Question

Question 2 a. Estimate the pH of distilled water produced in Jebel Ali desalination plant if the concentration of CO2 in the atmosphere is about 300 ppm assuming water temperature is 25 °C. Write your assumption clearly b. Assuming, due to increasing air pollution (especially at the vicinity of the gas combustion turbines and generators), the concentration of CO2 increases to 800 ppm in Jebel Ali Free Zone. Therefore, desalinated water reaches equilibrium according to this condition. Comment on the quality of water entering DEWA pipelines (mainly about pH) both from health and corrosion points of view.

Answer 1

Ans a) CO2 presence will decrease the pH due to formation of Carbonic acid ,

CO2 = 300 ppm

=> CO2_aq = K_HP_CO2 = 0.0338 M/atm x 0.0003 atm

= 1.014 x 10^-5 M

Following reaction occur :

CO2 + H2O ---> H⁺ + HCO3^- (K_al = 4.45 x 10^-7)

K_al = [H⁺] [HCO3^-] / [CO2]

4.45 x10^-7 = [H⁺]²/ [ 1.1014 x 10^-5]

[H⁺] = 2.21 x 10^-6

pH = - Log [H⁺]

= 6 - Log(2.21)

= 5.655

Ans b) Now , CO₂ concentration = 800 ppm

   => CO2_aq = K_HP_CO2 = 0.0338 M/atm x 0.0008 atm

=> CO2_aq = 3.104 x 10^-5

   K_al = [H⁺] [HCO3^-] / [CO2]

[H⁺] = (4.45 x 10^-7 x 3.104 x 10^-5 )^0.5

[H⁺] = 3.716 x 10^-6

=> pH = 6 - Log(3.716)

= 5.43

If water with pH of 5.43 enter DEWA pipeline, it is not only harmful for drinking and consumption but it will also increase the rate of corrosion of pipeline. The acidic water can lead to stroke, kidney disease, memory problem and high blood pressure. Acidic water can also leach metals from pipe and fixtures such as copper , lead and zinc which contribute to corrosion of pipeline