Basic Remarks on Acidity.

This Review provides a unified view on Brønsted acidity. For this purpose, a brief overview of the concepts acidity, acid strengths, and pH value is given, including problems, proposed solutions, and the use of the pH /pHabsH2O scale as a unifying concept. Thereafter, some examples of the accessibility and application of unified pH values are given. The Review is rounded off with the analogy of acid-base chemistry to redox chemistry with the introduction of the unified redox scale pe . The combination of pH and pe values in the protoelectric potential map (PPM), as elaborated in ongoing studies on the thermochemistry of single ions, provides a means to classify and to compare all possible acid-base/redox reactions in a medium-independent and, thus, unified fashion.