Quality- from adequacy to fitness for purpose

Scientific research and consequent progress, leads to the introduction of new concepts, new theories and new experimental methods and procedures. Their implementation, presentation and discussion, bring the associated need to choose and adopt the most appropriate terminology. The choice of the right words, definitions, or descriptions of all kinds, is far from being a minor task, demanding the best knowledge of the experts. As time goes by, this often requires revision and upgrading. In this work, Analytical Chemistry is the chosen scientific discipline, from where relevant examples are being taken and presented. Expression of acidity in terms of pH is addressed, covering a selection of developments where the author has been an active player along half a century, ranging from Chemistry to Metrology, hence involving expertise from both fields. Whilst much has been done, more is needed and continues to happen.