pH / Acid-Base Calculator
Convert between pH, pOH, and hydrogen ion concentration. Explore the pH scale with common substances.
Common Substances
What is pH?
pH is a logarithmic scale that measures the acidity or basicity (alkalinity) of a solution. It ranges from 0 to 14, with 7 being neutral (pure water). Values below 7 are acidic (higher concentration of hydrogen ions H+), and values above 7 are basic (higher concentration of hydroxide ions OH-). Each whole number change represents a tenfold change in acidity.
The pH scale was introduced by Danish chemist Soren Sorensen in 1909. The formula is pH = -log10[H+], where [H+] is the concentration of hydrogen ions in moles per liter. Because it is logarithmic, pH 3 is ten times more acidic than pH 4 and one hundred times more acidic than pH 5.
The pH scale
- pH 0-2: Strongly acidic. Battery acid (0), stomach acid (1-2).
- pH 3-4: Moderately acidic. Vinegar (2.5), orange juice (3.5).
- pH 5-6: Weakly acidic. Coffee (5), milk (6.5).
- pH 7: Neutral. Pure water at 25 degrees Celsius.
- pH 8-14: Basic/alkaline. Baking soda (8.3), bleach (12.5), drain cleaner (14).
How to use this tool
Enter a pH value to calculate the hydrogen ion concentration, or enter a hydrogen ion concentration to calculate the pH. The tool also shows the corresponding pOH value and hydroxide ion concentration. A visual scale indicates where the value falls relative to common substances.
Why pH matters
pH affects chemical reactions, biological processes, and material properties. Blood pH is tightly regulated between 7.35 and 7.45 — outside this range, enzymes malfunction and cells die. Soil pH determines which nutrients plants can absorb. Pool water pH (7.2-7.8) prevents equipment corrosion and eye irritation. Industrial processes like food production, water treatment, and pharmaceutical manufacturing require precise pH control.
Worked example
A solution has a hydrogen-ion concentration of 1 × 10⁻³ mol/L. pH = −log₁₀[H⁺] = −log₁₀(10⁻³) = 3, so it is acidic. To find the hydroxide concentration, use pH + pOH = 14, giving pOH = 11 and [OH⁻] = 10⁻¹¹ mol/L. Each whole pH unit represents a tenfold change in acidity.
Common mistakes
The most common mistake is forgetting that the pH scale is logarithmic, so pH 4 is ten times more acidic than pH 5, not 25% more. Another is assuming pH always runs 0–14; concentrated strong acids and bases can fall below 0 or above 14. Temperature also shifts neutrality — pure water is neutral at pH 7 only at 25 °C.
Frequently asked questions
Can pH be negative or above 14?
Yes. Very concentrated strong acids can have pH below 0, and very concentrated strong bases can exceed pH 14. The 0-14 range assumes dilute aqueous solutions at 25 degrees Celsius. Concentrated sulfuric acid has a pH around -1, and concentrated sodium hydroxide solution can reach pH 15.
Why is the pH scale logarithmic?
Hydrogen ion concentrations in common solutions span many orders of magnitude — from about 10 moles per liter (strong acid) to 10^-14 moles per liter (strong base). A logarithmic scale compresses this enormous range into a manageable 0-14 scale. Without the log scale, comparing the acidity of vinegar and battery acid would require very large or very small numbers.
Can pH be negative or above 14?
Yes. The 0–14 range simply covers common dilute solutions. A very concentrated strong acid such as 10 mol/L HCl can have a negative pH, and a concentrated strong base can exceed 14. The scale is defined by the logarithm of ion activity, which is not capped at those limits.