What are Chlorine tests really telling us, and how do we apply that?


LifeTime Supporter
In The Industry
Jul 2, 2012
Saugerties, NY
As the pool season winds down here in NY, I have been primarily taking care of hot tubs for the past few weeks. I've been paying close attention to test results for "Free Chlorine" and "Total Chlorine", and I've started to notice some trends that I didn't realize were there until recently.

A while back, I learned about Bromine pools/tubs. My understanding is that Bromine does the sanitizing, but converts to Bromide as it is consumed. When it runs out, we add an oxidizer that will turn the Bromide back into Bromine. The oxidizer is consumed in the process. So I started thinking and realized I don't fully understand what the Oxidizer is doing.

My relatively new understanding of Bromine made me realize that there is no need to regularly add Bromine tablets to a tub. Once the Bromine is there, it simply needs to be re-activated with an oxidizer. I use MPS as an oxidizer, and if I test the water immediately after adding it, I will see high "Total Chlorine" and usually no "Free Chlorine". After about 5 minutes, the test will show the presence of higher "Free Chlorine". I interpret this to be the re-activation process of Bromine I just described.

So what does this all have to do with the title of my post? I've been referring to the tests as "Free Chlorine" and "Total Chlorine" even though I'm dealing with Bromine. The test is the same test for Chlorine as it is for Bromine as it is for MPS (MPS shows up as Total Chlorine"). So the test isn't really telling me how much Chlorine is present. For that matter its not telling me how much Bromine or MPS either. I suspect it is measuring something like Oxidation Potential. To be clear, I'm referring to the tests used in Test Strips. I'm not sure if they work on the same principles as the liquid tests. So what are they actually measuring that we interpret as "Free Chlorine" and "Total Chlorine"?


Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
Tucson, AZ
I think MPS shows up as Combined Chlorine. Most of us only test for FC and CC with the FAS-DPD test, the TC (sum) is not useful. And you should know by now that we do not trust test strips at all.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
See this document for technical details about various chlorine tests. As shown in Figure 2.1, DPD (N,N-diethyl-p-phenylenediamine) is an organic dye that reacts with chlorine, bromine, and some other oxidizers including chlorine dioxide (ClO2), ozone (O3), and hydrogen peroxide (H2O2). It also indirectly reacts with monobromamine (NH2Br) because this is in equilibrium with monobromammonium ion (NH3Br+) that can briefly form an active bromine ion (Br+) that reacts with the dye. This means that inorganic combined bromine measures as Free Chlorine (FC) in that DPD test.

Monopersulfate (MPS) is different in that it does not generally react with DPD directly (at least not quickly enough for the test unless it is at very high concentrations).

As for the Combined Chlorine (CC) test, that adds potassium iodide to the sample. Weaker oxidzers, including some combined chlorine as well as MPS, oxidize the iodide to iodine and the iodine then reacts with the DPD dye.

The above talks about reacting with the DPD dye where the intensity of color represents the quantity of the reacting oxidizer in the water, in chlorine (ppm Cl2) units if you are using a chlorine test. The FAS-DPD test is a little different in that you back-titrate until the color goes away from the dye. It's a more accurate test then trying to compare the DPD color visually and it does not bleach out at higher chlorine levels. As for what happens with MPS, that gets more complicated because the answer depends on whether there is any oxidizer making the sample turn pink/red for the Free Chlorine (FC) part of the test. If there isn't, then the MPS will show up during the Combined Chlorine (CC) part of the test. If there is, then it will show up in the FC part of the test. The reason is that FAS-DPD will get used up by the MPS before it is able to turn DPD from pink/red to colorless.

If you are using MPS and want to distinguish it from chlorine or bromine in such tests, you can use the Taylor K-2042 test which contains a reagent that will react selectively with MPS so you can essentially measure it separately by doing the chlorine or bromine test twice -- once with the MPS interfering and again without such interference.