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Thread: How to test pH with a high chlorine level?

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    How to test pH with a high chlorine level?

    I read somewhere on this forum that if your chlorine level is high, your pH reading tends to be the same purplish color (appears to be 7.7 to me). My chlorine level on my pool is currently 17 (CYA is 160... been slowly trying to lower it). I added muriatic acid according to the pool calculator to lower the pH to 7.4, and the next day, still the same purplish color.

    So I'm guessing the high chlorine level is affecting it. How can I test the pH accurately with a high chlorine level like mine? I've been using the TFTestKit.
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    Mod Squad JohnT's Avatar
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    Re: How to test pH with a high chlorine level?

    You really can't. Wait until your chlorine level comes down before you attempt to adjust or test the pH.
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    JesseJames's Avatar
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    Re: How to test pH with a high chlorine level?

    You might have to wait a while. My FC was 14 in December. It finally got to 5 today.
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    Re: How to test pH with a high chlorine level?

    There are chlorine neutralizers available. pH can then be tested. I am pretty sure Leslies carries it.

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    Re: How to test pH with a high chlorine level?

    There is a chlorine neutralizer already in the Taylor (and TF-100) kits for the TA test. The R-0007 reagent is sodium thiosulfate which is a chlorine neutralizer. This is done to neutralize the chlorine for the TA test. However, such neutralizer solutions are themselves high in pH and the reactions with chlorine are inconsistent in their effects on pH (depending on factors such as the TA level). So while you can add some R-0007 before adding the pH reagent, you may still get a "too high" reading, though at least you won't get the bleached-out half-indicator result of purple. You can certainly add a couple of drops of R-0007 to see what it does -- just don't trust the final result.

    The Taylor R-0004 pH Indicator solution has a proprietary combination of chlorine neutralizers already in it that try and keep the pH the same when chlorine gets neutralized, but there's only enough to neutralize roughly 10 ppm FC or so of chlorine. They didn't want to add more since they didn't want to adversely affect results by this neutralizer solution (i.e. too much could affect the pH). Handling 0-10 ppm FC seemed reasonable.
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    Re: How to test pH with a high chlorine level?

    Quote Originally Posted by chem geek
    There is a chlorine neutralizer already in the Taylor (and TF-100) kits for the TA test. The R-0007 reagent is sodium thiosulfate which is a chlorine neutralizer. This is done to neutralize the chlorine for the TA test. However, such neutralizer solutions are themselves high in pH and the reactions with chlorine are inconsistent in their effects on pH (depending on factors such as the TA level). So while you can add some R-0007 before adding the pH reagent, you may still get a "too high" reading, though at least you won't get the bleached-out half-indicator result of purple. You can certainly add a couple of drops of R-0007 to see what it does -- just don't trust the final result.

    The Taylor R-0004 pH Indicator solution has a proprietary combination of chlorine neutralizers already in it that try and keep the pH the same when chlorine gets neutralized, but there's only enough to neutralize roughly 10 ppm FC or so of chlorine. They didn't want to add more since they didn't want to adversely affect results by this neutralizer solution (i.e. too much could affect the pH). Handling 0-10 ppm FC seemed reasonable.
    Ok, tried a drop of R-0007 and the pH reading went really high (above 8.2), so what you said about the alkalinity of R-0007 is right on. I guess I may have to bite the bullet and drain a lot of my pool in order to get CYA down so my chlorine doesn't have to be so high in order to get an accurate pH level. Bummer. Thanks for the tips!
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    Re: How to test pH with a high chlorine level?

    Yes, that is one of the problems of having such a high CYA level. To prevent algae growth with chlorine alone, you need a high FC level, but that makes the pH test give inaccurate results. A pH meter won't be affected by this, but they are expensive and can be finicky if you don't clean them and store them back in their proper solutions. Nevertheless, I wonder if anyone can recommend a relatively inexpensive pH "stick" meter that gives decent results.

    Another technique would be to get rid of the chlorine in a known way such as exposing a sample (or a bucket of pool water from which you will take a sample) to UV or by adding a small amount of ammonia to it. If you then measure the result, the pH will be lower than it was, but with the other water chemistry parameters we can calculate what it was before this procedure.
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    Re: How to test pH with a high chlorine level?

    chem what do you think the difference in pH level is? + or -? if chlorine is high like over 10ppm say 15 0r 20ppm and pH tested is 7.6

    is the real pH lower or higher? and by how much?

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    Re: How to test pH with a high chlorine level?

    The real PH is lower. Not sure if you can really pinpoint how much with any accuracy.
    Helpful links: Pool School; CYA/Chlorine Chart
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    Re: How to test pH with a high chlorine level?

    The quote from Taylor's Pool & Spa Water Chemistry booklet on the issue of a high FC causing a false high pH reading is as follows:

    FALSE READINGS: high levels of chlorine (usually > 10 ppm) will quickly and completely convert phenol red into another pH indicator (chlorphenol red). This new indicator is a dark purple when the water's pH is above 6.6. Unfortunately, some pool operators mistake the purple color for dark red and think the pool water is very alkaline and wrongly add acid to the pool.

    When a sanitizer level is not extreme, only some of the phenol red may convert to chlorphenol red. However, purple + orange (for example, pH 7.4) = red. This error is more subtle as no purple color is observed and the operator does not suspect that a false high pH reading has been produced. Some operators neutralize the sanitizer first by adding a drop of chlorine neutralizer (i.e. sodium thiosulfate). However, thiosulfate solutions have a high pH and, if heavily used, may cause a false higher sample pH.
    I am figuring out a speculative approach in this thread to working around this problem by using ammonia to convert the chlorine to monochloramine. This might prevent the oxidation of the dye and give a better pH reading that would be artificially high by a small known amount given other water parameters. [EDIT] I'll work on Jason's suggestion of using distilled water which seems far simpler and may work quite well. [END-EDIT]
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    Re: How to test pH with a high chlorine level?

    had a thought. What if you dilute the sample 1 to 1 with distilled water. The FC level will be half of what it was before, and the PH shift from the dilution should be quite small because the TA level of distilled water is very low. It should even be possible to calculate what the PH shift from the dilution will be and correct for it.
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    Re: How to test pH with a high chlorine level?

    Quote Originally Posted by JasonLion
    had a thought. What if you dilute the sample 1 to 1 with distilled water. The FC level will be half of what it was before, and the PH shift from the dilution should be quite small because the TA level of distilled water is very low. It should even be possible to calculate what the PH shift from the dilution will be and correct for it.
    chems onto something. Jason thats not a bad idea and is a simple solution.

    when I see pH of say 7.8 or 8.0 I only drop a small amount as pH bounce back increases the more you drop the pH. so will only drop to like 7.6 or 7.5 but not if FC is like this very high I am rethinking maybe I dont drop as low with these high FC run pools. Hmmm

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    Re: How to test pH with a high chlorine level?

    That's an even better idea than using the ammonia. The key to making dilution with water work is that the water really does need to be distilled water because it must be unbuffered. Otherwise, it will have a large effect on pH, so one cannot use tap or filtered water since they have too much TA (i.e. buffering).

    Distilled water cuts down all concentrations by an equal amount. However, water itself will dissociate to fill in for any reduction in hydrogen and hydroxyl ions with their product remaining the same. Mostly though, the main pH buffer relationships have their ratios the same so they keep the pH the same. I'll work on this, but I think the pH remains essentially the same so would be a good approach, assuming one dilutes to get the FC below 10 ppm -- say to 5 ppm FC for safety.
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