About the pH Calculator
pH measures how acidic or basic a solution is, on a scale that usually runs from 0 to
- It is defined from the concentration of hydrogen ions, [H⁺], in moles per litre:
the more hydrogen ions, the lower the pH and the more acidic the solution. pH matters everywhere from chemistry labs and swimming pools to soil, food, blood and the oceans.
This pH calculator works four ways. It finds pH from a hydrogen ion concentration, from a hydroxide ion concentration, converts a pH value back into concentrations, and works out the pH of a weak acid from its concentration and dissociation constant Ka. It also gives pOH and says how acidic or basic the solution is compared with pure water.
How to Use the pH Calculator
Choose From [H⁺], From [OH⁻], From pH or Weak acid (Ka).
Enter the concentration in mol/L as a decimal — 0.001 for 1 × 10⁻³ — or the pH value.
For a weak acid, enter its concentration and Ka.
For strong acids such as HCl and strong bases such as NaOH, the concentration of the acid or base is the ion concentration, since they dissociate completely.
The Formulas
pH = −log₁₀[H⁺] [H⁺] = 10^(−pH)
pOH = −log₁₀[OH⁻] [OH⁻] = 10^(−pOH)
pH + pOH = 14 [H⁺] × [OH⁻] = 1 × 10⁻¹⁴ (water at 25 °C)
weak acid HA ⇌ H⁺ + A⁻:
Ka = x² ÷ (C − x)
[H⁺] = x = (−Ka + √(Ka² + 4 Ka C)) ÷ 2Step-by-Step Examples
0.001 M hydrochloric acid (strong).
[H⁺] = 0.001 mol/L
pH = −log₁₀(0.001) = 3.000.01 M sodium hydroxide (strong base).
pOH = −log₁₀(0.01) = 2.00
pH = 14 − 2 = 12.000.1 M acetic acid, Ka = 1.8 × 10⁻⁵ (weak).
x² ÷ (0.1 − x) = 1.8 × 10⁻⁵
x = [H⁺] = 0.001333 mol/L (1.33% dissociated)
pH = −log₁₀(0.001333) = 2.88pH 4.5 to concentration: [H⁺] = 10⁻⁴·⁵ = 3.162 × 10⁻⁵ mol/L, and pOH = 9.5.
The pH Scale
pH Example [H⁺] (mol/L)
0–1 battery acid 1 to 0.1
2 lemon juice, vinegar (~2.4) 0.01
4 tomato juice 0.0001
5.6 clean rain 2.5 × 10⁻⁶
7 pure water at 25 °C 1 × 10⁻⁷
7.4 human blood 4 × 10⁻⁸
8.3 baking soda solution 5 × 10⁻⁹
11 household ammonia 1 × 10⁻¹¹
13 bleach, oven cleaner 1 × 10⁻¹³Values for everyday substances vary with concentration and are approximate.
A Logarithmic Scale
Because pH is a logarithm, each whole unit is a tenfold change in hydrogen ion concentration. pH 3 is ten times as acidic as pH 4 and a thousand times as acidic as pH 6. That is why a drop in ocean pH of just 0.1 since pre-industrial times represents roughly a 26 percent rise in hydrogen ion concentration, and why blood pH must stay within a narrow range of about 7.35 to 7.45.
Strong and Weak Acids
Strong acids — hydrochloric, nitric, sulfuric (first proton) — dissociate completely in water, so the hydrogen ion concentration equals the acid concentration. Weak acids — acetic, citric, carbonic — only partly dissociate, reaching an equilibrium described by Ka. The smaller Ka is, the weaker the acid. For 0.1 M acetic acid, only about 1.3 percent of the molecules give up their hydrogen, so the pH is 2.88 rather than the 1.00 of a strong acid at the same concentration.
Neutral Is Not Always 7
pH 7 is neutral only at 25 °C. The self-ionisation of water increases with temperature, so at 37 °C neutral water has a pH of about 6.8, and at 100 °C about 6.1 — still neutral, because hydrogen and hydroxide ions remain equal. This calculator uses the standard 25 °C relationship pH + pOH = 14. Very concentrated acids and bases can also fall outside the usual 0 to 14 range.
Measuring pH
In the laboratory, pH is measured most accurately with a pH meter, which uses a glass electrode sensitive to hydrogen ions. The meter must be calibrated regularly with buffer solutions of known pH, typically 4, 7 and 10, and the electrode rinsed between samples. For quicker, rougher readings, indicator paper or universal indicator changes colour across the pH range, from red in strong acid through green near neutral to purple in strong alkali. Many natural substances are indicators too: red cabbage juice turns pink in acid and green-yellow in alkali. Pool and aquarium test kits use indicator dyes and a colour chart, and garden soil test kits work the same way.
Buffers
A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It contains a weak acid and its conjugate base, such as acetic acid and sodium acetate, which mop up added hydrogen or hydroxide ions. Blood is buffered mainly by carbonic acid and bicarbonate, which is how it stays within its narrow range. The pH of a buffer is given by the Henderson–Hasselbalch equation, pH = pKa + log₁₀([A⁻] ÷ [HA]), so a buffer made from equal amounts of acid and conjugate base has a pH equal to the acid's pKa. Acetic acid, with a pKa of about 4.74, makes buffers near pH 4.7.
pH in Everyday Life
Most plants grow best in soil with a pH between about 6 and 7.5, while blueberries and azaleas prefer more acidic soil. Swimming pools are kept at about pH 7.2 to 7.8 so that chlorine works well without irritating eyes. Skin is slightly acidic, at around pH 5.
Understanding Your Result
The headline is the pH, to two decimal places.
The acidic or basic line classifies the solution and compares it with pure water.
The ion concentrations line gives [H⁺] and [OH⁻].
The pOH line gives the pOH.
When Should You Use This Calculator?
Use it for chemistry homework on acids, bases and equilibria.
Use it to convert a pH reading into ion concentrations.
Use it to estimate the pH of a weak acid solution.
Common Mistakes
Forgetting the minus sign. pH is the negative logarithm.
Treating weak acids as strong. Use Ka for acetic, citric and similar acids.
Assuming pH 7 is always neutral. It depends on temperature.
Reading the scale as linear. One pH unit is a tenfold change.