Newbie: To shock or not to shock... That is the question

[SalemCastles]

According to Taylor you want to achieve Breakpoint chlorination (10 x the CC level) any time your CC reaches .2ppm I have been trying this at .5ppm and it seems to be working ok. I really don't think of it as "shocking" the pool when I'm only raising it to say 7ppm when I have a cc of .5 but I guess technically it is. My normal target range is 5-9 so you see I can achieve Breakpoint as a matter of my normal maintenance. Just another way of saying the same thing as the others perhaps.

[chem geek]

The 10x rule is wrong when applied to CC. The rule originally came from the amount of chlorine needed to oxidize ammonia where the molar ratio is 3 chlorine to 2 ammonia. However, chlorine is measured in ppm Cl2 units while ammonia is measured in ppm N units and there is a factor of 5.06 difference between these units. So the ratio in chlorine and ammonia units becomes (3/2)*5.06 = 7.59 where side reactions and getting over the dichloramine hump has actual quantities required be in the 8-10 range and that's where the 10x rule was born for the chlorine oxidation of ammonia in their respective units.

CC as monochloramine already has 1 of the 1.5 chlorine needed to oxidize it and CC is measured in chlorine ppm Cl2 units and not in ammonia ppm N units so there is no factor of 5. So 1 ppm CC only needs somewhere in the 0.5 to 0.7 ppm FC amount to get fully oxidized. Now there are other forms of CC including monochlorourea that need more chlorine, but still on the order of 2-3x the CC level. Besides, breakpoint chlorination is continuous so long as there is ANY measurable FC in the water and nothing gets "stuck" if you don't use enough FC -- you simply add more and it picks up where it left off (more or less).

If you are measuring 0.5 ppm CC using a 10 ml sample size in your FAS-DPD chlorine test, then that is really <= 0.5 ppm and you should use a 25 ml sample size instead to see if it's really <= 0.2 ppm which is more likely.

Now, all that said, it is true that a higher active chlorine level (FC/CYA ratio) will oxidize CC faster, but just don't think there is anything magical or special about "10x".