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A water sample contains the following species: \(\mathrm{CO_3^{2-}}, \mathrm{Na^+}, \mathrm{H^+}, \mathrm{PO_4^{3-}}, \mathrm{Al^{3+}}, \mathrm{H_2CO_3}, \mathrm{Cl^-}, \mathrm{Ca^{2+}}, \mathrm{Mg^{2+}}, \mathrm{HCO_3^-}, \mathrm{Fe^{2+}}, \mathrm{OH^-}\). Concentrations of which species are required to compute alkalinity?

1. \(\mathrm{CO_3^{2-}}, \mathrm{H^+}, \mathrm{HCO_3^-}, \mathrm{OH^-}\)
2. \(\mathrm{CO_3^{2-}}, \mathrm{H^+}, \mathrm{H_2CO_3}, \mathrm{HCO_3^-}\)
3. \(\mathrm{CO_3^{2-}}, \mathrm{H_2CO_3}, \mathrm{HCO_3^-}, \mathrm{OH^-}\)
4. \(\mathrm{H^+}, \mathrm{H_2CO_3}, \mathrm{HCO_3^-}, \mathrm{OH^-}\)

Correct answer: \(\mathrm{CO_3^{2-}}, \mathrm{H^+}, \mathrm{HCO_3^-}, \mathrm{OH^-}\)

Solution

Alkalinity is determined mainly by hydroxide, carbonate, and bicarbonate species, while hydrogen ion contributes negatively. Species like sodium, chloride, calcium, and magnesium do not directly enter the alkalinity expression. Hence the required concentrations are those of \(\mathrm{CO_3^{2-}}, \mathrm{H^+}, \mathrm{HCO_3^-}, \mathrm{OH^-}\).

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