Why Test for Total Chlorine?

Working for Taylor Technologies has some unique advantages; however, there is one disadvantage: I know too much about pool/spa water chemistry and what to look out for.

Maybe I’m just overly cautious now, but I can remember back when I first started here and learned all about chlorine chemistry in pool water. I’d take that knowledge with me wherever I went. Unfortunately, for my family, that also meant taking it with me on vacation.

When my kids were much younger and we went on vacation where there was a pool, I wouldn’t let them get in until “Daddy did his thing.” And my “thing” was to test the water to make sure it was okay (i.e., safe) for them. More often than not, my fears were unfounded and the two of them happily jumped in. On the other hand, there were pools that…well…you simply wanted to run away from. These pools were usually cloudy green from the start (ugh — algae) or smelled like chlorine.

Wait…what? Smelled like chlorine? But I thought chlorine in pool water was a good thing.

While true, the real issue is what “form” of chlorine is in the water! Any form of chlorine added to water has the same chemical reaction: It forms HOCl (hypochlorous acid) and OCl- (hypochlorite ion). HOCl + OCl- = free chlorine (the active sanitizer and oxidizer that is doing the job of killing bacteria/germs and removing organics). However, when free chlorine has done its job and becomes depleted, what remains is combined chlorine (aka chloramines). Smelly and irritating, combined chlorine is nasty and can (if at high levels) irritate mucous membranes, skin, eyes, etc. In some extreme cases, rashes can develop on skin. Combined chlorine is removed by performing breakpoint chlorination. This process removes the combined chlorine, allowing the free chlorine (non-smelly or irritating) to take over.

When doing a standard color-matching test for chlorine, you begin with a test for free chlorine, usually by adding five drops of DPD #1 plus five drops of DPD #2. You then match the developed color to a color on a comparator block. To determine combined chlorine, take that same sample and add five drops of DPD #3. Match the developed color to a color on the comparator for the total chlorine reading. To determine the combined chlorine value, subtract the free chlorine value from the total chlorine value (TC — FC = CC). If the combined chlorine value is >0.2 ppm, then you should perform breakpoint chlorination. An alternative method would be to use a colorimeter, which can provide a more precise reading.

So, now you know the importance of testing for total chlorine as part of your normal testing regime, in addition to a free chlorine test to ensure that the pool or spa water is properly sanitized and oxidized.

Then your kids can get in…