curiosity about chlorine

[Guest]

Just wandering but how does only 3 ppm of chlorine keep that much water clean? Its 3 parts chlorine for every 1,000,000! That's not that much!

 

[chem geek]

The concentration is actually lower than you described because with Cyanuric Acid (CYA) in the water, the active chlorine concentration which is called hypochlorous acid is really only at around 0.05 ppm or even somewhat less, though the 3 ppm FC is a total capacity or reserve of chlorine. The active form of chlorine is fairly reactive and it looks a lot like water as you can compare it here to water here. So it's able to get into cells of bacteria, viruses and algae fairly easily and able to react with the chemicals inside such cells to kill them by disrupting normal cell processes (see this thread for more info).

As for keeping the water clean, it's able to essentially "chemically burn" (known as oxidize in chemistry) some chemicals including the main ones in your sweat and urine, turning them into carbon dioxide and nitrogen gases, and water. When it's all done, the chlorine becomes chloride which is part of regular sodium chloride salt. For every bather-hour in a pool, it takes around 2 fluid ounces of 6% bleach to get rid of the bather waste so most of the chlorine you add to the pool gets used up by other means -- mostly getting broken down by sunlight and by chemically reacting with other things that may get into the pool.

Even though numbers such as 0.05 parts per million may sound very small, there are over 2 million trillion (2x10¹⁵) of these active chlorine (hypochlorous acid) molecules in one teaspoon of pool water. The number of collisions of molecules in solutions is very high even at such low concentrations. Water and chlorine molecules in an 85ºF pool are traveling at speeds of around 1450 and 850 miles per hour, respectively. Even with the active chlorine so dilute, there are roughly 6700 collisions per second for each such molecule running into another on average (the frequency of running into water is much, much higher and I'm using the term "collisions" very loosely since solutions are much more complicated than gasses).

 

[Guest]

I just noticed an article About measuring FC/TC with a DPD test at a range of 0-10 (mines max is 3) with dilution. How would I do that? Mine uses this white pill that I drop into the THIN tube. Then shake and compare colors. If my max is 3 on my tube how would I do it?

 

[PaulR]

You would need to dilute a sample of pool water, using chlorine-free water. Distilled water would be chlorine-free; municipal tap water typically is not.

Thoroughly mix one part pool water with two parts chlorine-free water (e.g. 1/4 cup pool water, 1/2 cup chlorine-free water). Test the mixture; multiply the answer by 3. This will give you a range of 0-9, although you lose some accuracy. (The mention of a 0-10 range is because many DPD tests start with a range of 0-5, so you dilute 1:1 instead of 1:2.)
--paulr

 

[Guest]

For every bather-hour in a pool, it takes around 2 fluid ounces of 6% bleach to get rid of the bather waste so most of the chlorine you add to the pool gets used up by other means -- mostly getting broken down by sunlight and by chemically reacting with other things that may get into the pool.

So when you say "every bather-hour" is that per person or every hour that the pool is used? So theoretically for 4 people in 1 hour it would use up 1 cup of 6% bleach, and 2 people for another hour 1/2 cup. 1 1/2 cups of bleach would raise my pool's FC by 4 ppm. Not that I am assuming this is an exact measurement to go by but it would give me an idea of how long I need to run my SWG each day after we've been in the pool.

 

[PaulR]

Yes, 4 people in the water for 1 hour is 4 bather-hours.
--paulr

 

[chem geek]

Just note that most chlorine usage in a residential pool exposed to sunlight is from breakdown from the UV in sunlight, not from bather load. There is also some chlorine usage from slow oxidation of CYA, perhaps around 0.15 ppm FC per day. So don't just use the bather load formula alone and just monitor and trust your FC readings and adjust your SWG on-time so that they are stable. The bather-load formula can be useful for incremental adjustment estimates above a normal baseline such as for incremental manual chlorine addition after a heavier-than-normal bather load.

 

[Guest]

And just wandering, how long AFTER swimming can I expect the chlorine to have OXIDIZED bather waste? In other words how long should I wait after swimming to check my pools numbers?

 

[chem geek]

The answer to that question is complicated because the ammonia in your sweat and urine will get oxidized in hours (at 77ºF it's 90% oxidized in 3-1/2 hours with the FC near 10% of the CYA level), but the urea in your sweat and urine can take days, though having exposure to the UV in sunlight may reduce that time significantly and higher water temps accelerate the oxidation of urea quite a bit such that spas usually have urea all oxidized within 24 hours.

I wouldn't worry about such details. Just measure your FC at the low point during the day which would be before your pump comes on in the morning (with the SWG) and make sure it has at least the minimum FC level. As for checking for CC, the best time is also just before people start using the pool, so usually in the morning if it is used all day. In both cases, however, you need to have the pump circulate the water at least a little bit or you need to manually do some stirring so you get a decent reading. Take your water sample about a foot below the water surface.