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Thread: Acid column myth

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    Acid column myth

    Here is a link to a paper on the myth of lowering TA by the 'slug' or acid column method. Interesting reading!

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    Yeah, I just saw that today myself (we must have both read the link from another pool forum about the same time). The paper says, in much greater detail, what we've been saying all along and backs it up with experiments to boot. It's too bad the JSPSI organization and journal look like they have gone away. In its absence we have myths.

    And take a look at the bottom of this link in the same website for an article that describes how the kill times for bacteria track hypochlorous acid concentration and that the chlorinated isocyanurates are not effective disinfectants but act as a reservoir for chlorine. This is consistent with other studies. What would be more interesting is a similar study for algae prevention since killing bacteria is easy and needs very little hypochlorous acid. We have the experience of hundreds of various pool forum users showing that disinfecting chlorine level (i.e. Ben's chart) is what prevents algae growth (there was one user with extremely high phosphates who recently reported otherwise, but that's the only exception I've seen).

    And there is this link that has an article about the Langelier Saturation Index and its applicability to pools and says it is a misconception that the index only applies to closed systems. Lots of articles here pretty much saying the same things talked about on the Pool Forum and here. On this same page at the bottom is an article that says that the LSI is not a corrosion index but scale dissolving tendency, though I would consider the dissolving of calcium carbonate from plaster to be "corrosion", though it is not metal corrosion.

    The last article in this link talks about the relative buffering of carbonates, borates and cyanurates.

    This link has a wealth of useful info including interesting tidbits of chlorine not breaking down urea very quickly and ozone not being effective at oxidizing ammonia, urea or monochloramine.

    At this link there is an article that refers to keeping CYA in check not only to maintain chlorine disinfection capability, but also for algae control.

    At this link there is reference to thermal decomposition of chloroisocyanurates which is useful info since that is a different process than breakdown from sunlight. But something about this doesn't make much sense as CYA levels do not drop and the same article refers to 14% per day decomposition but only 1% per day drops in FC. Notice also the article with equations for calculating pH and TA from acid/base additions that sounds like a simplification of my spreadsheet that would be useful for the online calculator and for BleachCalc.

    And this link a little ways down says the following:

    Hydrolysis of hypochlorite ion (C10- + H20 D HOCI + OH-) temporarily increases pH. However, the effect, which is significant (-0.2 pH units) with a shock dose, will disappear when the chlorine dissipates (Wojtowicz, JSPS1, vol. 3, no. 2, p. 34, 1999).

    So, what happened to John A. Wojtowicz who wrote most of these articles?

    Richard
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    Very interesting article. It is amazing what happens when a bit of science is introduced.

    If I read it correctly, and I don't pretend to understand it all, aeration is not necessary to reduce TA but simply accelerates the process. Thus us SWG owners that continually add acid should see TA drop over time. Unfortunatly, high TA and PH have always been a problem for me and TA always seem to settle around 100 ppm.

    I noticed something interesting the last time I tried to lower the TA in my water. I added enough acid to lower the PH to about 7.1 and TA dropped to 70 ppm. But in three days, the PH was up to about 7.3 and TA was at 90 ppm which is still lower than where it started but higher than the first day after adding acid. This time of year my fill water TA is about 110 ppm but I wouldn't think that the pool lost enough water in three days to affect the TA that much. Just wondering if there is any other reason that TA would increase without adding a bicarbonate.
    Mark
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    Mark,

    Yes, I think you are interpreting the article correctly. Outgassing always occurs so the tendency of the pH to rise (ignoring other pH changing factors) always occurs so long as the pool is out of equilibrium with the air in terms of carbon dioxide saturation. However, lower pH makes this equilibrium even more out of balance and higher TA also changes that balance. This chart I made a while ago gives the ratio of the actual concentration of dissolved carbon dioxide ( carbonic acid plus CO2(g) ) at various pH and TA to the equilibrium concentration [EDIT] minus 1 [END-EDIT] based on standard amounts of carbon dioxide in the atmosphere.

