How is the pH scale constructed?
The pH is determined experimentally with a glass electrode. A glass electrode is a small glass tube limited by a thin membrane and filled with an aqueous KCl solution with a silver electrode inside. There is also a similar silver electrode out of the tube, in a compartment in contact with the outer solution. When you dip a glass electrode into an aqueous solution, the experiment shows that the tension E read between the two electrodes is proportional to the logarithm of the H+ concentration of the external solution. It is :  E  = Constant + 0.059 pH. The Constant depends of the particular glass membrane.
And the experiment shows that pH is -log[H+] at low acidic concentrations, and is measured as


7 in pure water,  
3 in 0.001 M aqueous solutions
2 in 0.01 M aqueous HCl solutions,
11 in 0.001 M aqueous NaOH solutions 
12 in 0.01 M aqueous NaOH solutions.


These values are in agreement with the usual expression : pH = -log[H+], or : pH = 14 - log[OH-]. At higher concentration, the measured pH values are not exactly equal to the logarithm of [H+]. The measured pH is said to be the logarithm of the "activity" of H+. 

The pH does not go only from zero to 14. It may be smaller than zero in concentrated solutions of HCl and of H2SO4.

In non-conducting solutions, the glass electrode does not give any stable values. The measured tension is fluctuating without any reason and can be ± 3, 5, 10 or 0, and this changes constantly.