It's been a few hours since you posted and there are no replys. First, let me say that I cannot answer your question regarding HOW to measure pH over that range. I may be able to give some insight into what that range means. As you no doubt know, pH is the negative log of the hydrogen ion concentration. Thus, for example, a 1 molar (M) concentration of hydrogen ion (H+) is pH 0. A pH of -2 would be a 100 molar concentration. I don't believe that is possible in an aqueous solution, as the concentration of water is only 55.6 M(i.e., 1000/18). In fact, a pH of -2 would imply to me some pretty odd chemistry and/or a very dense solution. That is, at a minimum, you would need 100 moles of a fully ionized acid in a 1-liter volume. Liquid HF, with a formula weight of 20, would have to have a specific gravity of 2 and be fully ionized (which I doubt) to give a pH of -2. Maybe Google can give some insight on what the pH of liquid HF or super acids (see: George Olah, who received the Nobel prize in chemistry) is considered to be.
The other end of the pH scale is a little easier (for me at least) to comprehend. A pH of 20 simply means that there is not a lot of H+ around, i.e., the extent of ionization is very low. Hydrocarbons are said, as I recall, to have pH 's in the range of 20 to 30. Again, such high pH's do not exist in water, but can be approached in other liquids, such as by dissolving sodium (metal) in liquid ammonia.
So, while it is possible to describe what the numbers would mean, it seems unlikely to me that your professor would accept as a final year project a simple definition. Secondly, it would seem a daunting task to try to design a single instrument to measure a dynamic range of 22 orders of magnitude. My first step would be to confer with the professor to be sure I understood the question correctly.
Good luck. John