I have recently come across the term "combined chlorine" as opposed to chloramines. In one context I was talking with a manufacturer's rep for a salt chlorine product. He told me that his product may not be suitable for some commercial pools because it causes test kits to register combined chlorine. He explained that combined chlorine is not the same as chloramines, which we know to be harmful. The company line is that not all combined chlorine is a bad thing, and his product has no negative effect on the sanitizer effectiveness in the pool.
In another context, I read a recent post regarding ozone where a contributor opined that when ozone and chlorine come into contact, the chlorine becomes "combined chlorine" and is therefore rendered useless. This statement makes no sense to me based on my experience with ozone pools, where I use a small amount of chlorine to fight algae. Maybe the chlorine in my pools is "combined" but it cerainly is not useless.
Even residential drinking water is sometimes sanitized with chloramines, so they are not entirely useless as a sanitizer.
I would love to have a better understanding of the nuances of the term "combined chlorine" as opposed to "chloramine".
The terms are often used interchangeably by the pool/spa industry. Technically, though, there can be chlorine combined with some organics not via amine groups (nitrogen with a lone pair of electrons) and therefore not be chloramines but that is rare. The more common chloramines or combined chlorines are the following:
Nitrogen trichloride aka Trichloramine (NCl3)
Haloacetic acids (HAA5)
Trihalomethanes (THM) including chloroform
Chlorinated amino acids
many other chlorinated organic compounds
There are also chlorinated organic compounds that measure as Free Chlorine (FC) and are not called combined chlorine or chloramines. This includes chlorine bound to Cyanuric Acid (CYA) which are chlorinated isocyanurates. These release chlorine quickly in the time of the FC test where half is released every 0.25 seconds. So while they do not provide for significant disinfection or oxidation, they act as a chlorine reserve to replenish chlorine that gets used up.
Combined Chlorine (CC) as measured in standard tests does not release its chlorine readily, not even as a reserve. It is released in the presence of a stronger reducing agent such as iodide (as used in most CC tests) where the CC oxidizes it to form iodine that then reacts with the DPD dye.
Other strong oxidizers may measure as FC or CC and not contain any chlorine at all. Non-chlorine shock, potassium monopersulfate (MPS), measures as CC in the DPD test and as FC in the FAS-DPD test (if there is any chlorine present as well). There is a chemical that can be added to remove such interference in these tests.
As for ozone and chlorine, these do react with each other, but they do NOT form chloramines or combined chlorine. Details of their reaction is in the following sources:
where ozone reacts with chlorine to form oxygen gas, chloride ion and chlorate ion. In a pool with an ozonator, the ozone mostly reacts with chlorine in the chamber and somewhat near where it is introduced into the pool. It does not last and generally the amount of ozone is so low compared to the volume of pool water that one does not notice the reduction in chlorine (except in residential spas where the ozonator's increase in chlorine demand is usually quite noticeable). The chlorine residual in the pool is what keeps algae from growing. The ozone only helps for killing algae that is circulated through the ozonator, but does nothing for any algae growing on surfaces.
Is it true that when ozone & chlorine react, hydroxyl radicals are formed?
No, that is not true. As shown in the two papers I referenced, the following are the reactions of ozone with chlorine (hypochlorite ion) and their percentages:
O3 + OCl- ---> 2O2 + Cl- (77%)
Ozone + Hypochlorite Ion ---> Oxygen Gas + Chloride Ion
2O3 + OCl- ---> 2O2 + ClO3(-) (23%)
Ozone + Hypochlorite Ion ---> Oxygen Gas + Chlorate Ion
As I wrote, ozone and hypochlorite ion react to form oxygen gas, chloride ion and chlorate ion. There are no hydroxyl radicals produced by this reaction.
Now it is true that ozone on its own will break down into hydroxyl radicals (and other oxygen species) above a pH of 7. This has nothing to do with chlorine and occurs due to instability of ozone itself in the presence of more hydroxyl ions (higher pH). The production of hydroxyl radicals is part of what makes ozone effective since ozone itself does not react with as many chemicals as hydroxyl radicals. Hydroxyl radicals, however, are very short-lived (microseconds).
Note that an outdoor chlorinated pool exposed to sunlight also produces hydroxyl radicals. This is what happens when UV breaks down chlorine as described in the following paper:
HOCl + hν ---> OH• + Cl•
Hypochlorous Acid + UV ---> Hydroxyl Radical + Chlorine Radical
OCl- + hν ---> O•(-) + Cl•
Hypochlorite Ion + UV ---> Atomic Oxygen Ion Radical + Chlorine Radical
O•(-) + H2O ---> OH• + OH-
Atomic Oxygen Ion Radical + Water ---> Hydroxyl Radical + Hydroxyl Ion
This is why outdoor chlorinated residential pools with the proper FC/CYA ratio that is high enough to prevent algae growth can be so clear and remain so clean. The chlorine breakdown from UV into hydroxyl radicals helps to oxidize additional chemicals that chlorine alone is not able to handle. However, since there is a limited amount of chlorine that breaks down from sunlight each day, this process is most noticeable in residential pools due to their low bather loads. A high bather load commercial/public pool will still benefit from some supplemental oxidation.
Combined chlorine is typically the result of not enough free chlorine in the pool. A high bather load, or "pool accident" can overwhelm the FC and create the CC condition, for example.
We always test the FC and CC when we do our water analysis with the Taylor K2006 test kit. We know that if we have a CC of more than 1.0 ppm we need to shock the pool to remove the CC condition and return the pool to a safe condition.
Saltwater chlorine generators produce high chlorine levels at low pH near the chlorine generation plate so the portion of water that passes through that region gets super-chlorinated and will tend to oxidize more quickly intermediate chloramines such as monochloramine and dichloramine and may oxidize some organics and organic chloramines faster. The downside is that higher active chlorine levels, especially at lower pH, form more nitrogen trichloride which is very volatile and irritating.
Hydroxyl radicals are produced in greater numbers using boron-doped diamond electrodes as found in the Adamant Technologies Oxineo system as well as the HotSpring Spas ACE Salt Water Sanitizing System.
The microflocculation is described in the "Chemistries of Ozone..." paper I linked to:
One of the major advantages of partial oxidation of organic materials present in pool/spa waters by ozone is that in becoming partially oxidized, the organic materials become much more polar than they were originally. Polar groupings such as carbonyl (>C=O), carboxyl (-COOH), and hydroxyl (-OH) groups are formed in the carbonaceous structures by oxidation. In the presence of polyvalent cations (i.e. calcium, magnesium, iron, aluminum and manganese), these polar groupings can combine with the polyvalent cations to produce complexed materials which are higher in molecular weight, thus becoming insoluble, and which can be removed readily by filtration. This type of process is called "microflocculation" (flocculation of soluble micropollutants), and has been described in detail by Maier (1984).