Breakpoint Chlorination

[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