Kathy Hood of Marlowe Environmental Services introduces hypochlorous acid and hypochlorite
Free available chlorine microbial control
Kathy Hood of Marlowe Environmental Services introduces hypochlorous acid and hypochlorite
Microbiological control in water systems is critical to protecting human health , as well as inhibiting the formation of microbial slimes . This can not only reduce water flow and heat transfer , it can also promote underdeposit corrosion .
One of the biocides most used to combat potential pathogens is free available chlorine . In 1997 , Life magazine stated , “ The filtration of drinking water plus the use of chlorine is probably the most significant public health advancement of the millennium .” Swimming pools also commonly use chlorine-based disinfectants to kill microorganisms , thereby reducing the risk of waterborne diseases .
The ability of chlorine to act as a disinfectant depends on factors such as the concentration of free chlorine residual concentration , contact duration , pH levels and water temperature . Increasing the concentration , contact time and water temperature all improve biocidal efficacy . Free chlorine has biocidal efficacy against algae , bacteria and viruses .
HOCL & OCl-
Hypochlorous acid ( HOCl ) and hypochlorite ( OCl- ), both of which are delivered by Marlowe Environmental Services , are two forms of free available chlorine that exist in aqueous solutions . They are widely used for sanitation , water treatment and various industrial applications .
However , they differ significantly in their chemical structure and reactivity . HOCl is a weak acid ,
Figure 1 – pH effectiveness in HOCL production
whereas OCl- is the conjugate base of hypochlorous acid . It is these differences in their chemical make-up that affect their biocidal efficacy .
HOCl is the more active as a biocide . Its efficacy has been known for over a century , with reports of its use as a disinfectant dating back to World War I . The acid is neutral in charge , allowing it to penetrate the cell membranes of microorganisms more readily .
Microbial cell membranes usually have a phospholipid bilayer , incorporating hydrophilic tails facing inward and hydrophobic heads facing outwards . Both sides of the membrane are negatively charged , meaning that it repels charged molecules and prevents their passive diffusion into the cell . Once inside the cell , HOCl disrupts critical biological processes by oxidising key cellular components , such as proteins , enzymes and nucleic acids , leading to the rapid inactivation of the microorganism .
In contrast , OCl- is negatively charged and therefore less effective at penetrating microbial cell membranes . As a result , its biocidal action is slower and less efficient than HOCl . Studies have shown that at a given concentration of free chlorine , solutions with a higher proportion of HOCl exhibit significantly greater antimicrobial activity than those where OCl- is the prevalent form .
The US National Research Council ’ s Safe Drinking Water Committee found that HOCl is approximately 50 to 80 times more effective than OCl- in killing pathogens , although figures can vary . The dissociation of sodium hypochlorite solutions into HOCL and OCl- , depending on the pH , is welldocumented . This is illustrated by the following equations :
NaOCl + H 2
O →HOCl + NaOH HOCl → H + + OCl-
The ratio of HOCl and OCl- is pHdependant . HOCl is very limited at pH
42 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981