Chasville said:
So, that being that, why was WB using the numbers he was using? Where did they come from? Why do his numbers indicate that Ozone is a less effective oxidier than chlorine?
:
I had based some of my justification/decision on the use ozone (and its expense) from the published 2.0 number. Now, if that is the wrong number to use to make a sensible comparison with, then I'd like to know the truth.
:
I took some time to peruse through the link CG provided that has a chart of redox potentials for may reactions and many materials. It seems that oxygen and ozone and chlorine "compounds" have many ways to go and various potentials. I see that the "marketers" seem to have glombed onto the most favorable numbers for their purposes, sans science (not too surprising). But, aren't the reactions with the greatest potential the ones most probable to occur? Or are we missing a number? I remember there being a curve, where the current state is at one level, then there is this wall and a slope on the other side. For instance, the thermite reaction between aluminum and iron oxide is incredibly exothermic, but the wall it has to overcome is pretty big. That was why the chemistry teacher used a magnesium "fuse" to set the correct temperature for the mixture to take off with.
If you read the post of mine that I linked to, I explained where waterbear was getting his chlorine number, using chlorine gas instead of hypochlorous acid. The +1.24V comes from the +1.229V for oxygen in the table I linked to (there are small variations in different tables as these numbers get remeasured and slightly improved over time), but it does not apply to ozone which has a higher potential (i.e. waterbear was wrong about that, but keep in mind that he make less mistakes than I do and we correct each other in any event).
You based your justification on a false single number to use. Your description of the "curve" is correct where the difference in potential (height level of the curve) for the reactants vs. the products is the thermodynamic (Gibbs) energy difference which is also related to the difference in electrode potential for redox reactions. It is NOT, however, a predictor of whether the reaction will occur because of the "hill" of activation energy that must be overcome and as you point out higher temperature can get you over that hill. However, the heat of reaction ("exothermic" that you mentioned) is just one component of the thermodynamic energy -- entropy is another factor.
The bottom line is that you should absolutely, positively, not base a purchase decision for ozone on a single redox potential number. Heck, if that were the case, you should just dump some fluorine (F
2) or Praseodymium ion (Pr
4+) or Terbium ion (Tb
4+) or xenon flouride (XeF) in the pool and potentially oxidize even more including the metal in your pool. The fact is that what makes hypochlorous acid so great as a fast killer of bacteria and viruses is that it looks a lot like water (compare
HOCl vs.
H2O), is a neutral molecule, and a fairly reactive oxidizer (but also for substitution reactions which are not redox reactions but disturb the normal functioning of proteins and other organic substances).
Chromic acid (H
2CrO
4) with sulfuric acid (aka Jones reagent) is a very strong oxidizer that will oxidize many organic compounds, yet the redox potential of HCrO
4- is only +1.35V. You simply cannot look at the standard electrode potential to predict actual chemical reactivity. You can only use it to predict when something cannot possibly happen (at least spontaneously without energy input as with electrolysis or photosynthesis).
Since you've got to have some chlorine in the water for disinfection anyway, you might as well look at it for oxidation as well. It's a reasonable oxidizer in most cases, especially for residential outdoor pools. You can, of course, buy more chemicals or equipment to your heart's content, but it is not necessary to do so.
Richard