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Thread: TA--What is it really?

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    TA--What is it really?

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    Ouch... my brain hurts now.
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    Quote Originally Posted by EskimoPie
    Ouch... my brain hurts now.
    I DID warn you not to read it at the very top if chemistry made you ill!

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    Re: TA--What is it really?

    Ok Obi-Wan, you are hired :P

    If I own the biggest pool in the world, you will be the "Man" in charge....

    Dang, I still hate chemistry...let me absorb this slowly. Honestly if 6-log is a great kill rate for bacteria, I would be MINUS 6-log as a chemistry dummy...

    Thanks WB.
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    Re: TA--What is it really?

    Thanks for the great writeup on TA and the carbonate buffering system. In my trying to simplify the equations for eventual use in The Pool Calculator, I found that essentially the carbonate ion can be ignored and that dissolved carbon dioxide is the dominant species over carbonic acid. The relative amounts of each species are as follows at varying pH and normal pool water TDS (the first set of numbers are relative ratio amounts while the second set are percentages of the total):

    [CO2(aq)] : [H2CO3] : [HCO3-] : [CO32-]
    at pH 7.0 .... 650 : 1 : 3379 : 2 .... 16.1% 0.025% 83.8% 0.05%
    at pH 7.5 .... 650 : 1 : 10677 : 23 .... 5.7% 0.009% 94.1% 0.20%
    at pH 8.0 .... 650 : 1 : 33772 : 226 .... 1.9% 0.003% 97.3% 0.65%

    Only the last two species above count towards Total Alkalinity (TA) and the last species (carbonate ion) counts twice as much as it can accept two hydrogen ions, but even so you can see that it is much smaller in concentration than the bicarbonate ion. So for practical purposes the primary equation that does the pH buffering near pool water pH is the following:

    CO2(aq) + H2O <---> HCO3- + H+
    Aqueous Carbon Dioxide + Water <---> Bicarbonate Ion + Hydrogen Ion

    where at a pH of 7.5, 94.1% is Bicarbonate Ion and 5.7% is Aqueous Carbon Dioxide. A similar analysis for the Cyanuric Acid buffering system shows that the following equation is the primary one:

    H3CY <---> H2CY- + H+
    Cyanuric Acid <---> Cyanurate Ion + Hydrogen Ion

    where at a pH of 7.5, 82% is Cyanurate Ion and 18% is Cyanuric Acid. Cyanurate Ion also counts toward Total Alkalinity which is why it is adjusted in the calculations for the saturation index because what is needed for that formula is the carbonate alkalinity (carbonate hardness).

    If there are Borates in the water, then the following equation is the primary one:

    B(OH)3 + H2O <---> B(OH)4- + H+
    Boric Acid + Water <---> Borate Ion + Hydrogen Ion

    where at a pH of 7.5, 97.6% is Boric Acid while only 2.4% is Borate Ion which is why this has a negligible effect on TA. At 50 ppm Borates, the TA is only increased by around 5 ppm at a pH of 7.5.

    One misconception is that TA is a direct measure of pH buffering. This is not true. Total Alkalinity (TA) is only a measure of pH buffering CAPACITY and even then, only in one direction, specifically against a lowering of pH. A different measure known as Total Acidity measures the pH buffering capacity against a rise in pH. The borates do not count much towards Total Alkalinity, but they count a lot towards Total Acidity meaning that they have a greater capacity against a rise in pH than a drop in pH. That doesn't mean they don't resist a drop in pH, but rather they don't have as much capacity and will "run out" sooner after which the carbonate buffer system is effectively the only one left.

    All of the above is the theory, but what is really important is what happens in practice. The following shows the effect on pH when adding 2 cups of 31.45% Muriatic Acid in 10,000 gallons:

    TA 50, CYA 0, Borates 0: pH 7.5 --> 6.96
    TA 150, CYA 0, Borates 0: pH 7.5 --> 7.26
    TA 100, CYA 0, Borates 0: pH 7.5 --> 7.17
    TA 100, CYA 80, Borates 0: pH 7.5 --> 7.26
    TA 100, CYA 0, Borates 50: pH 7.5 --> 7.26
    TA 100, CYA 80, Borates 80: pH 7.5 --> 7.31

    This shows that having 50 ppm more TA or 80 ppm CYA or 50 ppm Borates are roughly equivalent in terms of resisting a drop in pH. Now let's look at what happens when we add 7 ounces weight of lye (sodium hydroxide) which is a pure base roughly equivalent to 17.6 ounces weight of soda ash / pH Up (sodium carbonate) or 35.1 ounces weight of Borax though these latter two have other side effects on TA.

