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 (K_w) - 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

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

Next:  "Factors that Affect the Relative Strengths of Acids and Bases"