pH Solution Calculator

pH is a measure of the acidity or alkalinity of an aqueous solution. It is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = -log10[H+]. The pH scale runs from 0 to 14 — values below 7 indicate an acidic solution, 7 is neutral (e.g. pure water), and above 7 is basic (alkaline).

pOH is a measure of the hydroxide ion OH- concentration in a solution: pOH = -log10[OH-]. At 25°C the relationship pH + pOH = 14 holds. This means that if you know the pH, you can easily calculate the pOH and vice versa.

Use the formula: pH = -log10(cH), where cH is the H+ concentration in mol/L (mol/dm³). For example, for cH = 0.01 mol/L: pH = -log10(0.01) = 2. The solution is therefore acidic.

Common acidic solutions: lemon juice pH ≈ 2-3, vinegar pH ≈ 2.5-3.5, coffee pH ≈ 5, rain water pH ≈ 5.6. The lower the pH, the more acidic the solution and the higher the H+ concentration.

Common basic (alkaline) solutions: baking soda (NaHCO3) pH ≈ 8.3, washing soda (Na2CO3) pH ≈ 11, bleach pH ≈ 12-13, sodium hydroxide (NaOH) pH ≈ 14. The higher the pH, the more basic the solution.

Kw is the water ion product, equal to the equilibrium constant of the water auto-ionisation reaction: H2O ⇌ H+ + OH-. At 25°C, Kw = [H+] × [OH-] = 10^-14 (mol/L)². From this it follows that pH + pOH = 14 for pure water and dilute aqueous solutions at this temperature.

pH can be measured using indicator papers (litmus, pH paper strips), electronic pH meters with glass electrodes, and coloured indicators (phenolphthalein, methyl orange). The most accurate method is the electrochemical one using a pH meter.

The logarithmic scale arises from the enormous range of H+ concentrations — from 1 mol/L (strong acid) to 10^-14 mol/L (strong base). The logarithm compresses these values onto a convenient 0-14 scale instead of dealing with very small numbers. A change of 1 pH unit corresponds to a 10-fold change in H+ concentration.

Yes, in theory pH can exceed the 0-14 range at very high acid or base concentrations. For example, concentrated sulfuric acid may have a pH of about -3. However, the concept of pH is typically applied to aqueous solutions with H+ concentrations in the range 1-10^-14 mol/L.

pH is critical in biochemistry — enzymes operate within a narrow pH range, and human blood maintains pH 7.35-7.45. In agriculture, soil pH determines nutrient availability. In industry, pH controls reaction rates, product stability and corrosion processes.