The pH scale and concentration
The following table shows how concentration and pH are related by factors of 10.
[INSERT pH_CI_03_int]
[reinforcing the link between concentration and pH, and making it obvious that writing out all these zeroes is not as convenient as their logs and hence pH numbers]
Taking logarithms of the concentrations keeps the scale compact whilst remaining rich in information. Because
pH = –log [H+(aq)]
by rearranging the expression it follows that:
[H+(aq)] = 10–pH
Is it possible to have a pH number of less than 0?
Yes, a negative pH number, such as pH –1, represents a strong acid at very high concentration.
Is it possible to have a pH number greater than 14?
Yes, a pH number greater than 14 represents a strong base at high concentration.
Could we simply use the H+(aq) concentration to describe a solution rather than working out a pH?
Try the example of pH 4. Consider whether it is easier to say 'it's pH 4' rather than describe the solution as having 'a hydrogen ion concentration of 0.0001 mol dm–3'.
Does the version of the pH scale (eg does neutral mean pH 7?) depend upon the age of students and the stage being taught?
In school chemistry studies and lower school lessons (age 11–16) when getting to grips with new concepts such as acids, alkalis and pH, it is a reasonable simplification that most pH values encountered are between 1 and 14 and neutral is about pH 7. In upper school chemistry (age 16–19) and more advanced studies of pH, it's necessary to extend these ideas to the ionisation of water, Kw and equilibria. This is explored further in the next topic.
In aqueous solution, if the concentration of hydrogen ions exceeds the concentration of hydroxide ions, the solution is acidic. Conversely for alkalis, the hydroxide ion concentration exceeds the hydrogen ion concentration.
Examples of pH and concentration:
An acid has a pH of 3. What is the pH of an acid 100 times more concentrated?
100 is two powers of 10, hence 2 pH units more. This is pH 1.
5 cm3 of an alkali measuring pH 12 was diluted to 5000 cm3 with water. What is the new pH?
The alkali is 1000 times more dilute, ie 3 pH units. It is pH 9.