    TA 50, CYA 0, Borates 0: pH 7.5 --> 8.58
    TA 150, CYA 0, Borates 0: pH 7.5 --> 7.88
    TA 100, CYA 0, Borates 0: pH 7.5 --> 8.10
    TA 100, CYA 80, Borates 0: pH 7.5 --> 7.90
    TA 100, CYA 0, Borates 50: pH 7.5 --> 7.74
    TA 100, CYA 80, Borates 80: pH 7.5 --> 7.70

    From the above, you can see that Borates are a significant pH buffer resisting a rise in pH much more effectively than a lowering in pH and much more powerfully than the carbonate or cyanurate buffer systems. Here, having 50 ppm more TA is slightly better than 80 ppm CYA but 50 ppm Borates is even better at resisting a pH rise.

    Another way to look at these buffer systems is their change in resistance to pH at various pH. The following shows the amount of acid it takes to go from one pH to another using only the carbonate buffer system, so TA 100, CYA 0, Borates 0:

    pH 8.0 --> 7.9 ..... 2.3 fluid ounces
    pH 7.9 --> 7.8 ..... 2.4 fluid ounces
    pH 7.8 --> 7.7 ..... 2.6 fluid ounces
    pH 7.7 --> 7.6 ..... 2.9 fluid ounces
    pH 7.6 --> 7.5 ..... 3.4 fluid ounces
    pH 7.5 --> 7.4 ..... 4.0 fluid ounces
    pH 7.4 --> 7.3 ..... 4.8 fluid ounces
    pH 7.3 --> 7.2 ..... 5.8 fluid ounces
    pH 7.2 --> 7.1 ..... 7.0 fluid ounces
    pH 7.1 --> 7.0 ..... 8.4 fluid ounces

    You can see very clearly how the carbonate buffer system gets stronger at resisting changes in pH as the pH gets lower. Adding 80 ppm CYA to the system so we have TA 100, CYA 80, Borates 0 gives us the following where I just show a few of the points:

    pH 8.0 --> 7.9 ..... 2.8 fluid ounces
    pH 7.5 --> 7.4 ..... 5.9 fluid ounces
    pH 7.1 --> 7.0 ..... 11.1 fluid ounces

    The cyanurate buffer system adds some buffering in an uneven way (when combined with the carbonate buffer system) as a function of pH. If we had borates instead of CYA so we have TA 100, CYA 0, Borates 50, then this gives us the following:

    pH 8.0 --> 7.9 ..... 9.8 fluid ounces
    pH 7.5 --> 7.4 ..... 6.5 fluid ounces
    pH 7.1 --> 7.0 ..... 9.3 fluid ounces

    Here you can see that the borate buffer system is stronger at higher pH. Having both CYA and Borates so TA 100, CYA 80, Borates 50 gives us:

    pH 8.0 --> 7.9 ..... 10.4 fluid ounces
    pH 7.5 --> 7.4 ..... 8.3 fluid ounces
    pH 7.1 --> 7.0 ..... 11.9 fluid ounces

    The above gives you some sense for why the calculations predicting changes in pH at various TA, CYA and Borate levels are not trivial nor easily estimated. Nevertheless, I'm working on using the simplified dominant equations to come up with something that could be used in The Pool Calculator (someday).

    Richard
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    Re: TA--What is it really?

    Thanks Richard

    I admire people like you, you seems to seek perfection in what you fancy ....always analizing chemistry.
    I do the same for what I like to do, but chemistry is one that I practically have zero basic...

    If the pool calculator is up and running with new formula, would you mind posting it please.

    I been playing with the calculator. Its the CSI that I wanted to know ...fun playing with it.

    Thanks again...
    35,000 GL pool. In Ground. Concrete with all white ceramic tiles. Outdoor but shaded.
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    Re: TA--What is it really?

    The "accurate" pH & TA formulas are not in The Pool Calculator yet. I'm still working on simplifying the equations so as not to create a Java programming nightmare for Jason. I'm getting there... In the meantime, anyone who is a masochist can use my PoolEquations spreadsheet, though it is not for novice users. When The Pool Calculator gets updated in the future, I'm sure both Jason and I will let everyone know. He's done an outstanding job and it just keeps getting better.
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    Re: TA--What is it really?

