Free Chlorine & Total Chlorine?

[Thebiga]

Say you have a swimming pool and you just use the cheap chemical tester where you see how much Ph & how much Chlorine you have in the pool.

What is the difference on the total & free chlorine, Which does it measure or as long as it shows the right parts per million and your not getting any chlorine smell from the pool are you getting it pretty much right.

I thought that if you don't have enough chlorine you get the horrible smell that some public pools sometimes get.

Which is a good level for free and total chlorine in a pool. How often do you have to test for this, Daily, Weekly?

[Théière]

Morning Thebiga,

Testing, one of the most overlooked parts of pool maintenance but probably the most important.  With budget considerations the low cost test kits from supermarkets are at least a bit dubious from the colour charts to the chemical strengths.

The dip strips can also be way off on a few of the tests but fortunately Ph and Chlorine seem to be somewhere near.  Don't touch the pads at the end of the strips if you use them or the tablets as that can easily though off the test.  The drops test (OTO) is good for high levels of chlorine i.e. shock levels as they do not bleach out like the others. If you are in any doubt it is worth investing in an electronic tester, more later.

Test as often as required, more often if there have been a number of people in the pool.

Good level for chlorine, free chlorine 1- 1.5ppm.

Combined chlorine 0

But other factors such as Cyanuric acid levels  (CYA) play an important part on the free chlorine level. CYA can enter your pool via the multi action gallets you add, by the mid point of the season you will probably have too much CYA and then to manage your pool effectively you will need to be running the free chlorine level much higher around 5ppm.

[Poolguy]

John

I'll just add a bit there to your eloquent discussion.

Total chlorine is the measure of all of the chlorine in the pool.

Free chlorine is the measure of the chlorine NOT bound up with the Cyanuric acid (stabiliser). This is the important level and is not measured by the Drop Testers, which only measure Total Chlor.

Combine chlorine is the difference which is the chlorine bound up with the stabiliser.

If you need more definitions look at my lexicon here http://www.poolguy.fr/glossary.html

In all cases (even a little kids pool) I recommend the use of a photometer such as SCUBA + http://www.poolguy.fr/products/testing.html to test the water as it will give you the 4 parameters necessary for water balance.

Hope that this adds something to the post.

 

Andrew

 

 

 

 

end

 

 

[Thebiga]

Thanks people.

So if I have to much total chlorine (combined) I have to up the level of free chlorine and this I take it would mean adding say 'Eau de javel' to the water which I think has no stabiliser in.

But to get down the level of CYA means to unload some of the water which would bring down the amount of 'Combined chlorine' aswell.

Is this correct.

[Théière]

Yes pretty much,

Andrew filled a few holes in my post ( never post just before you are rushing out the door)

You need free chlorine to deal with bather waste and sanitise the water, Chlorine is good as it can do both pretty well but free chlorine gets broken down and becomes combined chlorine. So you can measure free chlorine with a DPD1 tablet test and total chlorine with the DPD3 tablet take result 1 from result 3 and you have a combined chlorine figure which you want as low as possible. DPD tablets can bleach out if the chlorine is high also test strips too that is why manufacturers tell you to complete the test by a certain time. OTO test does not bleach out so can be useful when shocking a pool providing you know the pre shock figures.

By raising the chlorine level to around 5-10 ppm (shocking) the large amount of free chlorine is able to oxidise the combined chlorine away (the bad smell part) leaving you with 1-1.5 ppm chlorine and ideally 0 combined chlorine.

You need CYA in the water to prevent the sunlight burning up your available free chlorine which it can do in as little as an hour or two. You do not however want too much CYA and as multi action gallets contain CYA you have to be careful not to over do it or you will need to increase the amount of free chlorine to counter the chlorine that is now bound to the CYA or drain off some water and replace with fresh. I suggest you add multi action gallets only until you reach 20-30 ppm CYA then switch to ordinary un-scented bleach from a brico at around 18 euros for 20 litres.

Your second sentence is correct

[chem geek]

Unfortunately, the terminology for what "Free Chlorine" means has changed over the years.  When the chlorine/CYA relationship was definitively determined in [url=http://richardfalk.home.comcast.net/~richardfalk/pool/OBrien.htm]this scientific paper[/url] back in 1974, the term Free Chlorine meant the hypochlorous acid (the killing form of chlorine) and the hypochlorite ion (not very effective at killing), but not the chlorine bound to CYA (technically called chlorinated isocyanurates).  This paper called the sum of the Free Chlorine and the chlorine bound to CYA as reservoir chlorine since it was chlorine held in reserve that was available (i.e. releaseable) as the hypochlorous acid got used up killing pathogens or oxidizing bather waste or other organics.

At some point in time, test kits started referring to "Free Chlorine" as what was being measured in the DPD or the FAS-DPD tests.  These tests actually measure what used to be called reservoir chlorine because the chlorine bound to CYA gets released in the timeframe of these tests.  So you are really measuring not only the active chlorine, hypochlorous acid, but also the chlorine in reserve as hypochlorite ion and mostly the chlorine bound to CYA.

Combined Chlorine (CC) is chlorine bound to ammonia (as chloramines) and organics (such as chlorourea) that get measured in the DPD test after adding potassium iodide (that's what DPD3 does).  Most chlorine bound to ammonia or organics will oxidize the iodide to iodine that then reacts with the dye in the test.  Technical details about these sorts of tests are in [url=http://www.hach.com/fmmimghach?/CODE:L70191473%7C1//true]this paper[/url].

It is the Combined Chlorine (CC) that can be smelly and irritating, especially the chloramines and of those most especially nitrogen trichloride.

One can consider CYA to be an active chlorine (hypochlorous acid) buffer, holding most of it in reserve.  This means that the amount of active chlorine is much lower when CYA is present and roughly speaking at a pH near 7.5 a pool with an FC and CYA is roughly equivalent to a pool with FC/CYA as FC and having no CYA.  So, a pool with 3 ppm FC and 30 ppm CYA, for example, has roughly the same active chlorine concentration as a pool with 0.1 ppm FC and no CYA.  Fortunately, it takes very little active chlorine to kill pathogens and to oxidize bather waste reasonably quickly, but this also means that pools that don't use any CYA are typically over-chlorinated which can lead to a greater production of nitrogen trichloride, faster oxidation of swimsuits, skin and hair, faster corrosion rates, etc.  This is one reason the German DIN 19643 standard specifies 0.3 to 0.6 ppm FC with no CYA (0.2 to 0.5 ppm if supplemental ozone is also used) though this is still at least 2-3 times higher than the FC as 10% of the CYA example I gave.  Higher active chlorine levels may produce more nitrogen trichloride (I write about this technically [url=http://www.troublefreepool.com/chloramines-and-fc-cya-t10257.html]here[/url]).

Richard