Acids and Bases :  pH

1909 - S. P. L. Sørenson - Danish biochemist who suggested the use of a logarithmic scale to express the concentration of the H3O+ ion

pH - The negative of the log of the H3O+ (hydronium) ion concentration:

pOH - The negative of the log of the OH- (hydroxide) concentration:

Water-dissociation equilibrium constant (Kw) - The product of the
equilibrium concentration of the H3O+ and OH- ions in an aqueous solution is
equal to 1.00 x 10-14 at 25C:

When the logs of both sides are taken:

NOTE: When the pH is doubled, the [H3O+ ] decreases by a factor of 100.
When the pH is quadrupled, the [H3O+ ] decreases by a factor of 10,000

• A substance is acidic if the pH is less than 7.

• A substance is basic if the pH is greater than 7.

• A substance is neutral if the pH is equal to 7.

In pure water, the concentration of the hydronium and hydroxide ions are equal.  At 25C:

Relationship Between[H3O+] and [OH-]
with pH and pOH of an Aqueous Solution

 [H3O+] (M) [OH-] (M) pH pOH 1.0 1.0 x 10-14 0 14 Acidic 1.0 x 10-1 1.0 x 10-13 1 13 1.0 x 10-2 1.0 x 10-12 2 12 1.0 x 10-3 1.0 x 10-11 3 11 1.0 x 10-4 1.0 x 10-10 4 10 1.0 x 10-5 1.0 x 10-9 5 9 1.0 x 10-6 1.0 x 10-8 6 8 1.0 x 10-7 1.0 x 10-7 7 7 Neutral 1.0 x 10-8 1.0 x 10-6 8 6 Basic 1.0 x 10-9 1.0 x 10-5 9 5 1.0 x 10-10 1.0 x 10-4 10 4 1.0 x 10-11 1.0 x 10-3 11 3 1.0 x 10-12 1.0 x 10-2 12 2 1.0 x 10-13 1.0 x 10-1 13 1 1.0 x 10-14 1.0 14 0

pH of 0.1 M Solutions of Common Acids and Bases

 Compound Name Formula pH hydrochloric acid HCl 1.1 sulfuric acid H2SO4 1.2 sodium bisulfate NaHSO4 1.4 sulfurous acid H2SO3 1.5 phosphoric acid H3PO4 1.5 hydrofluoric acid HF 2.1 acetic acid HC2H3O2 2.9 carbonic acid H2CO3 2.8 hydrogen sulfide H2S 4.1 sodium dihydrogen phosphate NaH2PO4 4.4 ammonium chloride NH4Cl 4.6 hydrocyanic acid HCN 5.1

 Compound Name Formula pH sodium sulfate Na2SO4 6.1 sodium chloride NaCl 6.4 sodium acetate NaC2H3O2 8.4 sodium bicarbonate NaHCO3 8.4 sodium biphosphate Na2HPO4 9.3 sodium sulfite NaSO3 9.8 sodium cyanide NaCN 11.0 ammonia NH3 11.1 sodium carbonate NaCO3 11.6 sodium phosphate Na3PO4 12.0 sodium hydroxide NaOH 13.0

Acid-base indicator - A weak acid or weak base which changes color when it gains or loses an H+ ion.

Examples of Acid-Base Indicators

 Indicator pH Range Color Change Methyl violet Thymol blue Bromophenol blue Methyl orange Methyl red Litmus Bromocresol purple Bromophenol red Bromothymol blue Cresol red Thymol blue Phenolphthalein Alizarin yellow 0.0 - 1.6 1.2 - 2.8 3.0 - 4.6 3.2 - 4.4 4.4 - 6.2 5 - 8 5.2 - 6.8 5.2 - 6.8 6.2 - 7.6 7.2 - 8.8 8.0 - 9.6 8.0 - 10.0 10.0 - 12.0 yellow  blue-violet red  yellow yellow  blue-violet red yellow-orange red yellow pink  blue yellow purple yellow red yellow blue yellow  red yellow blue colorless  pink yellow red-violet