Science of Water : Concepts and Applications

110 The Science of Water: Concepts and Applications
FIGURE 4.1 pH of selected liquids.
pH
pH is a measure of the hydrogen ion (H + ) concentration. Solutions range from very acidic (having
a high concentration of H + ions) to very basic (having a high concentration of OH − ions). The pH
scale ranges from 0 to 14, with 7 being the neutral value (see Figure 4.1). The pH of water is important
to the chemical reactions that take place within water, and pH values that are too high or low
can inhibit the growth of microorganisms.
High pH values are considered basic and low pH values are considered acidic. Stated another
way, low pH values indicate a high level of H + concentration, while high pH values indicate a low
H + concentration. Because of this inverse logarithmic relationship, there is a tenfold difference in
H + concentration.
Natural water varies in pH depending on its source. Pure water has a neutral pH, with an equal
number H + and OH − . Adding an acid to water causes additional positive ions to be released so that
the H + ion concentration goes up and the pH value goes down.
HCl ↔ H + + Cl −
To control coagulation and corrosion by water, the waterworks operator must test for the hydrogen
ion concentration to determine the water’s pH. In a test for coagulation, as more alum (acid)
is added, the pH value lowers. If more lime (alkali) is added, the pH value rises. This relationship
should be remembered—if a good fl oc is formed, the pH should then be determined and maintained
at that pH value until the raw water changes.
Pollution can change water’s pH, which in turn can harm animals and plants living in the water.
For instance, water coming out of an abandoned coal mine can have a pH of 2, which is very acidic
and would defi nitely affect any fi sh crazy enough to try to live in it. By using the logarithm scale,
this mine-drainage water would be 100,000 times more acidic than neutral water—so stay out of
abandoned mines.
√ Important Point: Seawater is slightly more basic (the pH value is higher) than most natural
fresh water. Neutral water (such as distilled water) has a pH of 7, which is in the middle of being
acidic and alkaline. Seawater happens to be slightly alkaline (basic), with a pH of about 8. Most
natural water has a pH range of 6–8, although acid rain can have a pH as low as 4.
COMMON WATER MEASUREMENTS
Neutral
Acidic Basic
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
1 M Gastric Oranges Urine Blood Sea Household 1 M
HCl juices Tomatoes Pure water ammonia NaOH
water
Water practitioners/regulators such as waterworks operators and USEPA along with their scientifi c
counterparts at the U.S. Geological Survey (USGS) have been measuring water for decades. Millions
of measurements and analyses have been made. Some measurements are taken almost every