Which of the phosphorus species will dominantly exist in the pH range of 3-7 in natural waters?

Correct option is C

The correct answer is (c) H₂PO₄⁻.
In natural waters with a pH range of 3-7, the dominant phosphorus species is dihydrogen phosphate (H₂PO₄⁻). The distribution of phosphate species in water depends on the pH value. At a pH range between 3 and 7:
· At lower pH (around 3), H₃PO₄ (phosphoric acid) is present.
· Between pH 3-7, H₂PO₄⁻ (dihydrogen phosphate) is the dominant species.
· At pH values above 7, HPO₄²⁻ (hydrogen phosphate) becomes dominant.
· Above pH 12, PO₄³⁻ (orthophosphate) dominates.
Information Booster:
1. pH and Phosphorus Species:
· pH < 3: Dominated by H₃PO₄.
· pH 3-7: Dominated by H₂PO₄⁻.
· pH 7-11: Dominated by HPO₄²⁻.
· pH > 12: Dominated by PO₄³⁻.
2. Phosphate Speciation:
· The speciation of phosphorus is crucial in understanding nutrient availability and eutrophication in water bodies.
3. Insoluble Compounds:
· Insoluble Fe/Al phosphate precipitates in acidic conditions.
· Insoluble Ca phosphate precipitates in alkaline conditions.
4. Environmental Impact:
· Phosphorus availability affects aquatic ecosystems, and its speciation influences the risk of algal blooms.
Additional Knowledge:
1. (a) Insoluble Fe/Al Phosphate:
· Forms under acidic conditions and low pH (<5).
· Common in acidic soils or waters where iron and aluminum are present.
2. (b) Insoluble Ca Phosphate:
· Precipitates in alkaline conditions (pH > 7).
· Common in calcareous soils and hard water.
3. (c) H₂PO₄⁻:
· Dominant in a moderate pH range (3-7).
· Available for plant uptake and critical in nutrient cycling.
4. (d) HPO₄²⁻:
· Predominant in slightly alkaline conditions (pH 7-11).
· Important in wastewater treatment and natural waters with higher pH.
Key Points:
1. H₂PO₄⁻ is the primary phosphate species in waters with pH 3-7.
2. Phosphorus speciation affects nutrient dynamics and eutrophication.
3. pH controls the solubility and availability of phosphorus compounds.
4. Different insoluble phosphates form under varying pH conditions.