Fundamental Chemistry and Hydraulics - Mathematics and Statistics for the Environment - page 355

14.1.2.7.12 Alkalinity
Alkalinity is the capacity of water to accept protons (positively charged particles); it can also be defined
as a measure of the ability of the water to neutralize an acid. Stated in even simpler terms: Alkalinity
is a measure of water's capacity to absorb hydrogen ions without significant pH change (i.e., capacity
to neutralize acids). Bicarbonates, carbonates, and hydrogen cause alkalinity compounds in a raw or
treated water supply. Bicarbonates are the major components, due to the action of carbon dioxide on
basic materials of soil; borates, silicates, and phosphates may be minor components. Alkalinity of raw
water may also contain salts formed from organic acids, such as humic acid. Alkalinity in water acts
as a buffer that tends to stabilize and prevent fluctuations in pH. It is usually beneficial to have signifi-
cant alkalinity in water because it would tend to prevent quick changes in pH. Quick changes in pH
interfere with the effectiveness of common water treatment processes. Low alkalinity also contributes
to corrosive tendencies of water. When alkalinity is below 80 mg/L, it is considered low.

14.1.2.7.13 Hardness
Hardness may be considered a physical or chemical parameter of water. It represents the total
concentration of calcium and magnesium ions, reported as calcium carbonate. Hardness causes
soaps and detergents to be less effective and contributes to scale formation in pipes and boilers.
Hardness is not considered a health hazard; however, lime precipitation or ion exchange must often
soften water that contains hardness. Low hardness contributes to the corrosive tendencies of water.
Hardness and alkalinity often occur together because some compounds can contribute both alkalin-
ity and hardness ions. Hardness is generally classified as shown in Table 14.6.

■ EXAMPLE 14.7
Problem: Find the molarity and normality of a solution if 22.4 g of Na 2 CO 3 are dissolved in water
and the solution is made up to 500 mL.

Solution : The molecular weight of Na 2 CO 3 is 106. Molarity is number of moles/volume in liters.
Number of moles is actual weight/molecular weight. The net positive valence of Na 2 CO 3 is 1 × 2 =
2. The equivalent weight of Na 2 CO 3 is 106/2 = 53. So,

Actual weight/molecularwt.
Volume

Actual weight
Molecularwt.

22 4 .

Molarity = = = = 042 . M

(L) × volume(L) 106 × 0 500 .

Actual weight
Equivalent weight

22.4

Normality = = = 0085 . N

× volume(L) 53 × 0.500

TABLE 14.6
Water Hardness

Classification mg/L CaCo 3

Soft 0-75

Moderately hard 75-150

Hard 150-300

Very hard Over 300

Source: Spellman, F.R., Handbook of Water
and Wastewater Treatment Plant
Operations , 3rd ed., Lewis Publishers,
Boca Raton, FL, 2013.