The dosage of lime and soda ash at a softening plant is based on which of the following alkalinities?

• A. hydroxide, carbonate, and bicarbonate alkalinity
• B. calcium carbonate and alkalinity
• C. calcium and magnesium carbonate alkalinity
• D. hydroxide and carbonate alkalinity

Answer: (A)

In lime-soda ash softening, the amount of reagents you need is driven by all forms of alkalinity present, because each form interacts with the reagents and affects precipitation and pH. Alkalinity exists as bicarbonate, carbonate, and hydroxide. Bicarbonate alkalinity must be neutralized or converted as part of the softening reactions, carbonate alkalinity provides the species that combine with calcium to form calcium carbonate, and hydroxide alkalinity comes from the lime itself, driving the pH rise and the precipitation chemistry. If you only consider one form (for example, carbonate), you’d miss how bicarbonate and hydroxide contribute to the buffering and the consumption of lime and soda ash, leading to improper dosing. Therefore, the dosage is based on hydroxide, carbonate, and bicarbonate alkalinity.