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GATE 2019 CE (CE2) – Question 20

Water and Waste Water Quality and Treatment · Water quality standards, parameters and water quality index · 1 mark · Multiple choice

A water sample contains $\mathrm{CO_3^{2-}}$, $\mathrm{Na^+}$, $\mathrm{H^+}$, $\mathrm{PO_4^{3-}}$, $\mathrm{Al^{3+}}$, $\mathrm{H_2CO_3}$, $\mathrm{Cl^-}$, $\mathrm{Ca^{2+}}$, $\mathrm{Mg^{2+}}$, $\mathrm{HCO_3^-}$, $\mathrm{Fe^{2+}}$ and $\mathrm{OH^-}$. Which concentrations are required to compute alkalinity as intended by these choices?

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

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

Explanation

For the conventional carbonate system, alkalinity in equivalents is $[\mathrm{HCO_3^-}]+2[\mathrm{CO_3^{2-}}]+[\mathrm{OH^-}]-[\mathrm{H^+}]$. Thus carbonate, bicarbonate, hydroxide and hydrogen ion concentrations are required: **A**. Other weak-acid systems can contribute in a fuller treatment; the options specify the carbonate-system expression.