What pH tells us
The pH value is a number from 0 to 14 that shows how acidic or alkaline a solution is. It depends on the concentration of hydrogen ions, [H+], in mol dm−3:
Key idea / 要点
pH = −log10[H+]
The higher the [H+], the lower the pH and the more acidic the solution.
- pH < 7 — acidic (high [H+], low [OH−])
- pH = 7 — neutral (pure water, [H+] = [OH−])
- pH > 7 — alkaline (low [H+], high [OH−])
Each drop of one pH unit means [H+] becomes 10 times larger. A solution of pH 2 has ten times the H+ concentration of a solution of pH 3.
Measuring pH
pH can be estimated with universal indicator (a colour chart from red at pH 1 to violet at pH 14) or measured accurately with a pH meter. Litmus only tells acidic (red) from alkaline (blue) but not the exact value.
Everyday examples help fix the scale in mind: lemon juice and vinegar are strongly acidic (pH about 2–3), rainwater is slightly acidic, pure water is neutral at pH 7, and household cleaners such as ammonia are alkaline (pH 11–13). Blood is kept very close to pH 7.4. Because the scale is logarithmic, a small change in pH represents a large change in [H+], which is why pH is such a compact and useful measure.
Worked example
Find the pH of 0.01 mol dm−3 hydrochloric acid.
HCl is a strong monoprotic acid, so [H+] = 0.01 = 1 × 10−2 mol dm−3.
pH = −log(1 × 10−2) = 2
Remember
- Lower pH ⇒ more acidic ⇒ higher [H+].
- A change of 1 pH unit is a 10-fold change in [H+].
- Concentration and strength are different: pH depends on the actual [H+] in solution.