Environmental Engineering Reference
In-Depth Information
DO Probe port
H 2 O Sample port
Diffuser
Pump
Flange
Cutting edge
0.27 m 2 over bottom velocity 0.8-1.2 fps
64.5 L.
FIGURE 5.28 In situ sediment oxygen demand chamber. (From USEPA SESD-EAB, Operating procedure:
Sediment oxygen demand, U.S. Environmental Protection Agency, Region 4 Science and Ecosystem Support
Division, Ecological Assessment Branch, Athens, GA, 2007.)
is a method for determining the oxygen production and respiration rates of a waterbody based on a
graphical analysis of diurnal DO curves (Odum 1956). Essentially, the DO is monitored over a diel
period and plotted, correcting for changes in saturation and diffusion (measured) and gross primary
productivity and respiration. Grace and Imberger (2006) describe other open water community
metabolism methods.
5.6 pH
The pH is a master variable impacting water quality. pH refers to the puissance or the power of
hydrogen, or the negative log of the hydrogen ion concentration (H + ). So, a pH of 7 indicates that the
hydrogen ion concentration is 10 −7 mol.
In equilibrium, the product of the hydrogen ion and the hydroxide concentration (OH ) would
equal approximately 10 −14 (the disassociation constant), so that if the pH is equal to 7, the pOH is 7.
If the pH is less than 7, then the solution is acidic and if the pH is greater than 7, it is basic. Some
typical pH values are provided in Figure 5.29.
Changes in the pH result from the addition of acidic or basic materials to aquatic systems. A
change in the pH resulting from the addition of acids is not necessarily linear. Aquatic systems may
have the ability to buffer or resist changes that may occur. The most common buffering system is
due to the presence of carbonates and the buffering capacity is measured as carbonate alkalinity.
Dissolved carbonates typically exist in one of three forms: carbonic acid, bicarbonate, and carbon-
ate (Figure 5.30). As the pH changes, the dominant form of the dissolved carbonate also changes, in
a nonlinear but easily predictable manner (Figure 5.31). So, at a pH below 4.5, all of the carbonates
exist as carbonic acid (dissolved carbon dioxide). As the pH increases from 4.5 to 8.3, the balance
shifts to the right and creates bicarbonate, and above 8.3, more carbonate is formed.
The impact of the carbonate species relationships can be illustrated using the alkalinity titration
curve. For example, if we take a water sample that is alkaline (high pH) and add a strong acid to it
(such as sulfuric or hydrochloric acid), the pH will go down, but only gradually, until we reach a pH
of near 8.3, where there is a shift of the inorganic carbon from carbonates to bicarbonates. A color
indicator, phenolphthalein, is commonly identiied at this end point. The end point is the carbon-
ate system and the shift from carbonates to bicarbonates will reduce the change in pH as an acid is
added. This resistance to pH changes is called chemical buffering. Similarly, if we continue adding
acid, we will reach another inlection point at a pH of 4.5, where the species shift is from bicarbonates
to carbonic acid. A color indicator, methyl orange, is commonly identiied at this end point. The total
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