pH and pOH formulas
At 25 °C the ion product of water gives pH + pOH = 14, so pOH = 14 − pH. The concentrations are [H+] = 10^−pH and [OH−] = 10^−pOH, both in mol/L. A pH below 7 is acidic, above 7 is basic, and exactly 7 is neutral. For concentration of charged species in titrations, pair this with the normality calculator.
How to use this calculator
Enter a pH value (typically 0 to 14). The calculator subtracts it from 14 to get pOH and raises 10 to the appropriate negative power to get the hydrogen and hydroxide ion concentrations. Results assume a temperature of 25 °C, where the water constant Kw is 1×10⁻¹⁴.
Remember that the pH scale is logarithmic, so each whole unit represents a tenfold change in [H+]. A shift of two pH units is a hundredfold change in acidity, which is why small pH movements matter so much in biology and water treatment. If you only need the acidic side of the pair, the pH calculator is the shorter route.
Worked example
For a solution with pH 3: pOH = 14 − 3 = 11, [H+] = 10⁻³ = 1×10⁻³ mol/L and [OH−] = 10⁻¹¹ = 1×10⁻¹¹ mol/L. The low pH and high [H+] confirm an acidic solution. Both start from a concentration in moles per litre, which the molarity calculator provides.
Frequently asked questions
- What is the relationship between pH and pOH?
- At 25 C they always add to 14, so pOH = 14 - pH. This comes from the ion product of water, Kw = 1x10^-14.
- How do I get [H+] from pH?
- Take 10 to the power of negative pH: [H+] = 10^-pH, giving the hydrogen ion concentration in mol/L.
- Is pH 7 always neutral?
- Only at 25 C. Because Kw changes with temperature, the neutral pH shifts slightly above or below 7 at other temperatures.
- What pH range is acidic or basic?
- A pH below 7 is acidic, a pH above 7 is basic, and a pH of exactly 7 is neutral at 25 C.
- Can pH fall below 0 or rise above 14?
- Yes. The 0-14 range simply covers the concentrations met in ordinary aqueous solutions. Very concentrated strong acids can show a negative pH, and concentrated strong bases can exceed 14, though the simple formulas become unreliable at those extremes.