pH (hydrogen ion exponent) expresses the strength of acidity or basicity numerically. It is widely used in daily life and experiments.
Basic Knowledge
A very small fraction of water is always ionized. At 25 ℃:
[H+][OH-] = 1.0×10-14 (mol/L)2 (ionic product of water Kw)
In pure water, [H+] = [OH-] = 1.0×10-7 mol/L, so it is neutral.
pH is defined as follows:
pH = -log10[H+]
Thus, if [H+] = 1.0×10-n mol/L, then pH = n.
・pH < 7: acidic ([H+] is large)
・pH = 7: neutral
・pH > 7: basic ([H+] is small)
When pH decreases by 1, [H+] increases tenfold.
Ionic product of water (Kw = [H+][OH-] = 1.0×10-14 (25 ℃))
pH (pH = -log[H+]. Hydrogen ion exponent)
Neutral (pH = 7. [H+] = [OH-] = 10-7 mol/L)
Indicator (Changes color according to pH. Litmus, BTB, phenolphthalein)
BTB solution (Acidic = yellow, neutral = green, basic = blue)
Phenolphthalein (Red in basic solutions, transition range pH 8.3〜10)
Further Exploration (Background and Significance)
Basic examples of pH calculations:
・0.10 mol/L HCl (monoprotic strong acid, α=1): [H+] = 0.10 = 1.0×10-1 → pH = 1
・0.0010 mol/L HCl: [H+] = 1.0×10-3 → pH = 3
・0.10 mol/L NaOH (monobasic strong base): [OH-] = 0.10 → [H+] = 1.0×10-13 → pH = 13
Familiar examples: gastric juice pH ≒ 1〜2, lemon juice pH ≒ 2, rainwater pH ≒ 5〜6 (dissolved CO2), seawater pH ≒ 8, soapy water pH ≒ 10, bleach pH ≒ 12.
pH can be measured with pH test paper, BTB solution, a pH meter, and so on.
- pH = -log[H+]
- If [H+] = 10-n, then pH = n
- Pure water and neutral solutions have pH = 7 (25 ℃)
- A pH decrease of 1 = a tenfold increase in [H+]
- Kw = [H+][OH-] = 10-14
- For a strong base: convert [OH-] to [H+] before finding pH
- BTB: acidic = yellow, neutral = green, basic = blue
Points of Caution (Common Confusions)
① For strong acids and strong bases, concentration can be used directly by assuming α ≒ 1, but for weak acids and weak bases, pH cannot be found without considering the degree of ionization. ② To find [H+] in a basic solution, use Kw = 10-14. ③ pH can be a decimal as well as an integer (for example, pH 2.5). ④ Low pH = strong acidity; high pH = strong basicity. Do not reverse them.
Practice
- Find the pH of a 0.010 mol/L HCl aqueous solution.
- Find the pH of a 0.10 mol/L NaOH aqueous solution.
- State [H+] and [OH-] in pure water. (25 ℃)