pH Calculator
Calculate pH, pOH, and hydrogen ion concentration for acid-base chemistry. Free online pH calculator with classification, ion concentrations, and visual charts.
About This Calculator
The pH Calculator computes pH, pOH, and hydroxide ion concentration from the hydrogen ion concentration [H⁺]. Enter the H⁺ concentration in mol/L to instantly get the complete acid-base profile of your solution.
pH is defined as the negative logarithm (base 10) of hydrogen ion activity: pH = -log10[H⁺]. At 25 degC, a solution with pH = 7 is neutral, pH below 7 is acidic, and pH above 7 is basic. The pOH = 14 - pH, and [OH⁻] = 10^(-pOH) at 25 degC.
About pH Scale
The pH scale ranges from 0 to 14 for most aqueous solutions. Strong acids like HCl (1 M) have pH ≈ 0, while strong bases like NaOH (1 M) have pH ≈ 14. The pH scale is logarithmic, meaning each whole pH unit represents a tenfold change in H⁺ concentration.
Frequently Asked Questions
What is pH and how is it calculated?
pH = -log10[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic (alkaline).
What is the relationship between pH and pOH?
pH + pOH = 14 at 25 degC (298 K). This relationship comes from the ion product of water (Kw = [H⁺][OH⁻] = 1x10⁻¹^4 at 25 degC).
What is a strong acid vs a weak acid?
Strong acids (HCl, H2SO4, HNO3) dissociate completely in water. Weak acids (CH3COOH, HF, H2CO3) only partially dissociate, establishing an equilibrium between undissociated and dissociated forms.
What is the pH of common substances?
Pure water pH = 7, stomach acid pH ≈ 2, lemon juice pH ≈ 2.5, vinegar pH ≈ 3, coffee pH ≈ 5, milk pH ≈ 6.5, blood pH ≈ 7.4, seawater pH ≈ 8, bleach pH ≈ 12.
How does temperature affect pH?
pH changes with temperature because the ion product of water (Kw) increases with temperature. At 0 degC, neutral pH ≈ 7.47; at 100 degC, neutral pH ≈ 6.14. pH meters automatically compensate for temperature.