Chloronitramide anion

The chloronitramide anion, also known as chloro(nitro)azanide, is a chemical byproduct of the disinfectant chloramine first identified in 2024. It is present in the tap water of about 113 million people in the United States in varying concentrations. Its toxicity has not yet been determined, although it may be removable by an activated carbon filter. Although its molecular formula and structure were unknown, it was first recorded as a byproduct of chloramine in the early 1980s.

Chloramine is often used as an alternative to chlorine for water disinfection because of chlorine's harmful byproducts, and on the basis that clean water improves health much more than small concentrations of byproducts harm it. Other methods of disinfection exist, including ozone (popular in European countries) and UV light, but these cannot currently be used in the US because the law requires water to have small residual amounts of disinfectant to prevent re-infection.

One study of 40 locations in the US found a median chloronitramide anion concentration of 23 micrograms per liter in drinking water, with a first quartile of 1.3 and a third quartile of 92.

The chloronitramide anion was first detected as a UV absorbance interference during monitoring of chloramine and dichloramine in 1981. It was then shown to form during the decomposition of both chemicals. It was shown to likely be an anion in 1990. In the 1980s and 1990s methods of producing it in high concentrations were identified, and the molecule was shown through destruction to contain both nitrogen and chlorine. According to Julian Fairey, research on the compound slowed down in the mid-1990s after attempts to identify it were unsuccessful.

The structure of the molecule was finally identified in 2024 using a combination of techniques, first identifying the molecular formula, then creating a candidate structure, then confirming it.

Ion chromatography, a method of separating ions and ionizable polar molecules, was used to separate the chloronitramide anion from the many salts present in water samples containing it, which otherwise made it difficult to use mass spectrometry; the water salinity was higher than that of saltwater.^[why?]

Mass spectrometry was sufficient to determine the molecular mass of the ion, but it was too small for structure determination from the fragmentation pattern. The ion was found to have the molecular formula ClN₂O₂⁻¹ (containing two oxygen atoms, two nitrogen atoms, and one chlorine atom) by electrospray ionisation mass spectrometry. A candidate structure was confirmed by ¹⁵N NMR spectroscopy and infrared spectroscopy.

Research investigating the toxicity of the chloronitramide anion, as well as the reasons for its formation in high or low concentration in different places, is expected.

The identifying paper proposes that the chloronitramide anion is formed through the reaction of chloramine (or dichloramine, which forms in chloramine solution) with NO₂⁺, one of its degradation products. The formation of NO₂⁺ begins when dichloramine (NHCl₂) is hydrolyzed to form nitroxyl (HNO), which then reacts with dissolved oxygen (O₂) to form the unstable peroxynitrite (ONOOH). NO₂⁺ is one of the several reactive nitrogen species formed when peroxynitrite decomposes. The chloronitramide formed in this way then dissociates, losing the hydrogen, to form the corresponding anion.

The EPA web application Generalized Read-Across predicts chloronitramide anion predicts possible toxicity effects in the categories of chronic toxicity, prenatal development toxicity, multigenerational reproductive toxicity, subacute repeat dose toxicity, subchronic toxicity.