pH gradient

in electrophoresis
https://doi.org/10.1351/goldbook.P04546
The differential change of pH with distance (dpH/dI).
Source:
PAC, 1994, 66, 891. 'Quantities and units for electrophoresis in the clinical laboratory (IUPAC Recommendations 1994)' on page 895 (https://doi.org/10.1351/pac199466040891)