Homework SourseA wastewater is simulated as a solution containing 1027 M Na
A wastewater is simulated as a solution containing 10-2.7 M NaHCO3 and 10-3 M NH4Cl.
a. List the dissolved species that you expect to be present in solution.
b. Prepare a log C-pH diagram for the system, and calculate the pH of the solution.
c. In order to remove nitrogen by volatilizing NH3 gas, 148 mg/L Ca(OH)2 (lime) will be added to raise the pH and convert NH4+ to the NH3(aq) form. Calculate the pH after lime addition. (Assume that no CaCO3 precipitates or ammonia loss takes place during lime addition.
d. If CaCO3 precipitation did occur during lime addition, would it raise, lower, or not change the 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 NH3(aq) by volatilization raise, lower, or not change the pH of the solution? Explain your reasoning briefly.
Draw the log C – pH diagram in excel, and solve parts b and c using proton conditions. Give reasons for parts d and e.
Solution
Solution:
Solution:
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+ + CO32-
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 [CO32-]/ [HCO3-]
= 5.6 x 10-11
10-3 M NH4Cl
[NH3+] [HCl-] = 10-3
HCl -----> H+ + Cl-
NH4 -------> NH3 + H+
NH3 --------> NH4+ + OH-
pH = [NH3+] [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 - pKw
CT = [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 NaHCO3, NaOH acts as a strong base and H2CO3 is weak acid
NH4Cl is a dominant species
