The pH scale
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Basic concepts
pH can be measured by a hydrogen sensitive electrode also referred to as a pH meter. pH is defined as
the negative base 10 logaritm of the hydrogen ion activity {H+} and not the hydrogen ion
concentration [H+]:
pH = -Log10{H+}
Many people believe that pH can be calculated from [H+] but this is only the case in very dilute solutions.
There's a consensus among chemistry teachers that students should be taught that pH = -Log10[H+] until they reach university level where they are told (at least at chemistry departments) the truth that pH = -Log10{H+}.
If you read this and have been told that pH = -Log10[H+] please keep on reading. You will still be able to learn from the information provided here at pH scale dot net.
In the section about ionic strength there are examples and discussion about how [H+] is calculated from {H+} and why it is important to be able to do so in some situations.
The scale
The range of the pH scale is from 0 to 14. The pH of a 1 mol/L HCl is solution is approximately 0 (because of ionic interactions between H2O, H+ and Cl-, the pH is not exactly 0 - see the ionic strength section). Any solution with a pH less than 7 is by definition acidic.
Alkaline or basic solutions are those with a pH above 7 and a neutral solutions is one with a pH of exactly 7.
Below is a picture of a scale showing pH from 0 to 14 with examples of solutions with different pH:
Figure: The pH values of the shown solutions are approximate. E.g. the pH of seawater is usually from 7.5 to 8.4.
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