In: Civil Engineering
The inactivation of a pathogenic bacterium by HOCl follows Chick’s law, i.e. the rate of death is a 1st order reaction, where the death rate is proportional to the number of bacteria at any time. 90 % of the pathogen are killed after 5 min for a constant free chlorine concentration of 2 mg/L (assume low pH, where all free chlorine is present as HOCl).
a) (9%) What is the percent kill after 10 min at the same HOCl concentration?
b) (8%) What percentage of the free chlorine would be present as HOCl (not OCl-) at pH8 if the pKa for hypochlorous acid is 7.5?
c) (3%) Why and how would the disinfection time have to change to still achieve 90 % kill of the pathogen at pH 8 at the same free chlorine concentration? (no need to calculate, discuss in terms of order of magnitude)