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Thread: He wants to use Sodium Bicarbonate first, then Sodium Carbon

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    He wants to use Sodium Bicarbonate first, then Sodium Carbon

    TA itself is a source of rising pH due to carbon dioxide outgassing
    A work associate is performing a "hot start" or "hot start up" procedure on a pool. He lowers the pH very low to treat the plaster, and then raises the pH to acceptable levels. He wants to use Sodium Bicarbonate first, then Sodium Carbonate. I'm trying to tell him that when he's all done his total alkalinity levels will be way too high and the water will have high pH problems for weeks or months because of this.

    Can you tell me more about how Sodium Bicarbonate leads to Carbon Dioxide outgassing?

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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    Swimming pools are like soda, they have excess carbonation. When the CO2 comes out of solution (bubbles in soda) the PH goes up. You can read all kinds of technical details in this fairly technical topic. The rate at which CO2 outgasses depends on the PH, TA, and amount of aeration. Lower PH, higher TA, and more aeration means more CO2 outgassing, and thus more PH increases.

    This is why pools with SWGs or negative edges tend to have rising PH. The SWG creates hydrogen bubbles, which are a source of aeration, just as the waterfalls on negative edge pools are a source of aeration. With typical PH levels plus aeration from a SWG or negative edge there will be some CO2 outgassing at just about any reasonable TA level. The higher the TA level, the faster CO2 outgasses.
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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    Carbon Dioxide is acidic, correct? The pH rises because this acidic substance is leaving?

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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    Exactly. When dissolved in water CO2 forms carbonic acid, which is indeed acidic.
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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    I'm missing something. Doesn't Sodium Bicarbonate help to create Carbon Dioxide?

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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    Yes, sodium bicarbonate is where the CO2 is coming from. Instead of injecting CO2 (as is done with soda) we add sodium bicarbonate. Sodium bicarbonate adds some other things besides CO2, which more than cancel out the acidity of the added CO2. Over simplifying a little, TA is formed from a weak acid and a weak base, which together form a PH buffer. The weak acid is carbonic acid and the base is hydroxide ions. The carbonic acid can outgas as CO2, but the hydroxide ions stay in the water.
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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    A misconception I've had is thinking that when starting out with very low pH, either from using Trichlor pucks and not checking pH or TA or from an acid startup or treatment (i.e. very low pH), that using sodium carbonate would be better than using sodium bicarbonate in that it would raise the pH more quickly with less change in TA. It turns out that this isn't quite true because of the significant amount of carbon dioxide outgassing occurs when one adds either product. So basically both products under the conditions of very low pH act as if they were a strong base where the carbonate portion of their chemistry comes right back out of the pool as carbon dioxide. Of course, as the pH rises this behavior changes, but the sodium bicarbonate helps prevent an overshoot of the pH.

    Now it is true that if you were to continue to use such products as the pH is getting close to 7.5 then you could end up with too much TA, but if you stop at around 7.0 and check on the TA level, then you can use aeration to raise the pH with no change in TA or use sodium carbonate to raise both pH and TA. I think this is what he has in mind. The sodium bicarbonate during the first portion when the pH is low is just another alternative that might be lower cost depending on relative pricing though my next calculations seem to indicate that this is not the case. It takes twice as much sodium bicarbonate in molar amount for the same effect as sodium carbonate -- by weight it takes 1.585 times as much sodium bicarbonate at $1.52 per pound as sodium carbonate at $2.01 per pound so it would seem that sodium carbonate would be more cost effective than sodium bicarbonate, though it does have the risk of overshooting the pH if one becomes overly zealous with dosing.
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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    We're not given enough time for aeration.

    I tried using bicarb exclusively once but gave up when the TA reached 300 ppm and the pH still had not reached 7.0.

    I remember one time when I used soda ash exclusively that the pH reached 7.5 and the TA reached only 100 ppm. Soda Ash seems to work better for me.

    What is the proper pH level for an Acid Treatment? I think my work associate doesn't really know. It seems like his method is simply to add a whole bunch of acid until the test water is really yellow.

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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    See Start-up Chemistry for Plaster Pools. An acid startup is at a pH of 4.5. This is the point where the TA goes to 0 in the TA test and the post says this is about 3 gallons of acid per 10,000 gallons of water but if the TA starts at 100 ppm then it takes 2 gallons of acid per 10,000 gallons to lower the TA to 0 so the rest is in part used to react with the calcium hydroxide from the plaster and is closer to the 3-4 gallons typically recommended by plasterers doing an acid startup.

