Product that measures active chlorine?

[keene3b07]

http://www.iccontrols.com/product.php?Model=875-25

would this product measure the active chlorine level? By active i mean the chlorine that is not bound to cya.

 

[chem geek]

Yes, it would appear to measure hypochlorous acid. Hypochlorite ion is also unbound to CYA but it is not the main disinfecting form of chlorine (though it is an oxidizer for some chemicals). Amperometric sensors measure hypochlorous acid and some membrane-specific sensors do as well (while some measure both hypochlorous acid and hypochlorite ion but with different sensitivities so a pH sensor is also needed to calculate the species). Chemtrol (Santa Barbara Controls) has one as does Omega, ProMinent, Cole-Parmer, GF Signet, AquaSensors, Hach and many others.

 

[JasonLion]

From what I have read, membrane fouling is still a regular maintenance issue with the current generation of HOCl sensors, though they have improved significantly over the previous generation. Prices remain fairly high, $1,000+ for just the sensor, though they are no longer outrageous, as they were a few years ago.

 

[chem geek]

I just want to remind you that you can readily calculate the active chlorine level if you know the FC and CYA levels, though if you want more accuracy with varying pH and temperature, then you'll need to use my Pool Equations spreadsheet though for temperature dependence you need to set line 230 "Use Temp. Dependent Cl-CYA" to TRUE in columns B and C. Also, the far less expensive ORP sensors have a roughly logarithmically proportional relationship to the active chlorine level, though you need to calibrate them to your specific water and even then they can be a bit flaky. You can see in the ORP vs. Calculated HOCl graph that the correlation has a lot of error but for process control where you have a setpoint it can be reasonable. So you probably don't need the much more expensive HOCl sensors.

Why are you asking about such sensors? What is your intended use for them?

 

[keene3b07]

i thought that it might be important to know, since it is what is actually available to kill organics. I didn't realize you could just calculate it from knowing the cya and fc levels. I did have another question on the topic. Is unbound chlorine protected by cya at all? Or does it have the same half-life as chlorine in water with no cya?

 

[JasonLion]

The unbound chlorine is just chlorine, and is lost to sunlight fairly quickly. However, the levels are rather low, so the total amount that is lost is also low. As chlorine is lost, some of the bound chlorine will be released, replenishing the supply.

 

[chem geek]

You need to think of it this way. At any given point in time, most of the chlorine is bound to CYA and a small proportion is unbound. The unbound chlorine breaks down in sunlight the fastest (the same as if that amount was present without any CYA) with hypochlorite ion having a half-life near the water surface of 20 minutes while hypochlorous acid has a half-life near the water surface of 2 hours 10 minutes. At pH 7.5 with a 50/50 mix, the half-life is 35 minutes. The actual half-life is longer in a pool because of water depth where even without CYA in the water the chlorine itself shields lower depth since there is a limit to the number of ultraviolet photons. In a 4-1/2 foot pool, the half-life is about an hour.

I don't know the half-life of chlorine bound to CYA, but suspect that it is in the neighborhood of a few hours, but it seems that CYA's greater protection is in absorbing UV light directly shielding lower depths from chlorine loss so higher CYA levels shield more. See the table in this post for some rough data on how CYA protects chlorine loss from sunlight. Some basic research could readily sort this all out, but apparently no one has done it (i.e. getting an accurate CYA UV absorption/extinction coefficient in the 310-370 nm wavelength range where chlorine breaks down; most historically measured spectra only go up to around 260 nm).