In a solution, which of the following dissolved species would contribute positively to alkalinity?

(A) CO₃²⁻
(B) PO₄²⁻
(C) HCO₃⁻
(D) OH⁻
(E) HS⁻

Options:

Correct option is D

Alkalinity is the capacity of water to neutralize acids (i.e., a measure of buffering capacity). It is primarily caused by the presence of basic (alkaline) ions that can accept protons (H⁺). All of the given ions are weak conjugate bases of weak acids and contribute to alkalinity:

• (a) CO₃²⁻ (carbonate ion): Strong contributor to alkalinity as it can accept 2 protons.
• (b) PO₄²⁻ (phosphate ion): Acts as a base and can neutralize H⁺ ions.
• (c) HCO₃⁻ (bicarbonate ion): One of the most common natural buffers in water, major source of alkalinity.
• (d) OH⁻ (hydroxide ion): Directly neutralizes H⁺, hence contributes to alkalinity.
• (e) HS⁻ (hydrosulfide ion): Conjugate base of H₂S, acts as a weak base and contributes to alkalinity.

All five ions listed have proton-accepting (basic) properties and hence increase the alkalinity of a solution.

Information Booster:

Alkalinity in natural waters comes primarily from bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), and hydroxide (OH⁻) ions, but other weak bases like phosphate (PO₄³⁻) and hydrosulfide (HS⁻) can also contribute. These ions help resist pH changes when acids are added, playing a crucial role in water chemistry and environmental buffering systems.

Additional Information:

• Options (1), (2), and (3) exclude one or more valid contributors.
• Only option (4) includes all relevant alkaline species.