    Richard, this goes along with what I have seen in the many salt pools that have borates added to them in terms of pH stability. It seems to 'ride at about 7.7 for a very long period of time.

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    Re: TA--What is it really?

    Richard..

    So what you are saying and correct me if I am incorrect.

    When we take a TA reading we are in effect measuring the waters abilility to resist a PH change to the downside and is not really an ideal reading to measure the waters ability to resist a PH change to the upside.

    On the flip side measuring borate levels is a measurement of the waters ability to resist a PH to the upside and again not a ideal reading to measure the waters ability to resist a PH to the downside.

    Now this would tie in with my experience when dealing with quite a large number of pools as I have found that with ideal TA readings I struggle to get the PH to drop, when I say struggle I mean in comparison with which the waters seem to rise quite easily to high PH especially after a heavy rain (Rain PH 8.2 - 8.4). At the moment I have no borates in any of my pools but this is due to change early next year because I am constantly fighting a high PH due to 16% bleach use in my pools and nothing else expect acid to counteract the PH rise and Bicarb to counter lower TA due to Acid use.. I'll be really glad when I can get some borates into the pools.

    All interesting stuff as usual.. Thanks Richard.

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    Re: TA--What is it really?

    Freelancer, you are probably keeping your TA level too high. High TA can cause the PH to rise, especially where there is a source of aeration, such as rain or a SWG. Meanwhile, it is very unlikely that the bleach/liquid chlorine usage is a primary cause of rising PH. Some bleach will raise the PH, most here in the US won't, but that effect is normally quite small compared to PH increases from high TA levels.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: TA--What is it really?

    Freelancer,

    Well, the TA and Borates levels are mostly measuring a total capacity, not so much the absolute resistance level to a move in pH, though this is loosely related. Nevertheless, as Jason points out, your experience of having the pH rise is probably due to the TA level being too high and not to the fact that it doesn't buffer against a pH rise as well as Borates when both are at typically recommended levels.

    The thing to understand, that I believe is the most counterintuitive concept in pool water chemistry, is that though a higher TA level provides greater pH buffering, it also leads to a faster rise in pH unless one is using acidic sources of chlorine or otherwise adding acid to the pool. The reason is that TA is mostly a measure of the carbonates in the pool and a higher TA from carbonates has TWO effects: 1) it provides pH buffering to make pH more stable and 2) it is a SOURCE of pH rising more rapidly due to increased carbon dioxide outgassing. At higher TA, the latter effect outweighs the former (i.e. high TA makes the pH rise). The Borates also buffer pH, but they do not outgas carbon dioxide so do not cause the pH to rise..

    Basically, pools are intentionally over-carbonated, just like a carbonated beverage (though obviously not as carbonated). If you stir the drink, or aerate the water in the pool, the carbon dioxide is released faster. The pH rises when this happens while the TA stays the same (for technical reasons I won't get into here). So a way to reduce the rate of pH rise in pools is to lower the TA, as strange as that may seem, and to provide additional pH buffering via something other than carbonates such as by adding borates. This chart shows how far out-of-equilibrium a pool is in terms of the amount of carbon dioxide in the water vs. the amount that would normally be there given the amount of carbon dioxide in the air. A higher TA, lower pH and greater aeration all make things more out-of-equilibrium and that causes the carbon dioxide to go faster and have the pH rise more quickly.

    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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    Re: TA--What is it really?

    Thanks Jason and Richard for your replies.

    Actually I run my TA levels on the low side at around 70 ppm. Due to a large amount of my pools being communal I do have to use rather a lot of liquid Chlorine which has a very high PH 12.0+ plus the rain we get here in Spain can have a PH as high as 8.4. Quite often after a heavy downpour my PH's rise by as much as 0.6 in just one day. One of my pools with a TA of 60 rose from 7.8 to 8.4 in 24 hours after a downpour.

    I will however be very careful to keep my TA's on the low side and hopefully when I take receipt of my first half ton of Boric Acid the high PH problems will be a thing of the past. Delivery is due early next year, I've had to get it shipped in from the UK as the prices here are truly ridiculous. It was over 10 times cheaper including shipping and when you consider that I will be needing about 5 tons that is quite a saving.

    This forum is such a breath of fresh air where you can converse with people who actually know what they are talking about. It never ceases to amaze me how many pool sites there are on the Web and they just spout the same old drivel as fed to them by the manufacturers of the products that they sell. Mind you I should not complain because in my line of business it's very good for me.

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