Breakpoint Chlorination

[chem geek]

The consensus so far on this thread seems to be that people manually dosing chlorine shock from zero to 3 times per season -- most only once or twice. So assuming some of them also have kids with, well, #1 releases, my guess that a regular manual dosing of chlorine is pretty similar to what an SWG does may be correct. It's obviously not as good since even once or twice a day chlorine addition isn't the same as several hours each day, but it does show that, as CarlD says, "Stay ahead of your water!" really works.

Richard

[KurtV]

That small error in the BleachCalc alogorithm may explain why the Best Guess Table seems to sometimes understate the chlorine needs for reaching "shock levels". Some people may not actually ever be getting to the breakpoint if they're dosing based on BleachCalc. Or would that be overstating the effect?

I certainly don't want to sound ungrateful for Michaels fine software. It has made the pool maintenance part of my life much simpler on many occasions.

[chem geek]

I don't think it is BleachCalc that is the root cause of any underestimation of how much chlorine is needed to shock, though you are correct that if I am right about the error, then it will underestimate how much to add. I believe the much bigger potential problem (that we don't know for certain yet) is that Ben's table tracks reasonably well with the ppm HOCl theory for his "min" and "max" columns, but not well at all for the "shock" column. Look at my FC.htm chart and you can see that (ignoring very low CYA since the required FC is so low as to be impractical to maintain) the 0.02 ppm column roughly correlates to Ben's "min" while the 0.05 ppm roughly corrleates to Ben's "max", but there is no good correlation for his "shock" column. We don't know which way the error is, if any, but if I were to hazard a guess, I would say that the proper "shock" level is at around 0.5 ppm though possibly 0.2 ppm *might* do the trick in some cases. If 0.5 ppm is correct to kill algae, then the high CYA levels in Ben's chart underestimate the amount of chlorine truly needed.

Only real-world experience will tell what we should do to go forward -- if the ppm HOCl is valid for the shock to kill algae and if so, what level is appropriate.

Richard

[RavenNS]

side bar here:
since you're using metric & imperial for these calculations..
is this imperial gallons or US gallons?

( I always wonder which it is that you guys are actually using... it's so confusing...lol)

[waterbear]

Even though beachcals says metric or imperial Michael stated in one of his posts that it is US gallons. He said he will fix that in the next version.

[RavenNS]

Okay great,
I was specifically refering to the calculations on the previous page of this thread...
( Mls to Gallons & such)

- so when people here say, " my pool is Blah Blah gallons", they mean US Gallons (?) & that is the standard measurement for you USAers?

good to know; Us Canadians get a wee bit confused on the whole gallons issue

[waterbear]

another good reason why we should have all gone metric years ago!

[chem geek]

I've done some more research on what the "12.5%" number is supposed to mean and it is used inconsistently in two different ways. Sometimes it appears to be used in the way I wrote, namely as 12.5% by weight of sodium hypochlorite. But other times it is claimed to mean "12.5% available chlorine" where "available chlorine" is normally defined to be relative to chlorine gas (Cl2) at 100% available chlorine.

So, while the first weight definition would divide the weight of chlorine you add by 74.4424 g/mole NaOCl (and multiply by 52.46 g/mole HOCl), the second definition would divide the weight of chlorine by 70.9064 g/mole Cl2 (and still multiply by 52.46 g/mole HOCl). This at least explains how 12.5% NaOCl becomes 12.5% * 70.9064 / 74.4424 = 11.9% or about 12% available chlorine which I have also sometimes seen.

The fact is that most of the Material Safety Data Sheets (MSDS) that I saw for chlorinating liquid show a range of 11-15% so I don't think anyone expects the actual amounts to be precise. The 12.5% is somewhat of a "target" goal and at least I know that the MSDS means that this is the weight percent of the actual component listed which in this case is sodium hypochlorite (NaOCl).

The Clorox liquid bleach MSDS lists 5.25% sodium hypochlorite for most bleach products. Again, this would represent a weight percentage of sodium hypochlorite so following the BleachCalc program for volumes that you put into your pool would lead to lower ppm results than desired. Likewise, if one entered in a volume into BleachCalc then the reported ppm would be higher than what would actually be acheived. [EDIT] An update: Ultra Clorox Regular bleach says 6-7.35% and Clorox Regular Bleach lists 6.15% so the 5.25% that I saw was probably for older bleach -- the latest numbers come from the Clorox website. The density (specific gravity) is listed as approximately 1.1 for both products and the pH is listed as 11.4. [END-EDIT]

Interestingly, in this thread CarlD equates 1.88ml in 5 gallons of water as producing a ppm value that should be the same as in the original bottle. (5 gal) * (3785.4118 ml/gal) / (1.88 ml) = 10067 or about 10000 which is 1/100th of one million so the measured ppm would indeed measure the % HOCl in the original source if the densities were the same, which they are not (the chlorinating liquid is about 1.16 g/ml while pool water is closer to 1.00 g/ml). He found that it was closer to 14% for his 12.5% chlorinating liquid which is the opposite of what I would have expected. If we divide by 1.16 then his 14ppm becomes 12ppm which is a little more like it and implies an original 14.7% which is quite possible given the nominal 11-15% range.

Obviously, more work is needed to get to the bottom of this.

Richard

[CarlD]

Wow! And all I was trying to do was determine if the chlorine in the jug had degraded significantly!

The measure of 1.88ml/5gal can also be done in metric: 1ml/10liters--5 2 liter soda jugs.

Metric vs English (Imperial implies Imperial gallons)?

It's a mistake to say we abandoned the Metric system as too hard. It's creeping up on us in many ways. If you work on cars or motorcycles, almost all the wrenches you will use will be metric. 20 years ago I had no idea what a 12 mm wrench was, vs a 17mm. Today an image pops into my head automatically. Soda is sold in 2 liter jugs, 1 liter jugs and even 3 liter jugs. ALL medications are administered in metric measures and only sometimes, for liquids (like cough medicine) is there a teaspoon equiv. You can even use the bleach calculator formulae in metric: Just measure your pool in liters and your bleach in liters and it works out the same.

In the UK, they thankfully STILL keep one imperial measure--the British Pint! They also, curiously measure body weight in Stone, rather than pounds or kilos.

[chem geek]

Yet another update in this continuing saga. This thread seems to indicate that perhaps the "ppm Chlorine" definition is based on the Chlorine gas (Cl2) equivalent where chlorine gas is 100%. I know that this is what is used for what is termed "available chlorine" when comparing different chlorine sources, but I did not know if that is what is also used for measuring chlorine from the test kits. This seems doubful and strange, but this wouldn't be the first time an unusual convention is used in the pool industry. I'll keep y'all posted. [EDIT] So far, most every source I can find says that ppm chlorine has the "part" in part per million be the weight of chlorine and the context has always been HOCl + OCl- (CYA is not talked about so Cl-CYA is included as if it were HOCl/OCl-). [END-EDIT]

[SECOND-EDIT] I found the answer! It's at this website where it says "Although Cl2 does not exist in potable waters, for historical reasons FAC, CAC and total chlorine are still expressed as mg/L as Cl2" so I need to correct my spreadsheet. This will somewhat effect all of my graphs, tables, etc. calculating ppm HOCl from FC (the FC numbers will go up by 35% holding everything else constant). The points and principles won't change, but some of the specific numbers will. I'm sure Michael will be very relieved to know that the proper adjustment to his algorithm in his program is quite minor and can possibly be ignored. [END-SECOND-EDIT]

Richard