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Hydrogen Ion Concentration Calculator

Convert a pH value into hydrogen ion concentration, pOH, and hydroxide ion concentration.

How it is calculated

Starting from pH, the tool computes [H+] = 10−pH in mol/L, then pOH = 14 − pH and [OH−] = 10−pOH in mol/L. The constant 14 is the value of pH + pOH for water at 25 °C, so results for solutions at other temperatures are only approximate. The concentrations are shown in scientific notation with four significant figures, for example 3.162e-5 means 3.162 × 10−5. The pOH is shown with two decimals. The accepted pH range is 0 to 14 inclusive: a value below 0 or above 14 returns no result, even though very strong acids and bases can go beyond it. Because pH is logarithmic, each whole unit changes the concentration by a factor of 10.

Worked example

pH 4.5. Hydrogen ions: 10−4.5 = 3.162e-5 mol/L. pOH: 14 − 4.5 = 9.50. Hydroxide ions: 10−9.5 = 3.162e-10 mol/L. The tool shows [H+] of 3.162e-5 mol/L, [OH−] of 3.162e-10 mol/L and a pOH of 9.50. The product of the two concentrations is 10−14, which is a quick check that the figures agree.

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Common mistakes

  • Treating pH as a concentration that changes in a straight line. Going from pH 4.5 to pH 5.5 divides [H+] by 10, not by 1.2.
  • Reading the e-notation wrongly. 3.162e-5 is 0.00003162, and e-10 is a much smaller value, not 10 times smaller than e-5.
  • Entering a pH of 15 or −1 and expecting a number. The tool accepts only 0 to 14 and returns no result outside it.
  • Applying the result at a temperature far from 25 °C. The relation pH + pOH = 14 holds only near that temperature.
  • Confusing [H+] with [OH−]. The first line is the acid ions, and for pH above 7 it is the smaller of the two.

About this calculator

pH is defined as the negative base-ten logarithm of the hydrogen ion concentration, so reversing the definition gives [H+] = 10 raised to the power of negative pH, expressed in moles per liter. Because water’s autoionization constant at 25 °C fixes pH plus pOH at 14, this calculator also derives pOH as fourteen minus pH and then the hydroxide ion concentration [OH-] as 10 raised to the power of negative pOH, using the same logarithmic relationship.

Chemistry students, lab technicians, and environmental analysts use these conversions constantly: a water-quality lab reporting a stream’s pH needs the underlying ion concentrations to assess corrosivity or aquatic toxicity thresholds, and anyone preparing a buffer solution or interpreting a titration curve needs to move fluently between pH and molar ion concentrations.

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