A wastewater is simulated as an ideal solution containing 10^-2.7 M NaHCO_3 and
ID: 487405 • Letter: A
Question
A wastewater is simulated as an ideal solution containing 10^-2.7 M NaHCO_3 and 10^-3 M NH_4 CI. Prepare a log C-pH diagram for the system. Predict the dominant species in the equilibrium solution, and prepare a system tableau using those species as components. Write the TOTH equation and use it to determine the pH of the solution. The utility hopes to remove nitrogen from the solution by driving out NH_3 gas. To prepare for that step, 148 mg/L Ca(OH)_2 (hydrated lime) will be added to raise the pH and convert NH^+_4 to NH_3 (aq). Calculate the pH after lime addition, but before any NH_3 exists the solution.Explanation / Answer
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
c) TOTh equation
TOTH refers to the total component of hydrogen ions, H+ present in a system and doesn't concentrate on the species.
Therefore, the TOTH equation becomes,
TOTH = [H+] - [OH-] - [NH3+] - [NaCl-]
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