ETS said:
Well then, what is the correct percentage? How does it know when some of the chlorine is used up and when it has reached the correct percentage? I'm not sure I buy that explanation yet.
The reason I'm asking is because I work in a
Pool Store
as a water test technician and am trying to get a handle on this so I can give my customers a more accurate prescription for their algae problems. You see, our store is very busy ( I sometimes do over 100 tests a day) and don't have time to do a lot of sophisticated tests. So I can only measure FC up to 5.0. The computer program we use calculates gallons of chlorine to use for different levels of algae bloom, but I'm sure it doesn't factor in CYA levels. I have to do a lot of guessing.
Let me jump in here since I just noticed this thread.
Chemical Equilibrium
It's a chemical equilibrium between chlorine bound to CYA and hypochlorous acid which is the "active" form of chlorine. This can be described as follows:
HClCY
- + H
2O <<<---> HOCl + H
2CY
-
"Chlorine bound to CYA" + Water <<<---> Hypochlorous Acid + CYA
I show the arrows mostly going to the left because at typical CYA levels in a pool the vast majority of chlorine is bound to CYA. When I say "bound", this is technically a separate chemical species -- it's not just some sort of loose association. This chemical species has very little (if any) capability to kill pathogens, prevent algae growth, or oxidize bather waste. It is hypochlorous acid that is the chemical species that does all of these things. With chemical equilibrium, chemical species are colliding into each other frequently in all sorts of combinations, but most don't react. When the combinations shown above (on the left and on the right) collide, they sometimes form the items on the other side. This happens very quickly at normal temperatures, back and forth, back and forth. As for where things settle out in terms of percentages, this is a function of the ratio of reaction rates going from right-to-left vs. left-to-right and this ratio (for simple reactions) is known as the equilibrium constant for the reaction. The equilibrium constant for the above reaction at the concentrations of chlorine and CYA in pools is way toward the left. When there is 3.5 ppm FC with 30 ppm CYA and a pH near 7.5, 97% of the chlorine is bound to CYA, 1.5% is hypochlorous acid, and 1.5% is hypochlorite ion.
The rate at which the above chemical equation occurs is reasonably fast. If all of the hypochlorous acid on the right-hand-side were used up, then it takes around 4 seconds for half of the amount of "Chlorine bound to CYA" to get converted to hypochlorous acid. In practice, very little gets converted (unless chlorine is rapidly getting used up) and the reaction slows down and then essentially stops and this all occurs in far less than one second. In fact, because this reaction (as well as companion reactions that are only 0.25 seconds that I don't show for simplicity) occurs so quickly, chlorine tests aren't really measuring the hypochlorous acid level, but rather both chlorine species on the left and right above (plus hypochlorite ion which I didn't show). That is, the chlorine tests are measuring both active chlorine and that in reserve because as soon as they use up some active chlorine, the reaction moves to the right releasing more chlorine that then gets used up (reacting with dye, for example). The chemicals don't have to "know" anything -- they are just blindly colliding, but when the chlorine on the right gets taken away, the reaction from left-to-right is all that is occurring (i.e. there isn't any right-to-left reaction because there isn't any hypochlorous acid because it's reacted with dye in the chlorine test).
Chlorine/CYA Relationship
It turns out from the chemistry that the same ratios of FC/CYA have roughly the same amount of active chlorine, hypochlorous acid (technical details are in
this post). So if the CYA is doubled, then the FC has to be doubled to get the same level of disinfection, algae kill, oxidation, etc. If the CYA is doubled and the FC is kept the same, then the amount of active chlorine is cut in half. If this amount of chlorine kills algae more slowly than algae can reproduce (double in population), then the water will usually turn dull, then cloudy, then cloudy-green as an algae bloom develops.
This chart in the Pool School shows the level of chlorine needed to prevent algae growth at various CYA levels and also gives the amount need to shock the pool to kill algae in an algae bloom reasonably quickly.
You can read the introduction sections in the technical paper written in 1974 in this link that describes in more detail this "reservoir" chlorine concept. Just keep in mind that the terminology in those days was different and the "Free Chlorine" in that paper isn't what is measured in today's chlorine tests and the paper's definition excludes the chlorine bound to CYA which they call reservoir chlorine.
You can also look at the graphs in
this post that show the major chemical species concentrations vs. pH without CYA and with CYA.
Why Hypochlorous Acid Is Effective While Chlorine Bound to CYA Is Not
As for why hypochlorous acid is such an effective pathogen and algae killer, take a look at the molecule
here and notice how similar it is to water
here. This similarity is why this molecule rather easily passes through cell membranes to react with all sorts of chemical species (mostly containing nitrogen, which includes proteins, enzymes, DNA, RNA, etc.) disrupting cell processes that then kill them (see
this thread). Now take a look at Dichlor
here which is very similar to the main "chlorine bound to CYA" chemical species where the only difference is that one of the two chlorine is removed from being attached to a nitrogen and this makes the overall molecule negatively charged. Notice how much larger this molecule is and that it doesn't look anything like water. The primary chemical species with chlorine bound to CYA are negatively charged and are repelled by most cells since they generally have negatively-charged surfaces (on the outside). It is not only their charge and size that makes these chemicals less reactive, but the "eagerness" (oxidation potential) of the chlorine to release from the CYA ring is somewhat limited -- it does sometimes leave, but it mostly likes to stay where it is (remember the reaction I showed above with water and how mostly it's to the left hand side).
You might think that with chlorine getting released from the reserve quickly that this should count towards being effective, but that's not correct. You can think of an analogy of people fighting a war where you have a group of "active" soldiers on the front lines fighting hand-to-hand combat with an enemy. You have many more soldiers in the rear that are not directly fighting and are in "reserve". When some soldiers in the front lines get killed or injured, you can replace them with some from the reserve, but the rate at which you will be able to wound or kill the enemy is only dependent on the number of soldiers you have on the front lines doing the hand-to-hand combat. It doesn't matter how many you have in reserve. The amount in reserve only tells you how long you can continue to fight -- not the RATE at which you kill your enemy.
Does that help?
Richard
I think you may be confused about chlorine and CYA and their roles. A great place for you to start is Pool School. Then start reading the various threads in the forum sections, and it will all start to make sense. 
(& that is some feat for a butterfly) :lol:
NO.