Photochemical smog

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The capacity of the organic and biological matter in a sample of natural water to consume oxygen, a process catalyzed by bacteria present, is called its biochemical oxygen demand, BOD. It is evaluated experimentally by determining the concentration of dissolved 0 2 at the beginning and at the end of a period in which a sealed water sample seeded with bacteria is maintained in the dark at a constant temperature, usually either 20°C or 25°C. A neutral pH is maintained by use of a buffer consisting of two ions of phosphoric acid, namely H 2 P0 4- and HP0 4 2- : The BOD equals the amount of oxygen consumed as a result of the oxidation of dissolved organic matter in the sample. The oxidation reactions are catalyzed in the sample by the action of microorganisms present in the natural water. If it is suspected that the sample will have a high BOD, it is first diluted with pure, oxygen-saturated water so that sufficient O 2 will be available overall to oxidize all the organic matter; the results are corrected for this dilution. Usually the reaction is allowed to proceed for five days before the residual oxygen is determined. The oxygen demand determined from such a test, often designated BODs, corresponds to about 80% of that which would be determined if the experiment were allowed to proceed for a very long timewhich of course is not very practical. The median BOD for unpolluted surface water in the United States is about 0.7 mg O 2 per liter, which is considerably less than the maximum solubility ofO 2 in water (of8.7 mg/L at 25°C). In contrast, the BOD values for sewage are typically several hundreds of milligrams of oxygen per liter.
Decomposition of Organic Matter in Water Dissolved organic matter will decompose in water under anaerobic (oxygen free) conditions if appropriate bacteria are present. Anaerobic conditions occur naturally in stagnant water such as swamps and at the bottom of deep lakes. The bacteria operate on carbon to disproportionate it; that is, some carbon is oxidized to carbon dioxide, CO 2 , and the rest is reduced to methane, CH 4 : This is an example of a fermentation reaction, which in chemistry is defined as one in which both the oxidizing and the reducing agents are organic materials. Since methane is almost insoluble in water, it forms bubbles that can be seen rising to the surface in swamps and sometimes catches fire.
Page 1 and 2: Photochemical smog Photochemical sm
Page 4 and 5: PAN (Peroxyacytyl nitrate) is a kin
Page 6 and 7: Chemical Equation for Photochemical
Page 8 and 9: Pollution by Carbon monoxide (CO) C
Page 10 and 11: Symptoms of CO poisoning The initia
Page 12 and 13: Effect = function of CO concentrati
Page 14 and 15: Water Pollution
Page 16 and 17: Thus, water pollution is all about
Page 18 and 19: Any change in the dynamic equilibri
Page 20 and 21: Oxidation-Reduction Chemistry in Na
Page 24 and 25: Since anaerobic conditions are redu
Page 26: FIGURE 13-3 The stratification of a

Photochemical smog

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