•The Theoretical DefinitionpH = -log aH•aHis the free hydrogen ion activityin a sample, not total ions.•Insolutionsthatcontainotherions,activityandconcentrationarenotthesame.Theactivityisaneffectiveconcentrationofhydrogenions,ratherthanthetrueconcentration;itaccountsforthefactthatotherionssurroundingthehydrogenionswillshieldthemandaffecttheirabilitytoparticipateinchemicalreactions.•These other ions effectively change the hydrogen ion concentration in any process that involves H+. pH electrodes are an ISE for hydrogen.