ENCE 433 Dr. Alba Torrents

ENVIRONMENTAL ENGINEERING ANALYSIS

Homework Set # 3 SOLUTIONS

1.

These three acids are all strong acids and thus are 100% ionized in dilute solutions

Solution	[H3O^+]	pH
2.0 F HCl	2 M	-0.30
0.200 F HClO4	0.200 M	0.70
2.00x10^-4 F HNO3	2.00x10^-4 M	3.70

3.

The problem is requesting:

In order to solve this problem another independent equation is required because the one expression written above has two unknown values. The equilibrium expression for the NH₄⁺/ NH₃ system provides the second required equation:

Note that at this neutral pH, almost all of the total ammonia of a system exists as ammonium ion (NH₄⁺). In fact, only 0.5% exists as NH₃!

We must ensure that most of the total aqueous ammonia (NH₃ + NH₄⁺) is in the form of NH₃ which is the only aqueous species in equilibrium with gaseous ammonia, NH₃ (g). The amount of NH₃ (aq) at different pHs can be determined, as was done in the previous question

at pH = 6 the %NH3(aq) is 0.052%

at pH = 8 the %NH3(aq) is 4.8%

at pH = 10 the %NH3(,,q) is 83.3%

at pH = 11 the %NH3(aq) is 98%

Because only NH₃ (aq) is in equilibrium with NH₃, an operator would typically calculate the lime dosage to raise the pH to approximately 11 before running the waste stream through an air-stripping tower.

NH₃ is toxic to several fish species at concentrations above 0.2 mg/L. Thus wastewater discharges with a pH < 9 have most of the total ammonia in the less toxic NH₄⁺ form. This is one reason why some wastewater discharge permits for ammonia specify that the pH of the discharge must also be less than 9.