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Thread: Possible Solution to Measuring pH when FC is High

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    Possible Solution to Measuring pH when FC is High

    During shocking and in pools with very high CYA levels where the normal FC level must be very high to prevent algae growth, it becomes difficult to get an accurate pH reading because the high FC level causes a false reading. This is described in the Taylor's Pool & Spa Water Chemistry booklet 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've been thinking about possible approaches to working around this problem and one approach would be to add a measured amount of ammonia sufficient to convert all of the FC into monochloramine. If monochloramine does not convert phenol red into chlorphenol red because monochloramine is a weaker oxidizer than hypochlorous acid, then this approach may work. The formation of monochloramine from ammonia will somewhat raise the pH, but by a known amount that can be compensated for in the calculations if one knows the starting water chemistry parameters.

    To test this theory out, we first need to create a stock solution of ammonia that is at a concentration where adding drops of it to a sample of known size will give us the result we desire. The Church & Dwight product known as Parson's Ammonia All-Purpose Cleaner has an MSDS indicating that it is <3% ammonia. Hach has an Ammonia Standard Solution that is N 1000 mg/L (i.e. 1000 mg Nitrogen per liter).

    What we want is a solution whose concentration will neutralize the chlorine level in the sample. For simplicity, since high chlorine levels are typically measured using 10 ml samples where each drop of FAS-DPD titrating reagent is 0.5 ppm FC, we can make an ammonia solution where one drop from a 0.75 oz. Taylor bottle "neutralizes" 0.5 ppm FC. Taylor's dropper tips are standardized to 24 drops/ml. So the next piece of information we need to know is the sample size and this varies depending on the pH kit. For the Taylor K-2xxx series, the pH sample size is 44 ml while for the Taylor K-1xxx series (and the TF-100) the pH sample size is 7.5 ml. It takes an equal molar amount of ammonia to combine with chlorine to produce monochloramine, but in relative units of ppm N for ammonia and ppm Cl2 for chlorine, it takes 0.1975 ppm N of ammonia for every 1 ppm of chlorine. So for 0.5 ppm chlorine, it takes 0.0988 ppm N of ammonia.

    Since we want one drop from a Taylor bottle to "neutralize" 0.5 ppm chlorine and since there are 24 drops/ml, the dilution ratio for the 44 ml sample size is 44/(1/24) = 1056 while for the 7.5 ml sample size it is 7.5/(1/24) = 180. So we want the concentration of ammonia solution to be 0.0988*1056 = 104 ppm N for the 44 ml sample size and 0.0988*180 = 17.8 ppm N for the 7.5 ml sample size. This implies diluting the Hach standard solution (with distilled or at least filtered water) by 1000/104 = 9.6 to 1 for the 44 ml sample size and 1000/17.8 = 56.2 to 1 for the 7.5 ml sample size. For simplicity we can just make these 10:1 and 50:1 dilutions. We just need to ensure that we add enough ammonia to get most of the FC down -- adding a little less just means a low remaining FC level while adding a little more just means some excess ammonia, neither of which are a problem.

    So creating a stock solution is probably most easily done using a spare Taylor bottle since it has a nice calibrated dropper tip. Creating 20 ml of solution using the graduated cylinders in the kits, for the 10:1 (really 9.6:1) dilution for the 44 ml sample size, one would add 50 drops of the Hach standard and then fill up to the 20 ml line using distilled (or at least filtered) water. For the 50:1 (really 56.2:1) dilution for the 7.5 ml sample size, one would add 8-9 drops (it's 8-1/2) and then fill up to 20 ml. The resulting 20 ml of solution can be stored in a 0.75 ounce (22 ml) Taylor bottle.

    As for adjusting the pH reading, this mostly depends on the starting FC, TA and pH. (more on this later...will edit this post).
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    Re: Possible Solution to Measuring pH when FC is High

    Perhaps you could use sodium ascorbate as a chlorine neutralizer. It is mostly pH neutral. Once you know the chlorine level, you could add just enough sodium ascorbate to lower the chlorine below 5 ppm. You could use a correction factor for the effect of the ascorbate on the pH.

