The pH of a substance is a measure of its acidity or alkalinity just as a degree is a measure of temperature. A specific pH value tells the exact amount of acidity or alkalinity just as 25° tells the exact temperature rather than saying a solution is hot or cold. pH is the measurement of the ratio of acid to base(alkalinity) of a solution. pH is the effective acidity or alkalinity of a solution. pH is measured on a scale of 0 to 14 with a pH of 7 as the neutral point.

Scale
Sulfuric acid1N................................................................. 0.3
Blood, human.............................................................. 7.3 -7.5
Limes................................................................................. 2.0
Egg White ..................................................................7.6 - 8.0
Wines........................................................................ 2.8 - 3.8
Sodium Bicarbonate........................................................... 8.4
Oranges.................................................................... 3.0 to 4.0
Ammonia 1N ....................................................................11.6
Beers........................................................................ 4.0 to 5.0
Lime, saturated .................................................................12.4
Cheese..................................................................... 4.8 to 6.4
Caustic Soda.................................................................... 14.0

Additional pH values
In the term pH, stands for the world "power" while H stands for the element Hydrogen. The actual definition of pH is the negative logarithm of the hydrogen ion activity.

pH= log (H+)

The pH value of an aqueous solution is directly related to the hydrogen (H+) and hydroxyl (OH-) ion concentrations. Acids are those substances which break down to give hydrogen ions while bases break down to give hydroxyl ions. If the OH- concentration is greater than the H+ concentration, the solution is alkaline and will have a pH greater than7. If the H+concentration is greater than the OH- concentration the solution is acidic and will have a pH less than 7. Solutions with an equal amount of H+ ions and OH- ions are neutral and have a pH of 7. In a neutral solution we have the following situation:

(H+) x (OH-)=10-7x10-7=10-14

In all solutions the product of H+and OH- is a constant. That is it the concentration of H+ions increase the concentration of OH-ions must decrease but the product will remain at 1 x 10-14.

pH scale
Molar hydrogen ion concentration (H+)
 pH  
Molar hydroxyl ion concentration (OH-)
100 1.0 0  
0.00000000000001 10-14
10-1 0.1 1   0.0000000000001 10-13
10-2 0.01 2 More 0.000000000001 10-12
10-3 0.001 3 acidic 0.00000000001 10-11
10-4 0.0001 4   0.0000000001 10-10
10-5 0.00001 5   0.000000001 10-9
10-6 0.000001 6   0.00000001 10-8
10-7 0.0000001 7 Neutral   0.0000001 10-7
10-8 0.00000001 8  
0.000001 10-6
10-9 0.000000001 9   0.00001 10-5
10-10 0.0000000001 10 More 0.0001 10-4
10-11 0.00000000001 11 alkaline 0.001 10-3
10-12 0.000000000001 12   0.01 10-2
10-13 0.0000000000001 13   0.1 10-1
10-14 0.00000000000001 14   1.0 100

The pH scale shows the active hydrogen ion and hydroxyl ion concentrations.

pH scale in a pdf format

The more hydrogen ions present in solution the lower the pH and the more acidic the solution. A solution of pH 1.0 is one million times more acidic than one of pH 7.0. Equal amounts of hydrogen and hydroxyl ions are present at pH 7.0. It can be seen that above pH 7.0, the hydrogen ion concentration becomes insignificantly small because the hydroxyl ion concentration become dominant.
Note that each pH unit differs from the one above or below it by a factor of ten. Each change in one pH unit corresponds to a tenfold change in hydrogen ion or hydroxyl ion concentration. Thus a solution at pH 5.0 will have ten times more active hydrogen ions than a solution of pH 6.0.
The hydrogen ion activity is expressed in moles per liter because in aqueous solutions (H+) concentration is one ten millionth moles per liter, or 10-7 molar.


pH Defined