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Thread: Is it possible to measure active chlorine?

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    pabeader's Avatar
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    Question Is it possible to measure active chlorine?

    I'm just wondering..
    Bob - Palm Beach by San Juan Pools. approx 5000 gals., Pentair 320 cartridge filter (all new guts installed by me), Goldline SWG, 'New to me' Kreepy Krauly Sand Shark, Intermec 104 Timer Test kit: TF-100 w/Speed Stir

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    Re: Is it possible to measure active chlorine?

    Yes it is, but the sensors for doing so are expensive. The technique is called amperometric and is usually done with a membrane specific to letting only hypochlorous acid (HOCl) pass through or sometimes both HOCl and hypochlorite ion (OCl-) depending on the membrane. The technique is essentially controlled electrolysis where the voltage applied to the working electrode is controlled relative to a reference electrode for optimal conditions of a small overvoltage and what is measured is the resulting current between the working electrode and a counter electrode where migration of HOCl to the working electrode is diffusion-limited and proportional to the hypochlorous acid (HOCl) concentration. It is the combination of measuring current in carefully controlled conditions combined with selectivity via a membrane that makes this technique fairly accurate and stable, especially when compared to ORP that measures potential difference (not current) and gets more interference and does not use selective membranes. Because it requires diffusion, the amperometric method takes from a few up to 30 minutes for the electrode to stabilize (how long depends on the specifics of the sensor).

    ORP is also a rough proxy for measuring HOCl, but it is indirect, inconsistent, has more interference, and doesn't follow chemical theory such as the Nernst equation as it should. Yes, it has rough logarithmic dependence, but the implied number of electrons in the half-reaction at the electrode is only 0.82 for Chemtrol or 0.64 for Oakton instead of the expected 2. So sensors themselves are inconsistent and require calibration. This document describes some of the differences between amperometric readings vs. ORP and why ORP has problems though some of their points are incorrect (ORP doesn't have 2 electron dependence so is not quite as insensitive as they claim and it does not seem to depend on chloride ion level also inconsistent with theory).
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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    Re: Is it possible to measure active chlorine?

    Well that's not going to work for what I wanted. I was hoping for a "Yes, put 3 drops of (magic potion #1) in before you test for FC"

    Oh well.

    As always, Thanks! CG
    Bob - Palm Beach by San Juan Pools. approx 5000 gals., Pentair 320 cartridge filter (all new guts installed by me), Goldline SWG, 'New to me' Kreepy Krauly Sand Shark, Intermec 104 Timer Test kit: TF-100 w/Speed Stir

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    Re: Is it possible to measure active chlorine?

    You CAN calculate the active chlorine level from the FC, CYA, pH and temperature using the PoolEquations spreadsheet, though for warmer temperatures you'd need to set line 230 "Use Temp. Dependent Cl-CYA" to TRUE in columns B and C.

    At a pH of 7.5, a rough rule-of-thumb is that the active chlorine (hypochlorous acid) level is half of the FC/CYA ratio.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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