    As for the sodium carbonate vs. sodium bicarbonate, what you describe is what I would have expected, but I was told in this post that sodium bicarbonate works fine, but that thread was in the context of avoiding metal staining. So your experiences differ so the best approach may not be the same in every situation.
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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    I'm just following along since one of the companies I've spoken to about replaster says they will use this Hot Start.... and they think it won't bother the heater as the system runs through it, but at a pH of 4.5!!! That's gotta hurt the heater.
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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    There must obviously be some effect on the heater, but the PH is only low for one to three days, so the effect is small.
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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    My undertanding of this chemistry issue is to slowly add soda ash first to a pH of about 7.2, then add bicarb as needed to increase the TA in proper range if needed, which 09659 indicated. That way you don't over shoot the TA.

    Chemgeek, I do not understand the claim that adding soda ash to acidic water (pH between 5.0 to 7.0) creates carbon dioxide. When soda ash reacts with the already existing high content of carbon dioxide (which is why the pH is low), it creates bicarbonate. (Soda ash reacts first with carbonic acid to form bicarbonate, which then reduces the CO2 content, not increase it). When adding sodium bicarbonate, it is my understanding that it doesn't react with CO2, it simply increases TA and thus, then you have to wait for the CO2 to off-gas. I do not understand how adding sodium bicarbonate increases the CO2 content, unless the pH is below 4.5. I know that equilibriums are established, but if I am missing something, please let me know.

    And I would never allow pH 4.5 go through my copper based heat exchanger.

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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    Adding either soda ash or baking soda when the pH is very low will create carbon dioxide and some will outgas. The pKa of carbon dioxide / carbonic acid vs. bicarbonate is around 6.3 so at that pH they are equal in concentration while below that pH most of the carbonates are carbon dioxide (between carbonic acid and carbon dioxide there is a fixed relationship independent of pH where there is 650 times as much carbon dioxide as carbonic acid).

    It is true that adding soda ash should move the pH with less change in TA which is why I originally proposed doing that first until I saw the post from James that I linked to. Technically, adding soda ash is identical to adding baking soda and lye in combination because

    NaHCO3 + NaOH ---> Na2CO3 + H2O
    Sodium Bicarbonate + Lye ---> Soda Ash + Water

    The main concern with using soda ash is that it can raise the pH locally too high (once the pH is up a bit) so if there are metals in the water then that can cause metal staining. If there are no metals in the water, then this should not be a problem.
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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    In regards to a low pH and low alkalinity (TA) of pool water and how to properly raise both of them, the following is my perspective and how I see this topic.

    I think Chem geek was correct the first time (in another post he linked to) that the most effective and most cost efficient way to raise the TA and pH on a very low pH and TA pool situation (as in a Zero Alkalinity Acid Treatment, and a 4.5 pH), is to add soda ash (sodium carbonate) first to re-balance to a pH of around 7.0, and then adding Bicarb (sodium bicarbonate) to finish raising the TA to the proper range of around 80 ppm.

    As James suggested in the other linked post, probably the safest way to re-balance the water is to calculate the correct amount of Bicarb to add to achieve an 80 ppm of TA, and then allow the low pH to rise naturally. That program however, requires some time to allow the pH to rise to the proper balanced level.

    As was also suggested by James in the other post, it is true that there are potential issues of precipitating copper (if present) and calcium when using soda ash as opposed to adding Bicarb. But as Chem geek also indicated correctly, if draining the pool is not an option, it may be better to precipitate these two metals (by adding Soda ash) and have them filtered out before they adhere to the surface. Of course, vinyl and fiberglass pools would be good candidates for that program.

    The bottom line on this issue is that there are different ways to re-balance low pH and low TA water effectively. There can be pros and cons to several ways. And adding and using some borax can be an extra option. And note that Soda ash is 60% stronger than Bicarb in adding alkalinity, and therefore will require a lesser amount to raise the alkalinity (and pH).

    The following is my understanding on the issue of carbon dioxide (CO2) and what Bicarb and soda ash does when added to water with a low pH and low TA.

    I am going to ignore the involvement of carbonic acid formation for simplicity sake. It is understood that whenever CO2 (carbon dioxide) is formed, there will be a very small fraction of carbonic acid formed establishing an equilibrium (as Chem Geek mentioned) and that it participates in the various reactions discussed below.

