Final answer:
The percentage of total disinfection power of hypochlorous acid in drinking water disinfection exists as 97% in its acid form at pH 6 and 76% at pH 7. The percentages are calculated using the Henderson-Hasselbalch equation and the known pKa value for HOCl.
Step-by-step explanation:
When Cl2 gas is added to water during the disinfection of drinking water, it reacts to form hypochlorous acid (HOCl) and its conjugate base (OCl-). The disinfection power of HOCl is significantly higher than that of OCl-. Given that the pKa of HOCl is 7.5, to calculate the percentage of the total disinfection power existing in the acid form at a certain pH, we can use the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
Where [A-] is the concentration of the conjugate base (OCl-) and [HA] is the concentration of the acid (HOCl). Rearranging for the ratio [A-]/[HA] gives:
[A-]/[HA] = 10(pH - pKa)
At a pH of 6, which is lower than the pKa, the acid form predominates. Conversely, at pH 7, closer to the pKa, the ratio of acid to base forms is closer to 1. To find the percentage of disinfection power in the acid form, we can calculate the proportion of the acid form relative to the total of both forms.
The percentage of disinfection power from HOCl can be expressed as:
% Disinfection power from HOCl = (relative concentration of HOCl / (relative concentration of HOCl + relative concentration of OCl-)) x 100
At pH 6, this leads to approximately 97% of the disinfection power in the acid form, while at pH 7, it results in about 76%.