Can someone explain why chlorine is a more effective disinfectant in a lower pH environment? Technical is fine, I've been brushing up on my chemistry for a year. But a subtitled description of what's happening would be fine.

Basically, if you you have a swamp with a 25 drop base demand, is it better to clear the pool up first or raise the pH and alkalinity first?

My understanding is that the more H+ in the water (lower pH) the more quickly that OCL- will convert back to HOCL and keep killing algae. Is there a point where the water is too acidic?

On a side note, does algae have a harder time growing in an acidic environment? I've seen cases where someone with a 40 drop base demand has "a tiny bit of green algae on the floor of the pool" become a swamp overnight when pH is raised before the addition of chlorine.

Thanks chums.

If there is no Cyanuric Acid (CYA) in the water, then the amount of active chlorine, hypochlorous acid (HOCl), is dependent on the pH due to the following chemical equilibrium:

HOCl <---> H⁺ + OCl^-
Hypochlorous Acid <---> Hydrogen Ion + Hypochlorite Ion

However, with CYA in the water, most of the chlorine is bound to CYA and this effectively acts like a hypochlorous acid buffer so pH swings have far less of an effect on the HOCl concentration. The situation without and with CYA is shown in the graphs in this post.

If the pH is very low in a pool, it is generally best to raise the pH before doing anything else. This is to protect plaster and equipment, though in practice the pH has to be pretty low to be a problem in the short-term. If you are going to shock the pool, then the pH need not be raised above 7.2 since shocking is going to raise the pH at least temporarily anyway.

Algae usually grows better in somewhat alkaline conditions, mostly because more readily absorbed algae nutrients are available, such as having more ammonia vs. ammonium ion and having more phosphate ion vs. hydrogen phosphate ions compared to lower pH as shown in the following chemical equations.

NH₄⁺ <----> H⁺ + NH₃
Ammonium Ion <----> Hydrogen Ion + Ammonia

HPO₄^2- <----> H⁺ + PO₄^3-
Hydrogen Phosphate Ion <----> Hydrogen Ion + Phosphate Ion

Even under ideal conditions, however, it still takes algae 3 to 8 hours to double in population so if you raise the pH you've still got plenty of time to kill off the algae faster than it can reproduce. Waiting 12 hours could have the algae grow by a factor of 16 if plenty of nutrients are available.

Richard