    When adding sodium bicarbonate (Bicarb) to water having a pH of 4.5 (approximately) and above, will not create or form a “measureable” amount of additional CO2. It simply adds bicarbonate alkalinity and raises the pH of the water. When enough Bicarb is added to raise the pH to 6.4; that will be where there are equal “ppm” amounts (a 1 to 1 ratio) of bicarbonate alkalinity and CO2. For example, at that point (pH 6.4) there may be 10 ppm of CO2 and 10 ppm of bicarbonate alkalinity (TA), or 50 ppm of CO2 and 50 ppm of bicarbonate. When the ratio of bicarbonate to carbon dioxide is 5 to 1, then the pH is 7.0, i.e. 50 ppm of bicarbonate and 10 ppm of CO2, or 100 ppm of bicarbonate and 20 ppm of CO2.

    As everyone knows, in time the CO2 out-gasses and is lost to the atmosphere thereby causing the pH to further rise before and after the addition of Bicarb.

    When soda ash is added to water with a pH 4.5 and above, it reduces the CO2 content, not create more. It does this by reacting carbonate (from soda ash) with the CO2 in the water to form bicarbonate alkalinity and also increases the pH significantly (more than Bicarb does). Adding more soda ash will continue to reduce the CO2 content until a pH of 8.4 is achieved. At that point, there will not be a “measureable” amount of CO2 in the water.

    Those who perform a Zero Alkalinity treatment often add too much acid to the pool and the pH may go as low as 3.0. When the pH goes below 4.5 (about), the TA is considered to be a “negative” amount or is defined as mineral acidity, and contains a more measureable amount of hydrogen ions (acid).

    Adding Bicarb or soda ash to water with a pH of 3.0 will cause the formation of some additional CO2, due to the reaction with higher content of hydrogen ions (a strong acid). But once enough Bicarb or soda ash is added and raises the pH to around 4.5 and above, the amount of hydrogen ions becomes negligible and the reactions with Bicarb or Soda ash essentially does not increase the CO2 content, and therefore, can be ignored.

    To help put this into perspective, when starting out with 80 ppm of TA (assuming no CYA) and performing a Zero Alkalinity or No-Drain acid bath, adding acid to lower the TA to zero (and pH of 4.5) creates about 70 ppm of CO2. (I hope I did my math right). And again, that excess CO2 is always out-gassing, and therefore is constantly being lost and reduced over time.

    Depending on how long the water is allowed to remain in an acidic condition, the content of CO2 may be somewhat high, or could be very low. This can be affected by at least two factors; the natural off-gassing of the CO2, and the fact that the acidic (and aggressive) water will have been dissolving calcium carbonate from the plaster surface (if a plaster pool), which will reduce the CO2 content and add alkalinity (and CH) to the water. So once Bicarb or Soda Ash is added to start the re-balancing program, the pH could be anywhere from 3.0 to 7.0 and the TA may be a negative number, or contain 5 to 10 ppm, or even higher.

    A pH meter would be helpful in determining how low the pH is, and along with a TA test, the correct dosages of chemicals could be determined for proper re-balancing.

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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    Using chem geek's spreadsheet, I calculate that starting with a PH of 4.5 and a TA of 0, and assuming no CO2 outgassing occurs, adding enough soda ash to raise the PH to 7.0 will raise the TA to 125. Doing the same thing with Borax instead of soda ash will only raise the TA to 73. Based on that I don't see any point in using any sodium bicarbonate, and Borax looks like a much better choice than soda ash. Note: this conclusion depends on aiming for TFP target levels. If you are a trichlor user, you will want the TA up around 125+, and soda ash becomes a much more appropriate choice.

    If lots of acid was added and now PH/TA needs to be raised it is normally because the desired amount of plaster has been dissolved. If you are in that situation you want to raise the PH rapidly so no further plaster will dissolve. Thus the raise TA and let PH come up on it's own approach will not be appropriate in this situation. Similar reasoning applies in most other plausible situations. Presumably, one way or another, you are only raising PH/TA because you don't want the PH to remain low and continue to dissolve plaster. If you did want more plaster dissolved, there would be no point in raising PH/TA.
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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    For acid startups, sometimes after I add the soda ash, the water:

    -Immediately becomes extremely cloudy and stays that way for days. The filter frequently needs cleaning.

    -Becomes extremely cloudy hours later.

    -Becomes cloudy initially, but becomes clear again in seconds or minutes and stays that way.

    What's happening?

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    Re: He wants to use Sodium Bicarbonate first, then Sodium Ca

    Calcium is precipitating out of the water as a very fine dust, which clouds up the water. Over time of it gets caught in the filter. If the PH, TA, and CH levels are low enough it will dissolve back into the water after a little while.

    This is generally the result of allowing one or more of PH, TA, and CH to get out of range high, though it can occasionally happen even when the water is properly balanced.
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