The oxidation of ammonia, urea, and other components of sweat and urine as well as dead skin, etc. happens continuously so long as there is any Free Chlorine (FC) present. The idea of "breakpoint" was coined when adding chlorine to ammonia in order to oxidize it and has to do with how much needs to be added before there is sufficient buildup of monochloramine before one gets over the hump where just a little more chlorine results in oxidation of the monochloramine that releases more chlorine so the reaction takes over on its own from there (i.e. no further chlorine needs to be added). However, during this buildup, there will be no measurable FC since chlorine combines with ammonia very quickly to form monochloramine.

In other words, the 10x rule has more to do with "how much chlorine is needed in total" point of view. However, the formula is totally wrong when applied to Combined Chlorine (CC) because the units of measurement are totally different. Ammonia is measured in ppm Nitrogen units while all chlorine, both FC and CC, are measured in ppm Chlorine gas (Cl2) units which is about a factor of 5.06 higher. The 7.6 comes from 1.5 chlorine-to-ammonia in molar units multiplied by this factor of 5.06. When starting with CC, there is no factor of 5.06. Also, monochloramine already has 1 of the 1.5 already accounted for.

Even if one looks at monochlorourea, the amount of chlorine needed to get over the hump for oxidation would not be more than 3x. In other words, the 10x rule is completely wrong. Also, it's not really relevant since one does not get "stuck" if one doesn't use enough chlorine -- one can simply add more to continue the reactions where they left off.

The levels for shocking have nothing to do with breakpoint chlorination and elimination of CC. They are levels for a reasonably fast clearing of a pool that has possible algae growth. Higher levels of chlorine make any reactions with chlorine proportionately faster. Such levels can also be used to accelerate the reduction of CC, but usually this isn't necessary as the CC will either go down at normal chlorine levels if there is exposure to sunlight or it won't go down quickly at all even at fairly high chlorine levels (this is known as persistent CC).

Richard