Alkalinity in water – forms present in natural and treated supplies Which forms of alkalinity may be present in a water sample (depending on its chemistry and treatment history)?

Introduction / Context:
Alkalinity is the acid-neutralizing capacity of water and is important for corrosion control, coagulation optimization, and biological stability in distribution. It typically arises from carbonate species and added bases.

Given Data / Assumptions:

• Water may be raw (natural) or treated.
• Species considered: HCO3-, CO3^2-, OH-, and combinations introduced by caustic addition.

Concept / Approach:
At typical water pH values, alkalinity is predominantly as bicarbonate (HCO3-). With higher pH (e.g., after lime/soda ash/caustic dosing), carbonate (CO3^2-) and hydroxide (OH-) fractions can appear. “Caustic alkalinity” is often used to denote the portion attributable to strong base additions producing hydroxide/carbonate alkalinity.

Step-by-Step Solution:
Recognize bicarbonate as the dominant species in most natural waters.Account for carbonate and hydroxide at elevated pH during softening/corrosion control.Therefore, all listed forms may occur depending on conditions.

Verification / Alternative check:
Standard titration with phenolphthalein and methyl orange indicators partitions total alkalinity into hydroxide, carbonate, and bicarbonate components.

Why Other Options Are Wrong:
Any single form alone ignores the pH-dependent speciation found in practice.

Common Pitfalls:
Equating alkalinity with hardness; they are distinct although both can involve carbonate species.

Final Answer:
All the above