The pH scale

pH can be measured by a hydrogen sensitive electrode also referred to as a pH meter. pH is defined as the negative base 10 logarithm 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.

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 pH scale was developed by a research scientist at Carlsberg as a tool for understanding proteins, and their components, aminoacids, better. The inventor was Soeren Soerensen, who lived from 1868 to 1939.

To begin with they thought they measured the hydrogen concentration, it was only later it was found out that it was in fact the hydrogen activity. Never than less pH, and the pH scale remains an effective tool for evaluating the acidity of solutions and the state of equilibriums in the solution.

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.

There are much more than discussions about the pH scale at this webpage, so please take your time to read or download the pdf files or read the webpages.

Web references

pH scale explained - Elmhurst College, Illinois
Dissociation constant
John Kyrk - it is essential to be a bit interactive here
pH - with emphasis on acid rain
General Chemistry Resources
Dissociation constant