ENCE 433 Dr. Alba Torrents

ENVIRONMENTAL ENGINEERING ANALYSIS Fall 1996

Homework Set # 4 SOLUTION

Carbonate system and alkalinity

• 1.

The concentration of CO2 in water in equilibrium with air is obtained from Henry's law constant:

And the two ionization constants for carbonic acid are:

Since Ka1 is almost 10,000 times larger than Ka2, we can ignore the second stage of ionization of carbonic acid and write: [H3O+] = [HCO-]

Note that in this solution [CO2] is a constant, because the CO2 in solution is in equilibrium with the air which has a constant partial pressure of CO2 .

We can verify the assumption we made when we neglected the second stage of ionization by calculating the [CO3-2] in this solution using Ka2 .

We can summarize the concentration of all species as:

2.

textbook pag 175

3.

pH = 6.3

Alk = 1 x 10-3 eq./L

Before equilibration with CO2

After equilibration with CO2 with P(CO2) = 3.16x10-4atm:

pH = ?

CT = ?

Solve the alkalinity equation for open system:

Trial and error : pH = 8.3

CT = 9.9x10-4 M

CT decreases and some CO2 gets released to the atmosphere