value at which the net electric charge of an elementary entity is zero. 
is a commonly used symbol for this kind-of-quantity. It should be replaced by 
because it is a 
determined under that particular condition.
Source:
PAC, 1994, 66, 891
(Quantities and units for electrophoresis in the clinical laboratory (IUPAC Recommendations
1994))
on page 894
PAC, 1972, 31, 577
(Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix
II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry)
on page 618