Shock (breakpoint chlorination) question

[leblar13]

Hello,

I have read elsewhere that to shock the pool a person should use 7.6 (rounded to 10) ppm per ppm of combined chlorine.

it seems the rule of thumb on here is between 10 and 15 ppm.

Any thoughts.

Andre

[Watermom]

There is no one size fits all rule. It depends on your readings. Post a complete set of current water testing results taken with a drops-based test kit. Also, tell us what type of pool you have and the volume and somebody here can better advise you.

BTW - Welcome to the forum.

[waterbear]

Recommendation for the amount of chlorine to reach breakpoint have been lowered recently by some "experts". However, the rules for breakpoint make the assumption that the only CC present is monochloramine, which is fairly easy to destroy. In real life this is not the case and there are several species of chloramine present and some are quite hard to break down and require a combination of high chlorine levels, expose to sunlight, and breakdown products being allowed to gas off (in other words the pool needs to be uncovered.)

Also, the recommended levels given here for shocking are not necessisarly the same as the levels to reach breakpoint.
It often takes higher FC levels to kill algae, deal with high ammonia levels from bacterial breakdown in a closed pool, or dealing with some of the other problems that shocking is recommended than it does to reach 'breakpoint' so the higher FC levels are a bit of a 'safety net'.
Too high a shock level is not a problem (with the exception of a vinyl liner pool) within reason, too low is often a source of frustration and failure.

How high you need to shock is more dependent on the amount of CYA in the water than any other factor, btw.

[chem geek]

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

[leblar13]

Great info thanks

Love to Learn

[CarlD]

Bottom line is: If you follow Ben's Best Guess Table for shock levels based on your CYA measurement, you'll not have to worry about CC breakdown.

The rule of thumb about vinyl liners is that with 0 CYA, you shouldn't let FC go over 15 or you could bleach or damage the liner. Note: "could". If you have CYA in your pool, you can go to the Best Guess level without fear. So I tend to think that you should always consider shocking to near 15 no matter what.

This spring I opened to an algae bloom--for the first time. Lousy weather and other commitments made my opening very late. I dumped about 4 gallons of 12.5% LC in--roughly reaching an FC of 25--and my pool was mostly clear in 24 hours and completely clear in 48. But my liner is old and ugly and STILL has footprints from the nincompoops who did the install, so I didn't care if it bleached out--it didn't and I've been fine since.