pH value is the best known parameter for characterizing acidity of a solution. The pH of a neutral solution is around 7. In acidic solutions, such as battery acid, pH is around 0. In alkaline solutions, such as certain bleaches and drain cleaners, the pH value can be as high as 14. One may ask: why is the solution neutral at pH 7? What is the origin of this reference point? The answer is: it originates in the properties of water as solvent and the pH scale ranging from 0 to 14 with 7 as the neutrality point is limited to aqueous (i.e. containing water as solvent) solutions only. Luckily, many objects of interest either are (or can be regarded as an approximation) aqueous solutions. But by far not all. Different solvents have different pH scales. For example in acetonitrile (a widely used solvent on organic and analytical chemistry) the neutral pH is ca 19, in sulfuric acid ca 1.5.
The pH values in different solvents, expressed in their own scales, are absolutely noncomparable. The pH 19 in acetonitrile corresponds by its acidity pretty well to pH 7 in water. Nevertheless, this is more a coincidence than anything else: the pH 1.5 in sulfuric acid is not even near neutrality in terms of aqueous pH. On the contrary, this solution is so acidic that such acidity cannot be realized in water at all. Its equivalent pH in water would be negative: ca -20.
Why is that so? The reason lies in chemical properties of the different solvents. Contrary to the common knowledge the pH value of a solution does not correspond to the negative logatrithm of hydrogen ion H+ concentration in the solution but to the negative logarithm of H+ activity in the solution: pH = -log[a(H+)]. In different solvents the same number of H+ ions can have vastly different activity because they are bound to solvent molecules with different strength. This causes their different “efficiency” in making solution acidic. This in turn means different efficiency in catalyzing reactions, reacting with bases, etc.
The above mentioned noncomparability is very inconvenient: using the conventional pH scales in different solvents it is impossible to make comparisons of the behavior of the solutions in different chemical and technological processes.
Recently a very important step was made towards achieving comparability of acidities in different solvents: a universal pH scale based on the chemical activity (chemical potential) of H+ was created that allows expressing the acidities of solutions in different solvents using a single scale of so-called pHabs values. This pHabs scale embraces the pH scales in different solvents as is seen in the picture on the left. More information can be found in the original publication in Angewandte Chemie Int. Ed. The beauty of this approach is in its complete universalitry: in addition to liquids it can be applied also to gases, solids, gels, etc. The main remaining problem is the experimental realization of this scale: direct comparison of acidities in different solvents is not easy. Work towards this is in progress.