Correct option is B
Introduction
The pE–pH (redox–acidity) relationship governs which inorganic nitrogen species dominate in natural waters.
Nitrogen shifts between oxidised and reduced forms depending on the electron activity (pE) and the pH of the environment.
Information Booster
The correct answer is NO₂⁻ (nitrite). On a pE–pH stability (predominance) diagram for inorganic nitrogen in natural water, nitrite occupies a narrow intermediate field that appears specifically under alkaline conditions (pH 8–14) combined with moderately oxidising, intermediate-oxygen conditions (small positive pE values of about 0–4).
Nitrite is a transitional, thermodynamically less stable species that sits between fully oxidised nitrate (high pE) and reduced ammonium (low pE).
It only becomes the predominant species within this restricted zone of mildly oxidising alkaline water, because at higher pE it oxidises to nitrate, at lower pE it reduces toward ammonium or nitrogen gas, and its stability field is enlarged toward the alkaline (high-pH) region.
This matches the described conditions precisely, making NO₂⁻ the answer.
Additional knowledge
· NO₃⁻ (nitrate) — Incorrect for these conditions. Nitrate is the most oxidised nitrogen species and predominates under strongly oxidising (high positive pE) conditions with abundant oxygen, not the intermediate pE range of 0–4.
· NH₄⁺ (ammonium) — Incorrect. Ammonium is the reduced form of nitrogen, predominating under low pE (reducing, oxygen-poor) conditions and, being acidic in character, more prevalent at lower pH; it does not fit the mildly oxidising alkaline zone.
· N₂ (nitrogen gas) — Incorrect. Molecular N₂ occupies a large stability field at intermediate-to-low pE across a wide pH range, but it is favoured under more reducing conditions than the specified small positive pE window where nitrite dominates.
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