Hypochlorous Acid (HOCl)
From Sodium Dichloroisocyanurate (NaDCC / SDIC)
- Sodium dichloroisocyanurate (NaDCC or SDIC) — also known as sodium troclosene — is an organic chlorine donor.
- On dissolving in water, NaDCC establishes a dynamic, pH-dependent equilibrium between free available chlorine (FAC) — present as hypochlorous acid (HOCl) and hypochlorite ions (OCl⁻) — and available chlorine temporarily held by chlorinated isocyanurate species. The distribution varies with pH, temperature and water chemistry: WHO reports that an NaDCC solution at 1 mg/L total available chlorine and pH 7 holds ≈48.1% as HOCl, 12.8% as OCl⁻, 26.8% as monochlorocyanurate and 11.8% as dichlorocyanurate.
- HOCl provides the primary rapid antimicrobial activity by oxidising and inactivating microorganisms and reacting with other oxidisable contaminants.
- As FAC is consumed, the equilibrium shifts and the chlorinated isocyanurate species release additional available chlorine — replenishing FAC until the reservoir is exhausted. A practical advantage over chlorine sources without an isocyanurate-bound reservoir.
- The reversible association with cyanurate species also reduces sunlight-driven chlorine degradation and extends the persistence of the available-chlorine residual.
- HOCl is electrically neutral, so it interacts with microbial envelopes more readily than negatively charged OCl⁻, then oxidises essential cellular components including membranes, proteins and enzymes.
- Once all available chlorine is released and consumed, no active chlorine remains — the water contains primarily cyanurate species (monosodium cyanurate and/or cyanuric acid, depending on final pH), chloride ions (Cl⁻) as the final reduction product, dissolved sodium and naturally occurring minerals. Under normal use, chlorate is not a primary reaction product.
250 mg/L HOCl
≈ 2.9 × 1021 HOCl molecules per litre — as grains of beach sand, a cube roughly 6 km × 6 km × 6 km.
2.5 mg/L HOCl (potable dose)
≈ 2.9 × 1019 molecules per litre — as grains of sand, a cube roughly 1.3 km × 1.3 km × 1.3 km.
