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.

pHIon 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:

ProductpH
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

IndicatorpH - RangeColor Cange
AcidBase
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

Related Topics

  • Basics - Basic Information as SI-system, Unit converters, Physical constants
  • Miscellaneous - Miscellaneous engineering related topics like Beaufort Wind Scale, CE-marking, drawing standards and more

Related Documents

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