Please Explain Breakpoint and "Pounds" of Sodium Hypoclorate

Breakpoint is simply how high you need to raise your free chlorine to get rid of the combined chlorine in the water. It is just another name for shocking. Breakpoint is easy to calculate. You need to raise your FC by 10 times the amount of CC present. For example if you have 2 ppm CC you would need to raise your FC 20 ppm over what it currently is to reach 'breakpoint'. This formula is really only applicable if there is NO CYA in the water. (And a properly maintained pool will be shocked when the CC is over .5 ppm so unless you have a severe algae bloom or a lot of nitorgen compounds in the water creating a huge chlorine demand then it's a moot point). Stabilized pools will require higher FC levels. The 'best guess chart' actually does a better job in stablilized pools unless the CC is very high.
As far as dosing liquid sodium hypochlorite it's really easy! For 12.5% strength each oz will raise 1000 gallons 1 ppm FC so if your have a 14000 gallon pool 14 oz will raise your pool 1 ppm. If you wanted to raise it 20 ppm you would need 14 x 20 oz = 280 oz = 2 gallons and 1 1/2 pints.
If you are using bleach you can use roughly double the amount of 6%. It will be close enough.
1 gallon of bleach will raise 10000 gallons approx. the pertenctage of the bleach (6% will raise it about 6 ppm and 5.25% will raise it about 5.25 ppm). It's not exact but it's close enough for government work! Remember, it's a pool, not rocket science!

IF you have a vinyl pool then you really can't apply the breakpoint forumula if your CC is high because you could bleach your liner. Raising your FC to the amount suggested in the 'best guess chart' AND KEEPING IT THERE until there is no CC present is just as effective, although it can be a bit slower and more labor intensive.

Waterbear, recently I had an experience that I'm having trouble fitting in what you wrote above.

Through FAS/DPD testing I found I had a FC of 10 ppm and a CC of .5 ppm. CYA is 25 ppm. I had shocked two nights before, up to 15 ppm. The next morning I had no CC. I was letting my TC drift back to 3 ppm.

If I only need 10x CC to reach breakpoint, which would be 5 ppm (.5 x 10), why did I still have CC when my FC tested at 10 ppm, 2 times the 5 ppm needed for breakpoint?

[If you had .5 ppm CC then you needed to raise the FC 5 ppm above the 10 ppm FC you measured the CC at or up to 15 ppm. Like I said above this works if there is NO CYA present. Also, uv light from the sun plays an important part in breakpoint chlorination, which is why breakpoint is very difficult to reach in indoor pools. The formula for breakpoint is really just a guide and not to be taken as gospel. If the CC is a result of ammonia in the water then it pretty much holds true. If there are other nitrogen compounds present then it can take a lot more chlorine to reach breakpoint.

CCs are constantly being formed so long as there is organic material in the water since CCs are simply chlorine combined with organics (or with ammonia). Additional chlorine breaks down CCs, but is relatively slow especially for some CCs while sunlight helps speed up the breakpoint process.

The 10x rule historically came from the stoichiometric relationship converted to ppm, but is based on Chlorine vs. Ammonia, NOT Chlorine vs. Combined Chlorine. The following is the best case set of reactions that occur:

Creation of Combined Chlorine:
HOCl + NH₃ --> NH₂Cl + H₂O
Hypochlorous Acid + Ammonia --> Monochloramine + Water

Breakpoint of Combined Chlorine:
2NH₂Cl + HOCl --> N₂(g) + 3H⁺ + 3Cl^- + H₂O
Monochloramine + Hypochlorous Acid --> Nitrogen Gas + Hydrogen Ion + Chloride Ion + Water

So you can see that it takes 1 molecule of hypochlorous acid to form combined chlorine but 1/2 a molecule per molecule of combined chlorine to break it. So to go from ammonia to nitrogen gas takes 1.5 molecules of chlorine.

Ammonia is measured (by convention) as ppm Nitrogen with a molecular weight of 14.0067 while Hypochlorous Acid is measured (by convention) as ppm Chlorine Gas with a molecular weight of 70.906 so in ppm terms we it takes 1.5 * 70.906 / 14.0067 = 7.59 ppm of Chlorine to "break" 1 ppm of Ammonia. This 7.6 factor is where the 10x rule originally came from because this is a minimal factor with an optimum range being around 8-10.

There are two problems with taking this 10x rule to the chlorine tests of FC and CC. First, is that CC is already combined chlorine so two-thirds of the chlorine needed to get to breakpoint has already been used to form the combined chlorine in the first place. Second, the CC measurement is in the same units as FC, namely ppm Chlorine Gas, NOT ppm Nitrogen (this is because the test simply forces the attached chlorine to become free chlorine again by substituting iodine in its place and then the free chlorine gets measured). So there is no molecular weight difference.

This means that technically it takes a minimum FC of one-half the amount of CC to "break" it. So why the 10x rule? I believe the main reason is Cyanuric Acid (CYA). Though chlorine combines with ammonia very, very quickly (seconds or less) and combines with various organics at different rates (minutes to hours or not at all for some organics), the breakpoint reaction is rather slow even with no CYA where it's roughly a half hour to get near completion with 1 ppm FC and 0.1 ppm Ammonia (here, the 10x rule applies since we're starting with ammonia measured as ppm Nitrogen). In the presence of CYA, the hypochlorous acid level is significantly reduced and that reduces reaction rates proportionately. At 30 ppm CYA and 3 ppm FC, there is about 33x less hypochlorous acid so if one uses a 10x rule then this is 20x (due to the one-half requirement) the amount needed to break the chlorine, but mostly compensates for the 33x smaller amount that would affect the reaction rate. In other words, the 10x rule roughly gets one to similar half-hour reaction rates (without sunlight as a catalyst) for typical breakpoint without CYA.

If it truly took 10x of FC amount to break CCs, then one should see a HUGE reduction in FC when CCs got broken and that simply is not seen. Even with sunlight helping as a catalyst, it still takes chlorine to break CCs. The fact that CCs really only take half their amount in FC to get broken is why you never see a huge drop in FC when CCs drop.

Richard

Chemgeek...thanks for your above post! interesting


take care all,
dan

I would like to add to Richard's (Chemgeek's) excellent post above that the 10x rule for breakpoint also assumes that monochloramine is going to be the mose prevalant chloramine present but in real life situations that is not the case unless the pH of the pool water is quite high when the chloramine is formed and ammonia is the only nitrogen compound present.