2( 35.5) Therefore, alkalinity needed = 0.0142(50) = 0.71 mg/L as CaCO3 Ans
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 Cl2 is Cl2/2 = 35.5. Consider another chlorine disinfectant such as NaOCl. To find its available chlorine, its unit of disinfecting power must also, first, be determined.
From this equation, the unit of disinfecting power of NaOCl is NaOCl/2 = 37.24. Therefore, the available chlorine of NaOCl is the ratio of the mass of chlorine to the mass of NaOCl that has the same unit of oxidizing power as chlorine, or available chlorine of NaOCl = 35.5/37.24 = 0.95 or 95%. In other words, NaOCl is 95% effective compared with chlorine.
Example 17.12 What is the available chlorine in dichloramine?
Solution: When dichloramine oxidizes a substance, its chlorine atom is reduced to chloride; And, as gleaned from its formula, the nitrogen must be converted to the NH4+ ion. Thus, the oxidation-reduction reaction using only half the reaction is
the unit of oxidizing power of dichloramine = NHCl2/4 = 21.48 and, available chlorine = 35.5/21.48 = 1.65 or 165% Ans
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