Environmental Chemistry

Hardness, Alkalinity & the Carbonate System

Total hardness as CaCO₃, alkalinity end-points and carbonate equilibria — the inorganic chemistry of natural waters.

PART 1

Topic Breakdown & Traps

Hardness is caused mainly by

Ca^{2 +}

and

Mg^{2 +}

and is expressed as an equivalent concentration of

CaCO_{3}

. Alkalinity is the acid-neutralising capacity, contributed by

HCO_{3}^{-}

CO_{3}^{2 -}

and

OH^{-}

. In the pH range of most natural waters (≈ 6.3–10.3) the carbonate system is dominated by bicarbonate.

Ion as CaCO₃:

mg/L as CaCO_{3} = mg/L of ion \times \frac{50}{equivalent weight}

Equivalent weights: Ca = 20, Mg = 12.15, CaCO₃ = 50.

Total hardness

=

(Ca as CaCO₃)

+

(Mg as CaCO₃).

Alkalinity

=

acid-neutralising capacity (mainly

HCO_{3}^{-}

between pH 6.3–10.3).

⚠
Always convert each ion to CaCO₃ equivalents before summing hardness.
⚠
Alkalinity neutralises acid, not base.
⚠
Bicarbonate dominates the carbonate system in typical natural waters, not carbonate or hydroxide.

PART 2

Q1MEDIUM2 Marks · NAT

Water contains 40 mg/L Ca²⁺ and 12.15 mg/L Mg²⁺. The total hardness as CaCO₃ (eq. wt Ca = 20, Mg = 12.15, CaCO₃ = 50) is _____ mg/L.

Q2BASIC1 Mark · MCQ

Alkalinity of a natural water is a measure of its capacity to:

Q3HARD1 Mark · MCQ

In the pH range 6.3–10.3 of most natural waters, alkalinity is predominantly due to: