Page 234 - Water and wastewater engineering

COAGULATION AND FLOCCULATION 6-11

Case I
Acid is added to carbonate buffer system a
* b
Reaction shifts to the left as HCO 3 is formed when H and HCO 3 combine
2
CO 2 is released to the atmosphere

pH is lowered slightly because of the availability of free H (amount depends on
buffering capacity)

Case II
Base is added to carbonate buffer system

Reaction shifts to the right
CO 2 from the atmosphere dissolves into solution

pH is raised slightly because H combines with OH (amount depends on
buffering capacity)
Case III
CO 2 is bubbled into carbonate buffer system

*
Reaction shifts to the right because HCO 3 is formed when CO 2 and H 2 O
2
combine
CO 2 dissolves into solution
pH is lowered

Case IV
Carbonate buffer system is stripped of CO 2
*
Reaction shifts to the left to form more HCO 3 to replace that removed by
2
stripping
CO 2 is removed from solution
pH is raised

a
Refer to Equation 6-2.
b
The asterisk * in the H 2 CO 3 is used to signify the sum of CO 2 and H 2 CO 3 in solution.
FIGURE 6-7
Behavior of the carbonate buffer system with the addition of acids and bases or the addition and removal
of CO 2 . (Source: Davis and Cornwell, 2008.)

carbonate (CaCO 3 ), which is very insoluble. As a consequence, it precipitates from solution. The
2 2
reaction of Ca with CO 3 to form a precipitate is one of the fundamental reactions used to
soften water.

Alkalinity. Alkalinity is defined as the sum of all titratable bases down to about pH 4.5. It is
found by experimentally determining how much acid it takes to lower the pH of water to 4.5. In