Water can be considered an acid because of its ability to ionize to a proton and a negatively charged hydroxide ion:
H-O-H H + + OH -
The frequency with which this occurs in pure water is very low. In fact, only one water molecule in 500 million will be ionized at any one time in pure water. This means that the molar concentrations of H + and OH - are approximately 10 - 7 each in pure water.
Hydrogen-ion concentrations of aqueous solutions range from greater than 1 M
to less than 10 - 14 M. For convenience, H + concentrations
are expressed on a logarithmic scale as pH. The pH of a solution is the negative
log of its H + concentration, or:
pH = -log [H +]
Therefore, a H + concentration of 10 -
7 M has a pH of 7.0. In pure water, there are always equal numbers
of H + and OH - ,
and the pH is defined to be neutral.(Note that neutral pH is not always
exactly 7.00. If you raise the temperature, more H2O
dissociates and the pH will be lower).
Acids contribute another source of H + causing
the pH to decrease from neutrality. Conversely, bases absorb H + from
solution and thus make the pH higher than neutrality.