Why the hydroxyl radical deserves correct notation

Abstract

The hydroxyl radical (HO•) is the central oxidant in water and wastewater treatment processes and in aquatic photochemical systems, governing contaminant transformation, by-product formation, and process performance. Despite its foundational role, the hydroxyl radical is denoted in multiple, often inconsistent forms across the literature. As these representations propagate across chemistry, environmental engineering, and mechanistic and photochemical modeling frameworks, they increasingly obscure structural meaning and complicate mechanistic interpretation. This is not a cosmetic issue. In aqueous systems where multiple radical families coexist (HO•, SO₄•−, CO₃•−, Cl•/Cl₂•− and related chemical species), dot placement encodes the location of the unpaired electron and implicitly defines the presumed site of reactivity. Ambiguous notation therefore propagates into mechanistic schemes, kinetic interpretations, reaction databases, modelling frameworks, and education. Here, we explain the scientific and IUPAC basis for writing the hydroxyl radical as HO•, clarify a persistent typographic confusion between the radical dot and the hydrate separator dot, and propose a practical pathway for standardizing radical notation across water research. Correct and uniform notation is a step toward greater mechanistic clarity, reproducibility, and interoperability in aqueous radical chemistry. © 2026 Elsevier Ltd.