Question

A wastewater is simulated as a solution containing \(10^{-2.7} \mathrm{M}  \mathrm{NaHCO}_{3}\) and \(10^{-3} \mathrm{M} \mathrm{NH}_{4} \mathrm{Cl}\)

a. List the dissolved species that you expect to be present in solution.

b. Prepare a log \(C-p H\) diagram for the system, and calculate the \(\mathrm{pH}\) of the solution.

c. In order to remove nitrogen by volatilizing \(\mathrm{NH}_{3}\) gas, \(148 \mathrm{mg} / \mathrm{L}\) Ca(OH) \(_{2}\) (lime) will be added to raise the \(\mathrm{pH}\) and convert \(\mathrm{NH}_{4}^{+}\) to the \(\mathrm{NH}_{3}(a q)\) form. Calculate the \(\mathrm{pH}\) after lime addition. (Assume that no \(\mathrm{CaCO}_{3}\) precipitation or ammonia loss takes place during lime addition.)

\(d\). If \(\mathrm{CaCO}_{3}\) precipitation did occur during lime addition, would it raise, lower, or not change the \(\mathrm{pH}\) of the solution? (Hint: Consider what species are removed from solution by this reaction, and what the response would be of the species that remain.)

e. Would removal of \(\mathrm{NH}_{3}(a q)\) by volatilization raise, lower, or not change the \(\mathrm{pH}\) of the solution? Explain your reasoning briefly.

Answer 1

Wastewater comprises of 10^-2.7 M NaHCO₃ and 10^-3 M NH₄Cl

NaHCO₃ + NH₄Cl -------> NaCl + NH₃ + CO₂ + H₂O

Log C-pH diagram:

10^-2.7 M NaHCO₃

[Na⁺] [HCO₃^-] = 10^-2.7

NaHCO₃ ------->Na⁺ + HCO₃^-

The possible reaction can be:

H₂CO₃ ------> H⁺ + HCO₃^-

   HCO₃^- -------> H⁺ + CO₃^2-

H₂O -------> H⁺ + OH^-

   pH = [Na⁺] [HCO₃^-] / NaHCO₃

pH = - log [OH^-]

= - log [10^-2.7] = 2.7

Equilibrium Constants can be expresed as:

K₁ (of eq 1) = [H⁺] x [HCO₃^-]/ [H₂CO₃]

= 4.3 x 10^-7

     K₂ (of eq 2) = [H⁺] x [CO₃^2-]/ [HCO₃^-]

= 5.6 x 10^-11

10^-3 M NH₄Cl

[NH_3⁺] [HCl^-] = 10^-3

HCl -----> H⁺ + Cl^-

   NH_{4 ------->} NH₃ + H⁺

   NH_{3 -------->} NH₄⁺ + OH^-

      pH = [NH_3⁺] [HCl^-] / NH₄Cl

pH = log [10^-3]

= -3

In a Log C-pH diagram, log C is plotted on the X-axis and Y-axis holds the pH value

Now, -pH = log [H⁺] and  log [OH^-] = pH - pK_w

C_{T =} [NH₃⁺] [NaCl^-]

and the graph can be plotted according to the values provided above.

b) NaHCO₃ + NH₄Cl -------> NaCl + NH₃ + CO₂ + H₂O

In NH₄Cl, ammonia (NH₃) is a weak base and the strong acid is HCl

In NaHCO_3, NaOH acts as a strong base and H₂CO₃ is a weak acid

  NH₄Cl is a dominant species