    As for your pH and TA rise, the outgassing would cause a pH rise with no change in TA so the TA rise must have come from something else. It could have been a combination of several items including the fill water you mentioned plus measurement error in both TA and pH and the time it takes for the overall pool to equilibrate after acid addition (which needs to dilute and mix throughout the pool). Also, if the pool plaster was doing any curing or was dissolving from the water being low in saturation index, then that would make both pH and TA rise, though 3 days is pretty short for that.

    Richard
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    Richard, on that chart, what do the green, red and orange numbers denote?

    I'm guessing red will show a tendency for PH rise, green for PH drop and orange should provide stability?

    By the way, all your charts should be stickied somewhere, if they aren't already

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    Most of all of my charts and technical info may be found at this thread in this Advanced Chemistry section.

    The color coding is a rough break of the amount of outgassing with red being very, very likely to cause a pool to rise in pH while orange is somewhat likely and green is less likely. This seems to work reasonably well for pools that do not have unusual sources of aeration. However, pools with lots of aerating water features, such as waterfalls, spillovers, etc. or pools with SWG systems have a far stronger tendency to rise in pH (due to the higher aeration) so even the green area isn't "stable" for such pools. Instead of green being a cutoff of a ratio of 10, in high aeration pools it appears that a reasonable cutoff is closer to 5 or even a bit lower in unusual situations.

    In all areas of the chart with a number higher than "0", the tendency will be for the pH to rise. I was incorrect in my previous post when I talked about the chart being a pure ratio. It's actually that ratio minus 1 in order for the numbers to represent a relative outgas rate where "0" is no ougtassing and "1" is outgassing with a 2:1 ratio of actual vs. equilibrium amounts of carbon dioxide (gas and carbonic acid) in the water. If I had extended the chart to even higher pH or lower TA, the numbers would turn negative indicating a tendency for carbon dioxide in the air to dissolve in the water and the pH to drop.

    Richard
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    FWIW - He doesn't really cover the effects of the variation in rate of CO2 loss at the surface.

    "First, excess carbon dioxide (from carbonic acid) will, over time, release into the air regardless of the pH, as predicted by Henry’s Law. It is not necessary to have a pH of 5.5 or 4.0 or below to accomplish this."

    This is misleading if the rates are such that at normal PH it takes months while at low PH it happens in minuets. I don't know what the rates are, but it seems at least possible that they are sufficently different. If they are, and if the surface water PH changed significantly, there could still be an effect.

    Of course none of that matters very much for debunking the acid clump method given that the acid tends to pool at the bottom if the pump is not running and to mix quickly if the pump is running. The PH change at the surface, where it could have an effect, would be minimal and the volume of water affected would also be minimal.
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    I'm certainly no chemist - heck, I don't even understand half of what the "science" of all of it is.

    However, I just went through 2 weeks last month of lowering my TA and I can attest that it was much easier and quicker to lower my TA using the acid ball rather than the drizzle method. I wish I still had my numbers to show the progress differences between the two methods. I was getting very frustrated doing the drizzle method.

    Maybe it was a quirk, I surely don't know - but that was my experience.
    Buggs

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    Quote Originally Posted by chem geek
    So, what happened to John A. Wojtowicz who wrote most of these articles?

    Richard
    Bumped off by Chemtura goons?

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    Is there a sweet spot in the chart where PH should tend to remain stable in normal operation?

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    JasonLion,

    You are right that he doesn't refer to rates, but IF the rates are first-order so are proportional to the concentration of carbon dioxide in each medium (water vs. air), THEN the chart I made will show the relative rate at different pH and TA. This is not an absolute rate since one needs to know more about the physical aeration, diffusion processes, but with such processes roughly constant then the chart is useful as a rough guide.

    Rangeball,

    I think I answered in an earlier post in this thread that there is carbon dioxide outgassing and a tendency for the pH to rise for the entire chart range, except for the "0" entry (and entries with higher pH and lower TA than that). However, as JasonLion pointed out, the rate means everything since if the outgassing is so slow that it takes months to see a pH change, then that's a good thing and should be considered "stable" for all practical purposes. All the chart tells you is that different pH and TA combinations that have the same number in the table should have the same rate of pH rise and that higher numbers should have correspondingly (proportionately) higher rates of rise in terms of relative (all else equal) outgassing rates -- the outgassing rates do not directly track with pH rise since the amount of rise is a function of TA level. I was thinking about modifying the chart to accommodate that but never got around to it (it would show that the decrease in the rate of pH rise didn't change very quickly, but the amount of acid needed to be added per unit time, say per week, dropped significantly).

