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6-12 WATER AND WASTEWATER ENGINEERING
most waters the only significant contributions to alkalinity are the carbonate species and any free
H or OH . The total H that can be taken up by a water containing primarily carbonate species is
2
2
]
Alkalinity [HCO 3 ] [CO 3 ] [ OH [ H ] (6-3)
where [] refers to concentrations in moles/L. In most natural water situations (pH 6 to 8), the
OH and H are negligible, such that
2
2
Alkalinity [HCO 3 ] [CO 3 ] (6-4)
2
Note that [CO 3 ] is multiplied by two because it can accept two protons. The pertinent acid/base
reactions are
HCO H HCO pK 6 35 at 25 C
.
2 3 3 1 a (6-5)
H HCO H CO 2 pK 10 33 at 25 C (6-6)
.
3 3 a 2
From the p K values, some useful relationships can be found. The more important ones are as
follows:
1. Below pH of 4.5, essentially all of the carbonate species are present as H 2 CO 3 , and the
alkalinity is negative (due to the H ).
2. At a pH of 8.3 most of the carbonate species are present as HCO 3 and the alkalinity
equals HCO 3 .
2
3. Above a pH of 12.3, essentially all of the carbonate species are present as [CO 3 ] and the
2
alkalinity equals [CO 3 ] [OH ]. The [OH ] may not be insignificant at this pH.
Figure 6-8 schematically shows the change of species described above as the pH is lowered
by the addition of acid to a water containing alkalinity. Note that the pH starts at above 12.3
and as acid is added the pH drops slowly as the first acid (H ) addition is consumed by free
hydroxide (OH ), preventing a significant pH drop, and then the acid is consumed by carbonate
2
(CO 3 ) being converted to bicarbonate (HCO 3 ). At about pH 8.3 the carbonate is essentially all
converted to bicarbonate, at which point there is another somewhat flat area where the acid is
consumed by converting bicarbonate to carbonic acid.
From Equation 6-4 and the discussion of buffer solutions, it can be seen that alkalinity serves
as a measure of buffering capacity. The greater the alkalinity, the greater the buffering capacity.
We differentiate between alkaline water and water having high alkalinity. Alkaline water has a
pH greater than 7, while a water with high alkalinity has a high buffering capacity. An alkaline
water may or may not have a high buffering capacity. Likewise, a water with a high alkalinity
may or may not have a high pH.
By convention, alkalinity is not expressed in molarity units as shown in the above equations,
but rather in mg/L as CaCO 3 . In order to convert species to mg/L as CaCO 3 , multiply mg/L as the
species by the ratio of the equivalent weight of CaCO 3 to the species equivalent weight:
⎛ ⎞
mg/L as CaCO 3 ( mg/L as the species) ⎜ EW CaCO3 ⎟ (6-7)
W
⎝ EW species ⎠