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Thread: high chlorine levels interfering with pH test readings

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    Join Date
    Jun 2014
    Bedford, TX

    high chlorine levels interfering with pH test readings

    This posting is more informational than inquisitive, as I found very little previously related information while searching this forum.

    Have been combating high pH levels since managing a pool for the very first time this summer and absorbing all I can about the Trouble Free Pool philosophy. Both K-1000 Basic Residential Pool and Spa Test Kit (R-0014 reagent) and 9056 Pool Test Kit (R-0004 reagent) are part of my toolkit since they each cover slightly different pH ranges. But neither provide a high enough pH scale to satisfy a curiosity of exactly where my pool pH plateaus without the constant need to add muriatic acid, so I sprung for yet a 3rd Taylor pH kit, the K-1285-6 (using cresol red reagent R-1003K) to allow pH readings above 8.2 temporarily purely as an experiment.

    I recognize that 7.8 is the commonly accepted maximum pH level for pools, and I really wish to keep below this level; however, I am willing to exceed this by a bit so long as the calcite saturation index remains balanced, and that no metals are present to come out of solution to stain the plaster.

    My pool chemistry (tested with TF-100 test kit/reagents) is/was:
    Free chlorine: 18
    Total Alkalinity: 110
    Calcium Hardness: 550
    Cyanuric Acid: 80
    Borates: 50
    Salt: 3400
    Copper: 0

    Yes, free chlorine at 18 as I recently emptied my remaining bleach containers into the pool since my newly installed salt water chlorine generator rendered the bleach obsolete. But because of this high chlorine concentration, I discovered its effect on some of the common pH test reagents.

    The pH reading given by the commonly used K-1000 and 9056 kits/reagents was 7.8; however, the pH reading given by relatively obscure K-1285-6 kit/reagent was 7.2. The large pH variance shown by the K-1285-6 kit perplexed me to the point of contacting Taylor for an explanation.

    reply from Taylor Technical Service rep:

    The only piece of water chemistry that will interfere with a pH test is a high sanitizer reading with the other option being compromised reagents. From the information that you provided me, your free chlorine reading is 18ppm - a significantly high reading. At that level of sanitizer during a pH test, the indicator may actually become purple from a chemical reaction of phenol red to chlorphenol red. The higher reading that you are obtaining using R-0004/R-0014 may be caused by some of the sample (or all) to be turning that purple color - providing a higher reading. I would trust the reading provided from K-1285-6 because a high sanitizer level does not affect this testing procedure.
    When I then asked what is the maximum chlorine concentration that both R-0004/R-0014 are expected to provide reliable pH readings, the Taylor Technical Service rep responded:

    R-0004/R-0014 may begin to change the indicator at 10ppm chlorine, but there are a lot of other factors that will contribute to this reaction taking place as with any chemical reaction. Namely, temperature, time, and other chemicals in the sample. If you routinely keep the chlorine level at 5-7ppm, you should not experience an interference with the pH testing procedure.
    I would like to note however, that even with this morning's fairly reasonable FC reading of 5.0, the difference in pH readings between the three kits/reagents were:

    K-1000 (R-0014 reagent): 7.6 pH
    9056 (R-0004 reagent): 7.6 pH
    K-1285-6 (cresol red reagent R-1003K): 7.4 pH

    Chlorine generator is not fully dialed in, so chlorine readings have routinely ranged between 8 and 10 since installing it. My takeaway from all of this is that the allegedly high pH readings that I (and possibly others) am combating in part stem from high chlorine concentrations interfering with pH readings provided by the two most commonly used pH reagents.
    Pool Details: 20,000 gal kidney shaped IG gunite; Filter: Hayward Micro-Clear DE (48 Sq Ft)
    Primary Pump: Single speed 1.5 HP (1.30 SF) Hayward (Century Centurion); Booster Pump: Single speed 3/4 HP (1.50 SF) Hayward (Century Centurion)
    SWG: Circupool Si-45 PLUS; Cleaner: Polaris 380

  2. Back To Top    #2

    Join Date
    Oct 2013
    Chapin, SC

    Re: high chlorine levels interfering with pH test readings

    Is/was? ? Exactly when was the FClevel at 18? If that tf-100 wasn't your own, you probably ought to consider getting one.
    Pool size: 24000gal inground Vinyl-Taylor k-2006 and k-1766 test kits and-speed stir
    Intermatic P1353ME digital timer w/freeze sensor
    CircuPool Si-45 SWCG System
    Polaris 280 vacuum/Polaris PB4-60 boost pump
    Pentair IntelliFlo VS 3hp Pump--Pentair sand filter

  3. Back To Top    #3
    Mod Squad jblizzle's Avatar
    Join Date
    May 2010
    Tucson, AZ

    Re: high chlorine levels interfering with pH test readings

    It is well known and often stated on the forum that the common pH test will read artificially high when the FC > 10ppm.

    That is why we always say to lower the pH to 7.2 before raising the FC above 10ppm for the SLAM process and then not to test the pH again until the FC drops.
    Jason, TFP Moderator
    18k IG pebblesheen pool, Hayward ProLogic P4 w/ T-15 SWG, Pentair 1HP 2-speed Superflo, Hayward 6020 DE filter
    500 sqft Heliocol solar panels, ThePoolCleaner, TF-100 test kit w/ SpeedStir
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  4. Back To Top    #4

    In the Industry
    duraleigh's Avatar
    Join Date
    Apr 2007
    Sebring, Florida

    Re: high chlorine levels interfering with pH test readings

    Welcome to the forum.

    I am willing to exceed this by a bit so long as the calcite saturation index remains balanced, and that no metals are present to come out of solution to stain the plaster.
    Seems reasonable at first but your CH is too high (CH is a metal) and despite what CSI may tell you, high pH and high CH is a recipe for calcium scale and I would suggest you bring both of those back within guidelines for a balanced pool.
    Dave S. - Forum owner
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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