pHjuk

pHjuk is a theoretical index used in chemistry to express the effective acidity of an aqueous solution by combining pH with corrections for non-ideality caused by ionic strength. It is not a separate measurement but a computed quantity intended to reflect hydrogen ion activity under non-ideal conditions.

Definition and concept: In solutions with significant ionic strength or unusual composition, the activity of hydrogen ions deviates from what is suggested by pH alone. pHjuk incorporates pH readings with an ionic-strength– and temperature-dependent correction obtained from a chosen activity-coefficient model (for example Debye-Hückel, Davies, or Pitzer). The goal is to provide a single value that captures non-ideality and facilitates comparisons across waters with different chemistries.

Calculation and interpretation: In practice, pHjuk is derived from measured pH, temperature, and ionic strength, using the selected model to estimate the H+ activity coefficient gamma_H+. The pHjuk value increases or decreases relative to pH depending on how gamma_H+ shifts with composition. At low ionic strength, pHjuk closely matches pH; as ionic strength rises, pHjuk diverges, reflecting non-ideal behavior and implying the activity of H+ differs from its nominal concentration.

Applications and limitations: pHjuk is used mainly in teaching and some environmental models to illustrate how non-ideality affects acidity and metal solubility. It is not a standardized measure in regulatory contexts, and results depend on the chosen activity model and parameters. Critics note that pHjuk adds complexity without universally accepted calibration.

History: The concept emerged in science-education resources in the 2010s as a pedagogical tool to discuss activity corrections and non-ideality in aqueous chemistry.

See also: pH, ionic strength, activity coefficient, Debye-Hückel, Davies, Pitzer.