    C5H5O5CH2ONa + HOCL --> C5H3O5CH2OH + Na+ + Cl- + H2O

    Sodium ascorbate + Hypochlorous acid --> Dehydroascorbic acid + Sodium + chloride + water

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    Re: Possible Solution to Measuring pH when FC is High

    Yes, that seems better. This actually isn't really pH neutral because the pH drops due to removal of hypochlorous acid (a weak acid that will be partly dissociated at typical pool pH so the remaining hypochlorite ion will form more hypochlorous acid causing the pH to rise). If one used ascorbic acid, then the pH would [EDIT] fall [END-EDIT] since it's an acid mostly dissociated (for the first hydrogen) at pool pH. A 50/50 mixture of sodium ascorbate and ascorbic acid would be roughly pH neutral using up hypochlorous acid near a starting pH of 7.5, but of course one doesn't really know the starting pH so can't really predict the ideal ascorbate/ascorbic mixture. So just using either sodium ascorbate or ascorbic acid and calculating what the original pH should have been is possible. The advantage of the ascorabate over ascorbic acid is that you can add excess beyond that needed to neutralize chlorine and the excess essentially won't change the pH.

    Also, any reducing agent would work, including ferrous ion such as using ferrous chloride as chlorine will oxidize it to ferric ion. Again, this would lower the pH, but one could calculate the original pH based on the other water parameters (such as the starting TA, FC and CYA).

    Jason's idea of using distilled (or deionized) water to dilute the water sample is probably the easiest and most practical. When I work out the exact effect on pH it just cuts down all concentrations by the dilution factor, but the effect of cutting down the net of [OH-]-[H+] is equivalent to adding a small amount of acid. I've worked this out and it's definitely a negligible effect with buffered water so Jason's dilution method is clearly the best when dealing with buffered water such as pool water. Diluting with distilled (or deionized, but NOT filtered or tap) water will work very well.
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    Re: Possible Solution to Measuring pH when FC is High

    Yall are making my head hurt! Possibly from the fact that I actually understand what you're saying. I've been on this forum too long!!
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    Re: Possible Solution to Measuring pH when FC is High

    By the way, JamesW, I forgot to give you a Welcome to TFP!
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    Re: Possible Solution to Measuring pH when FC is High

    How about sodium dithionite as a reductant?
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    Re: Possible Solution to Measuring pH when FC is High

    Yes, sodium dithionite as well as sodium thiosulfate are reducing agents, but unfortunately they don't react with chlorine in consistent ways regarding pH. This is part of the reason that thiosulfate alone isn't used to neutralize chlorine in good pH tests. Instead, a proprietary mixture of different reducing agents are used that hopefully keep the pH fairly consistent (some make the pH rise; others make it fall; the rates try to be matched by adjusting the relative concentration).
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    Re: Possible Solution to Measuring pH when FC is High

    What about measuring pH with a pH meter? It shouldn't be too expensive.
    They have hand held models as well. Cole Parmer carries one for $60 but there are cheaper models out there.
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    Re: Possible Solution to Measuring pH when FC is High

    Quote Originally Posted by Water_man
    What about measuring pH with a pH meter? It shouldn't be too expensive.
    They have hand held models as well. Cole Parmer carries one for $60 but there are cheaper models out there.
    Because hand held pH meters are crappy at best. Unless you know of reliable, affordable ones?

    I'd love to have an inexpensive, good quality, continuous inline pH meter that didn't have to be recalibrated every other day.
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    Re: Possible Solution to Measuring pH when FC is High

    Quote Originally Posted by mac4lyfe
    I'd love to have an inexpensive, good quality, continuous inline pH meter that didn't have to be recalibrated every other day.
    Cole Parmer makes reliable instruments. I'd try there, and call their tech support first. Here's an example of an inexpensive one ($60.)
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    Re: Possible Solution to Measuring pH when FC is High

    Those $60 single junction pH meters are not reliable or very accurate. Also, you need to buy a standardization buffer solution. Those portable double junction pH meters at for $200 or more at Cole Palmer are better because you can replace the electrode but now you would also want to buy an electrode storage solution too. These electrodes do not last forever either. The indicator drop test is probably better than a cheap pocket meter but I have no comparison experience either. I have used all sorts of pH meters for many years though.
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