH unit represents a tenfold change in concentration, not a fixed amount. The companion quantity pOH describes the hydroxide-ion concentration the same way, and at 25°C the two always sum to 14, reflecting the fixed product of hydrogen and hydroxide ions in water. A pH below 7 is acidic, above 7 is basic, and exactly 7 is neutral, the value of pure water where the two ion concentrations are equal.
مثال محلول
A solution with a hydrogen-ion concentration of 0.0001 mol/L (10⁻⁴) has a pH of −log₁₀(10⁻⁴) = 4, which is acidic — about the acidity of tomato juice. Its pOH is 14 − 4 = 10, and its hydroxide concentration is 10⁻¹⁰ mol/L, far smaller than its hydrogen-ion concentration, as expected for an acid.
pH Calculator: الدليل الكامل
Why the pH scale is logarithmic
Hydrogen-ion concentrations in real solutions span an astonishing range — roughly fourteen orders of magnitude from the strongest common acids to the strongest bases. Writing these as ordinary numbers would be unwieldy: 0.00000001 mol/L is hard to read and compare. Taking the negative logarithm collapses that vast range onto a tidy scale from about 0 to 14, where each step of one unit corresponds to a factor of ten in concentration. This is the same trick used by the Richter scale for earthquakes and the decibel scale for sound, and for the same reason — to tame quantities that vary over many powers of ten.
The logarithmic nature has an important practical consequence that often surprises people: small differences in pH represent large differences in acidity. A drop from pH 5 to pH 4 is not a modest 20% change but a tenfold increase in hydrogen-ion concentration; from pH 5 to pH 3 is a hundredfold. This is why a seemingly minor shift in the pH of blood, soil, or a swimming pool can have outsized effects, and why pH must be controlled far more tightly than the compact 0-to-14 scale might suggest.
Acids, bases, and the balance in water
Water itself slightly ionises, producing equal tiny amounts of hydrogen ions (H⁺) and hydroxide ions (OH⁻). In pure water at room temperature each sits at 10⁻⁷ mol/L, which is why neutral pH is 7. Adding an acid increases the hydrogen-ion concentration and lowers the pH; adding a base increases hydroxide ions, which suppresses hydrogen ions and raises the pH. The two are inversely linked because their product is fixed at 10⁻¹⁴ in water at 25°C — push one up and the other must fall.
This fixed relationship is why pH and pOH always add to 14 at that temperature, and why knowing the acidity automatically tells you the basicity. It also explains why the scale is not strictly bounded at 0 and 14: very concentrated strong acids or bases can push slightly beyond those limits. The 0-to-14 range is simply where almost all everyday solutions fall, from battery acid near 0 through lemon juice, coffee, pure water, seawater, and ammonia up toward the basic end.
Why pH matters everywhere
Acidity controls an extraordinary range of natural and industrial processes, which is what makes pH one of the most measured quantities in science. In the human body, blood pH is held within a razor-thin range around 7.4, and even small deviations are life-threatening, so the body deploys elaborate buffering systems to defend it. Enzymes, the catalysts of life, each work best at a particular pH and falter outside it, which is why the stomach is strongly acidic and the small intestine mildly basic.
Beyond biology, pH governs soil fertility and which plants can grow, the safety and taste of food and water, the corrosion of metals, and countless chemical reactions in manufacturing. Environmental concerns like acid rain and ocean acidification are, at heart, stories about shifting pH and its consequences for ecosystems. Measuring and adjusting pH — with indicators, meters, and buffers — is therefore routine across chemistry, biology, agriculture, medicine, and engineering. Converting between concentration and pH, as this calculator does, is the first step in understanding and controlling any of it.
الأسئلة الشائعة
How do I calculate pH from concentration?
pH is the negative base-ten logarithm of the hydrogen-ion concentration in moles per litre: pH = −log₁₀[H⁺]. A concentration of 0.0001 (10⁻⁴) mol/L gives a pH of 4. To reverse it, [H⁺] = 10^(−pH), so a pH of 4 means a concentration of 10⁻⁴ mol/L.
What does each pH unit represent?
A tenfold change in hydrogen-ion concentration, because the scale is logarithmic. A pH 3 solution is ten times as acidic as pH 4 and a hundred times as acidic as pH 5. This is why small pH differences correspond to large differences in actual acidity.
What is the relationship between pH and pOH?
At 25°C they always add to 14, because the product of hydrogen- and hydroxide-ion concentrations in water is fixed at 10⁻¹⁴. So pOH = 14 − pH. Knowing one immediately gives the other, and with it the concentration of both ions.
What pH is acidic, neutral, or basic?
Below 7 is acidic, exactly 7 is neutral (pure water at 25°C), and above 7 is basic or alkaline. The scale normally runs 0 to 14, though very concentrated strong acids or bases can push slightly beyond those ends.