    Richard
    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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    You always want to run a pool out of equilibrium so there will always be some tendency for PH changes due to CO2 outgassing. Given that, high PH and low ALK have far less of a tendancy to outgass than low PH and high ALK. This is why the recommended ALK for bleach/SWG pools is lower and also why the high PH approach (which isn't discussed much) works.

    Unless you have a dramatic source of aeration, a negative edge or large fountain/waterfall that is always on, numbers along the lines that are recommended here (PH 7.5 ALK 80-90) are reasonably stable. As you increase the average amount of aeration you need to compensate by either raising the PH or lowering the ALK or both, unless you want to be constantly fighting it.

    With a SWG most people end up constantly fighting it, adding acid frequently and baking soda occasionally. Since high PH and low ALK lead to their own issues that is often a reasonable approach. However, there are things you can do to avoid this. For example, by adding a borate buffer you can safely lower your carbonate buffer (ie raise borates to 50 ppm and lower ALK to perhaps 50). This takes noticable work, raising borates requires adding both borax and acid in fairly large quantities and it might pose a risk for pets, so it isn't currently recommended as a standard practice.
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    So I finally computed the theoretical relative pH rise associated with the relative CO2 outgassing I had calculated earlier in this chart and the result is shown in this chart. The relative scale has even less meaning in this chart in the sense that it is truly purely relative -- the numbers were the pH rise I calculated when using the outgassing relative rate numbers as a TA equivalent number (in the first chart, a "1" meant that the air has twice as much carbon dioxide as the water as far as Henry's Law is concerned, but in the new chart there is no equivalent meaning for the numbers in an absolute sense). Though I knew that the lower alkalinity would not only slow down outgassing but would be a weaker pH buffer and these two effects would cancel each other out to some degree, one can see from the chart that even going from a TA of 200 to 80 ppm at a pH of 7.5 only goes from a relative pH change of 0.97 to 0.76 or a decrease of 22% in the rate of pH rise. That's not very much. Now the original chart shows the relative outgassing rate going from 20.7 to 7.0 which is a decrease of 66% that is far more significant. This means that the required frequency of acid addition won't change very much at lower, but not really low, TA, but the quantity of acid added over time will be a lot less.

    This theoretical result does account for some people not seeing dramatic improvements until they lower the TA in their pools by quite a bit going down closer to 50 or 60 ppm if they only have 30 ppm CYA (similar to 70 or 80 ppm if they have 80 ppm CYA).

    As JasonLion points out, adding a Borate buffer has multiple benefits. It is a great buffer against rise in pH (not much capacity against lowering in pH, but that's OK since it's a rise we are fighting and are not using acidic sources of chlorine). This means that lowering the TA AND using a Borate buffer will significantly lower the rate of pH rise (due to the extra pH buffering) AND the amount of acid needed to be added over time (due to the lower TA so lower outgassing). In addition, the Boric Acid will inhibit algae so that chlorine won't have to and this lower chlorine consumption means that in SWG pools the SWG output can be lowered and that reduces its aeration (from hydrogen gas bubbles) which lowers the pH rise at its source, so to speak.

    At least now we know why the combination of the lower TA and the Borates is so powerful and why the lower TA alone isn't all its cracked up to be unless taken to an extreme. Of course, many users have been happy lowering their TA from levels of 150-200 or more down to even 80 or 70. And, of course, not every pool needs to have the TA lowered since it depends a lot on the aeration, whether the pool is covered, etc.

    Richard
    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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    Quote Originally Posted by chem geek
    So, what happened to John A. Wojtowicz who wrote most of these articles?
    As of two years ago he was supposedly retired and living in a suburb of Phoenix and being courted to write a book on swimming pools.
    CuriousMark

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