kinds of aquatic organisms require oxygen for their existence, water bodies should contain appreciable level of dissolved oxygen. Dissolved oxygen can decrease due to different reasons. Part of the oxygen coming from algal photosynthesis during the day for example is used up by the algae itself as part of their metabolic processes. Because of this, dissolved oxygen contribution through algal photosynthesis is not that efficient. The degradation of biomass coming from dead algae and other organic matter also consume dissolved oxygen. Therefore, the ability of a body of water to reoxygenate itself by contact with the atmosphere is an important characteristic. B) Carbon Dioxide in Water Carbondioxide is present in virtually all natural waters and wastewaters and is the most important weak acid in water. The CO 2 in water comes from the dissolution of atmospheric CO 2 and from microbial decay of organic matter. Carbon dioxide and its ionization products have extremely important influence upon the chemistry of water. Many minerals are deposited as salts of the carbonate ion. Although dissolved CO 2 is often represented as H 2 CO 3 just a small fraction of the dissolved CO 2 is actually present as H 2 CO 3 . Therefore, to make a distinction, nonionized carbon dioxide in water is designated simply as CO 2 . The equilibrium for dissolution of CO 2 in water is represented by the following equations CO 2 + H 2 O ⇌ HCO 3 - + H + (3.1) K a1 = [HCO 3 - ] [ H + ] = 4.45 x 10 -7 pK a1 = 6.35 (3.2) [CO 2 ] HCO 3 - ⇌ CO 3 2- + H + (3.4) K a2 = [CO 3 2- ] [ H + ] = 4.69 x 10 -11 pK a2 = 10.33 (3.5) [HCO 3 - ] Therefore, dissolved CO 2 at equilibrium contains: CO 2 , bicarbonate ion (HCO 3 - ), and carbonate ion (CO 3 2- ). The predominant species formed by CO 2 dissolved in water depends upon pH. This is best shown by a distribution of species diagram with pH as a master variable as illustrated in Figure 3.6.
From Figure 3.5 we can see that hydrogen carbonate (bicarbonate) ion (HCO 3 - ) is the predominant species in the pH range found in most waters, with CO 2 predominating in more acidic waters. 3.1.3 Acid-base phenomena in water In your general chemistry courses you have studied the various definitions of acids and bases given by different scientists. In this section, we use the definition given by Bronsted and Lowry. According to the Bronsted - Lowry definition, acids are described as those chemical species that lose H + ion in a reaction and bases as those chemical species that accept the H + ion lost by acids. The bicarbonate ion, HCO 3 - , is an important species in the acid-base chemistry of water. In water, it may act as either an acid or a base: HCO 3 - ⇌ CO 3 2- + H + (HCO 3 - acting as an acid) (3.6) HCO 3 - + H + ⇌ CO 2 (aq) + H 2 O (HCO 3 - acting as a base) (3.7) Acidity as applied to natural water and wastewater is the capacity of the water to neutralize OH - and alkalinity is the capacity to neutralize H + .
- Fall '19