Disinfecting power

In swimming pools disinfected by bromine, bromamide and bromimide can form. These compounds have about half the disinfecting power of HOBr giving bromine an advantage compared to chlorine. Chloramide and cblorimide have 80 to 100 times less disinfecting power than HOCl. Bromides. 

From Equation (17.9), the hypochlorous acid, HOCl, is formed, which is one of the chlorine disinfectants. If its formula is analyzed, it will be found that the chlorine has an oxidation state of -i-l, as we mentioned before. Note also that hydrochloric acid is formed. This is a characteristic in the use of the chlorine gas as a disinfectant. The water becomes acidic. Also, as we have mentioned, the chlorine molecule is a much stronger oxidizer than the hypochlorite ion and, hence, a stronger disinfectant. From Equation (17.9), if the water is intentionally made acidic, the reaction will be driven to the left, producing more of the chlorine molecule. This condition will then produce more disinfecting power. As will be shown later, however, this condition, where the chlorine molecule will exist, is at a very low pH hovering around zero. This makes the chlorine molecule useless as a disinfectant. 

Now, let us discuss the final fate of trichloramine during disinfection. In accordance with the chloramine reactions [Reactions (17.34) to (17.36)], by the time three moles of HOCl have been added, a mole of trichloramine would have been formed. This, however, is not the case. As mentioned, while the monochloramine decomposes in a stepwise fashion to convert into the dichloramine, its destruction into the nitrogen gas intervenes. Thus, the eventual formation of the dichloramine would be less in fact, much, much less, since, as we have found, formation of the gas is favored over the formation of the dichloramine. In addition, monochloramine and dichloramine, themselves, react with each other along with HOCl to form another gas N2O [NH2CI -1-NHCI2 + HOCl N2O -I- 4H" -I- 4CL]. Also, there may be more other side reactions that could occur before the eventual formation of the dichloramine from monochloramine. Overall, as soon as the step for the conversion of the dichloramine to the trichloramine is reached, the concentration of dichloramine is already very low and the amount of trichloramine produced is also very low. Thus, if, indeed, trichloramine has a disinfecting power, this disinfectant property is useless, since the concentration is already very low in the first place. This is the reason why combined chlorine is only composed of the monochloramine and the dichloramine. Also, it follows... 

Available chlorine. The strength of a chlorine disinfectant is measured in terms of available chlorine. Available chlorine is defined as the ratio of the mass of chlorine to the mass of the disinfectant that has the same unit of oxidizing power as chlorine. The unit of disinfecting power of chlorine may be found as follows in terms of one mole of electrons ... 

From this equation, the unit of oxidizing power of Cl, is CI2/2 = 35.5. Consider another chlorine disinfectant such as NaOCl. To find its available chlorine, its unit of disinfecting power must also, first, be determined. 

Each chlorination method provides the same disinfecting power on a pound for pound basis of available chlorine at the same pH. The choice of method depends primarily on the availability of each chemical and the construction and annual operating costs for the different systems. 

Bisphenols (not to be confused with the diphenols), are formed by two phenols linked with a bridge (-CHj-, -O-or -S-) in the ortho position, which makes the product easier to tolerate. Bisphenols are often chlorinated to increase their disinfectant power. Dichlorophene, tetrachlorophene... 

To more precisely determine the disinfectant power of a product, quantitative tests have been designed. In such a test, samples of untreated and biocide-treated organisms are plated on a nutrient medium. After incubation, the number of colony-forming units is counted and the reduction in viable cells is determined. The data are generally expressed as log reduction 1 log reduction, for example, corresponds to a product killing 90% of the initial inoculum. 

Prepare the phenol test dilutions from a standardised stock 5 per cent solution in distilled water of pure phenol (crystallising-point not less than 40-5°). Prepare the disinfectant test dilutions (usually in a series differing by 10 per cent or thereabouts) from an initial 1 or 2 per cent master dilution in distilled water. In making the master dilution always measure the disinfectant by pipette into the required volume of water and then mix thoroughly any other procedure might affect the quality of the emulsion formed and hence its disinfecting power. 

Concern over possible production of toxic organochlorine compounds by water chlorination processes has increased interest in ozonation. Furthermore, ozone is more destructive to viruses than is chlorine. Unfortunately, the solubility of ozone in water is relatively low, which limits its disinfective power. 

The disinfecting power of chlorine was tested in Japan by cultivating bacteria taken from the Jeddah seawater. The results were as shown in Figure 2.10 (Fiijiwara, 1999a). 

Disinfectant. Powerful oxidising agent in organic synth. Reagent for gc determination of cis-trans ratios in fatty esters. Liq. with unpleasant odour. Sol. H2O, EtOH, Et20. Fp +0.1°. Bp 105°. pK 8.2 (20°). 

Figure 22.9 shows the disinfection effect of a 60 S treatment after a 2 day cultivation for various media, for example, celery cabbage, grapes, lettuce, and perilla leaf. We obtained the result that the disinfection power was highest for ozone and decreased in the order ozone>chloride>water and also depended on the ozone concentration. Complete disinfection by ozone at 20 ppm was... 

---