OpenSpecEngineering

Converts between pH, pOH, the hydrogen ion concentration [H⁺] and the hydroxide ion concentration [OH⁻] of a water solution, at a water temperature you choose. It also gives the neutral pH at that temperature and says whether the solution is acidic or basic. Chemists, water treatment operators and lab technicians use it to turn a meter reading into concentrations, or the reverse.

Equations

pH = −log₁₀[H⁺]
pOH = −log₁₀[OH⁻]
pH + pOH = pK_w
neutral pH = pK_w/2

[H⁺] and [OH⁻] are molar concentrations in mol/L. K_w = [H⁺][OH⁻] is the ion product of water, and pK_w = −log₁₀K_w [1]. Choose Given to say which of the four values you know. The page finds the other three from these equations.

Inputs and limits

• Water Temperature sets pK_w. The default is the rounded textbook value K_w = 1.0×10⁻¹⁴ (pK_w 14) for 25 °C. The other choices, from 0 to 100 °C, use Marshall and Franck’s values [2] as tabulated by Harris [3]. Choose Custom to type your own pK_w, between 10 and 16.
• pH and pOH may run from −5 to 19. Concentrations must be positive.
• The equations treat concentration as activity. That is close for dilute solutions, below about 0.01 mol/L of dissolved salts. In strong acids, strong bases and brines the measured pH departs from −log₁₀[H⁺].

Common mistakes

• Neutral is pH 7 only near 25 °C. Hot water is neutral below 7 and cold water above it. Judge a hot sample against the neutral pH shown, not against 7.
• A pH meter with automatic temperature compensation corrects the electrode, not the sample’s chemistry. Still choose the sample’s own temperature here.
• One pH unit is a factor of ten in [H⁺]. Don’t average pH readings; average the concentrations.

Worked example

A process water sample reads pH 6.8 on a meter at 50 °C. Is it acidic, and what are the ion concentrations?

1. Choose Given pH.
2. Choose Water Temperature 50 °C (pKw 13.275). The pKw field fills in 13.275.
3. Enter pH 6.8.
4. Press Solve. The Solution shows: pOH: 6.475
   [H⁺]: 1.58489e-7 mol/L
   [OH⁻]: 3.34965e-7 mol/L
   Neutral pH (= pKw/2): 6.6375
   Classification: Basic

Check by hand: pOH = 13.275 − 6.8 = 6.475, and 10^−6.8 = 1.585×10⁻⁷. The product [H⁺][OH⁻] = 5.31×10⁻¹⁴ = 10^−13.275, as it should be. The sample is slightly basic even though its pH is below 7, because water at 50 °C is neutral at pH 6.64.