pH Definition An introduction to pH

pH can be viewed as an abbreviation for power of Hydrogen - or more completely, power of the concentration of the Hydrogen ion.
The mathematical definition of pH is a bit less intuitive but in general more useful. It says that the pH is equal to to the negative logarithmic value of the Hydrogen ion (H⁺) concentration, or

pH = -log [H⁺]

pH can alternatively be defined mathematically as the negative logarithmic value of the Hydroxonium ion (H₃O⁺) concentration. Using the Bronsted-Lowry approach

pH = -log [H₃O⁺]

pH values are calculated in powers of 10. The hydrogen ion concentration of a solution with pH 1.0 is 10 times larger than the hydrogen concentration in a solution with pH 2.0. The larger the hydrogen ion concentration, the smaller the pH.

• when the pH is above 7 the solution is basic (alkaline)
• when the pH is below 7 the solution is acidic

In pure neutral water the concentration of hydrogen and hydroxide ions are both 10^-7 equivalents per liter.

pH	Ion Concentration (gram equivalent per liter)	Type of Solution
0	1.0	Acid Solution - Hydrogen ions - H+
1	0.1
2	0.01
3	0.001
4	0.0001
5	0.00001
6	0.000001
7	0.0000001	Neutral Solution
8	0.000001	Basic (alkaline) Solution - Hydroxide ions - OH-
9	0.00001
10	0.0001
11	0.001
12	0.01
13	0.1
14	1.0

Some common Products and their pH Values

pH values in some common products:

Product	pH
Battery Acid	0
HCl in stomach acid	1
Lemon juice, vinegar	2-3
Orange juice	3-4
Acid rain	4
Black coffee	5
Urine, salvia	6
Pure water	7
Sea water	8
Baking soda	9
Ammonia solution	10-11
Soapy water	12
Bleach	13
Oven cleaner	13-14
Drain cleaner	14

Acid-Base Indicators

Indicator	pH - Range	Color Cange
		Acid	Base
Thymol blue	1.2 - 2.8	red	yellow
Pentamethoxy red	1.2 - 2.3	red - violet	colorless
Tropeolin	1.3 - 3.2	red	yellow
2,4 - Dinitrophenol	2.4 - 4.0	colorless	yellow
Methyl yellow	2.9 - 4.0	red	yellow
Methyl orange	3.1 - 4.4	red	orange
Congo red	3.0 - 4.2	blue-violet	red-orange
Bromphenol blue	3.0 - 4.6	yellow	blue - violet
Tetrabromphenol blue	3.0 - 4.6	yellow	blue
Alizarin sodium sulfonate	3.7 - 5.2	yellow	violet
α - Naphthyl red	3.7 - 5.0	red	yellow
p - Ethoxychrysoidine	3.5 - 5.5	red	yellow
Bromcresol green	4.0 - 5.6	yellow	blue
Methyl red	4.4 - 6.2	red	yellow
Bromcresol purple	5.2 - 6.8	yellow	purple
Chlorphenol red	5.4 - 6.8	yellow	red
Bromphenol blue	6.2 - 7.6	yellow	blue
p - Nitrophenol	5.0 - 7.0	colorless	yellow
Litmus	5.0 - 8.0	red	blue
Azolitmin	5.0 - 8.0	red	blue
Phenol red	6.4 - 8.0	yellow	red
Neutral red	6.4 - 8.0	red	yellow
Rosolic acid	6.8 - 8.0	yellow	red
Cresol red	7.2 - 8.8	yellow	red
α - Naphtholphthalein	7.3 - 8.7	rose	green
Tropeolin	7.6 - 8.9	yellow	rose - red
Thymol blue	8.0 - 9.6	yellow	blue
Phenolphthalein	8.0 - 10.0	colorless	red
α - Naphtholbenzein	9.0 - 11.0	yellow	blue
Thymolphthalein	9.4 - 10.6	colorless	blue
Nile blue	10.1 - 11.1	blue	red
Alizarin yellow	10.0 - 12.0	yellow	lilac
Salicyl yellow	10.0 - 12.0	yellow	orange - brown
Diazo violet	10.1 - 12.0	yellow	violet
Tropeolin	11.0 - 13.0	yellow	orange - brown
Nitramine	11.0 - 13.0	colorless	orange - brown
Poirrier's blue	11.0 - 13.0	blue	violet - pink
Trinitrobenzoic acid	12.0 - 13.4	colorless	orange - red