Standard Shocking Instructions

[rjordan390]

It appears that 10 ppm free chlorine is the standard for shocking pools. For instance, if the combined chlorine tests at 0.5, then 10 times that amount is 5 ppm. So why add additional shock treatment to bring it to 10 ppm free chlorine?

The shock instructions on the HTH product do not appear to account for the amount of CC and the proper dose and I did not put enough in during the evening and the combined chlorine increased. When I tested the next day, my CC went to 2 ppm to 2.5 ppm, testing twice. So my original CC must have been 1.5 ppm. Knowing that the family members will want to use the pool later that day, I shocked again at 10 am at 30 ppm; 5 ppm over what was needed because it was a sunny day.

The result at 3 pm was 3 ppm FC and 0.5 CC. Much better then what it was. Now I feel confident that when I shock treat in the evening, most of the CC will be gone when I use the following formula:

Example only; substitute your own volume & math figures.

(a) 2400 gals. X 8.34 (weight of 1 gal.) = 20,016 lbs. of water.

( 1,000,000 / 20,016 = 49.96 ppm that 1 lb. of chemical will represent in the pool.

© Take your combined chlorine result (say 0.5 ppm) and multiply by 10 = 5 ppm and divide that by 49.96 = .0.10 lbs. This tells you how many lbs. of available chlorine you will need to get to 5 ppm.

(d) Most if not all package shock treatments have less then 100% available chlorine. So you need to divide the lbs. needed by the amount of

percentage available chlorine stated on the package. (example) if 0.10 lbs are needed; / by 52% = 0.19 lbs. or 3.07 ounces.

This will give you the final result that you must weigh out of the package. If need be, you can convert lbs to ounces by X 16.

Thoughts anyone?

[chem geek]

It doesn't work that way. The 10x rule is wrong because it was based on chlorine oxidation of ammonia each measured in their own different units. Chlorine is measured in ppm Cl₂ while ammonia is measured in ppm N where there is a roughly factor of 5 difference between these. It takes 3 molecules of chlorine to oxidize 2 molecules of ammonia so a 1.5 to 1 ratio. 5*1.5 = 7.5 (it's actually 7.6 when using the more accurate number instead of "5"). Because of side reactions, this was rounded up to 10 for the 10x rule.

HOWEVER, when you measure Combined Chlorine (CC) it is in units of ppm Cl₂ so there is no factor of 5. Also, CC already has one of the 1.5 needed chlorine attached to it. So technically it only takes less than 1x the amount of CC to oxidize it, not 10x. A similar analysis for urea shows that it takes no more than 3x to oxidize it assuming that the CC is measuring monochlorourea.

The ammonia, urea, creatinine, amino acids and some other nitrogenous compounds are mostly being oxidized by chlorine on a continual basis. The only purpose for shocking is to speed up this process so there is no magic 10x number to follow. The higher the FC, the faster the CC will be oxidized. It also helps to expose the pool to sunlight because the breakdown of chlorine produces hydroxyl radicals that are more powerful oxidizers so help to oxidize some organics that would otherwise be too slow to oxidize by chlorine alone and could show up as "persistent CC" usually in indoor pools where there is no sunlight (this is why UV systems can be useful in such pools).

In your particular situation, an increasing CC means one of two things. If the FC was near zero, then you could have had ammonia or other precursors (including algae) such that adding more FC simply created more CC. If you had simply maintained an FC level and waited, then the CC would likely have dropped (unless it was algae growth and your FC was too low for your CYA level). The other possibility, if your FC wasn't near zero, is that you had algae growth because your FC was too low for your CYA level so the chlorine was producing more CC because there was more algae and you weren't getting ahead of its growth.

The bottom line is that there is no need for you to do "nightly shocking". If you were to properly MAINTAIN the appropriate Free Chlorine (FC) level relative to the Cyanuric Acid (CYA) level, you would not normally see any significant CC nor have any algae growth or unusual chlorine demand or dull/cloudy water. For a manually dosed pool, having an FC that is around 7.5% of the CYA level is a minimum that should prevent green algae growth regardless of algae nutrient (e.g. phosphate) level. If you do need to shock the pool because of improper maintenance or an unanticipated situation (storm blew in lots of organics into the pool or you had a pool party with kids urinating and didn't raise the FC in advance, etc.) then raising the FC to 40% of the CYA level gives a decent active chlorine level to kill existing green algae reasonably quickly, but it's not a magic number.

[rjordan390]

Thank you,

I'll have to be careful about the searches I do on pool subjects.