Library / Solution chemistry, equilibria and kinetics / SCH-12
Solution chemistry, equilibria and kinetics
Neutral pH at a specified temperature
pH of pure water at neutrality, where hydrogen and hydroxide activities are equal, from the water ion product Kw at the chosen temperature.
- Ref
- SCH-12
- Rev
- v1
- Field
- Solution chemistry, equilibria and kinetics
- Result
- pHneutral [pH]
Pure water is neutral when [H⁺] = [OH⁻] = √Kw, so pH = −½ log₁₀ Kw. At 25 °C Kw = 1.0 × 10⁻¹⁴ and the neutral pH is 7.00. Kw rises with temperature, so the neutral pH falls: it is 6.8 at Kw = 2.5 × 10⁻¹⁴ (about 40 °C). Neutral is not always pH 7, and a pH below 7 can still be basic at higher temperatures.
pH of pure water at neutrality, where hydrogen and hydroxide activities are equal, from the water ion product Kw at the chosen temperature. It is 7.00 at 25 °C and falls as the water gets warmer.
Calculator
- Hydronium at neutrality, √Kw1.581×10−7 mol/L
- Neutral pOH6.801
- Shift from pH 7−0.199 pH
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- With Kw = 2.5×10−14 the neutral pH is 6.801, 0.199 units below 7.
- Each factor of 10 in Kw moves the neutral pH by 0.5, so a tenfold rise in Kw lowers it from 7.0 to 6.5.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (pH) | Local step | Change in result |
|---|---|---|---|---|---|---|
| Kw Water ion product | 1e-15 to 1e-12 | 6 | 7.5 | 1.5 | +10% | −0.304 % |
Sweep of Water ion product
| Water ion product | Neutral pH (pH) |
|---|---|
| 1e-15 | 7.5 |
| 1.259e-13 | 6.45 |
| 2.507e-13 | 6.3 |
| 3.756e-13 | 6.213 |
| 5.005e-13 | 6.15 |
| 6.254e-13 | 6.102 |
| 7.502e-13 | 6.062 |
| 8.751e-13 | 6.029 |
| 1e-12 | 6 |
Sweep one variable
Variables and how each one contributes
Neutral pH
What the equation gives you. Everything else feeds it.
Water ion product
The water ion product at the chosen temperature (dimensionless, concentration scale). A larger Kw lowers the neutral pH. Raising it by 10 % (to 2.75×10−14) moves the result by −0.304 %, from 6.801 to 6.78 pH.
Worked example with the values above
- Take the log: log₁₀ Kw = log₁₀ 2.5×10−14 = −13.602.
- Multiply by −½.
- pH(neutral) = 6.801.
Assumptions and limits
- Dilute solutions in which concentrations stand for activities, with constants on the concentration scale.
- No other acids, bases or dissolved gases such as CO₂ unless included.
Common mistakes
- Using pKa where Ka is needed, or the reverse.
- Using the Kw of 25 °C at other temperatures.
- Ignoring water autoionisation for acids below about 10⁻⁶ M.
Statistics and uncertainty
Acid constants change with ionic strength and temperature, so results are best to about 0.05 pH units even with accurate constants. Calibration of a pH electrode contributes about 0.02 pH.
Where it is used
- Calculating pH of dilute acids, salts and intermediate ions.
- Finding which species dominates at a given pH.
Maths you need
- pH: pH = −log₁₀ [H⁺]
- Acid dissociation constant: Ka = [H⁺][A⁻]/[HA]
- Water ion product: Kw = [H⁺][OH⁻]