pH, which stands for *power of hydrogen*, is a measure of the acidity of an aqueous solution. It is defined as:

where [*H*^{+}] is the concentration of the aqueous hydrogen ion.

The concept was developed by the Danish chemist, Soren Peder Lauritz Sorensen in 1909.

###### Question

Why is pH defined on a logarithmic scale and why does it have a negative sign in front?

###### Answer

The most accurate way of measuring the concentration of *H*^{+} in a solution is using a glass electrode, a type of ion-selective electrode (see below diagram). The voltage, *E*, of the electrochemical cell containing the electrode is measured at 25^{o}C and the concentration of *H*^{+} is determined by the Nernst equation:

Scientists found that the values generated by the factor *log*[*H*^{+}] at different voltages are accurate over the range of -14 to 0 and decided to use it to conveniently express the concentration of *H*^{+} in a solution. As it is easier to deal with positive numbers (i.e. 0 to 14 instead of -14 to 0), pH is defined as –*log*[*H*^{+}], which makes eq2: