Now – what really happens when acid is added in a concentrated fashion to a pool? Since acid is noticeably heavier than water, the acid sinks to the bottom, and flows to the lowest part of the pool. If that lowest part happens to include an operating main drain or an operating suction-side cleaner, the concentrated acid dose winds up in the circulation system. Although the movement of the water will eventually dilute and blend the acid into the rest of the pool, the initial contact of concentrated acid can potentially etch the plaster it touches, and attack the circulation system it flows through. Even adding several smaller “slugs” of concentrated acid will result in this puddling of acid on the bottom – try adding dye to your acid and watch it. Its fun – as long as you are not worried about etching the bottom of the pool!Of course, etching the plaster and eating the components of the circulation system is generally considered a bad thing… yet the proposed treatment process (“slugging” the acid) recommends this potentially damaging treatment technique, all for the sake of a theoretical, unfounded attempt at chemistry manipulation. Even if it worked (which it doesn’t), would it be worth the risk?What is the right way? The best way to manually add acid to a pool is to pre-dilute the acid and to add it by “walking” it around the perimeter of the pool, pouring it evenly, close to the surface, and slowly enough to minimize splashing. When added in this fashion, the acid blends throughout the pool water faster, and the pool is protected from low pH. In fact, in extensive testing of the two methods, pH levels at the bottom of the pool with the “slugged” method were routinely below 3.0. In pools treated with the acid dilution and distribution method, the lowest pH levels measured next to the plaster were in the 7.1-7.2 range. (Repeat – once the acid addition was blended throughout the water, the end-result pH and total alkalinity reduction was identical, no matter how the acid was added.)After having conducted experiments in 1994 and documenting that the acid column program was an incorrect concept, all but one of the major sources we cited have revised their publications and removed the myth from their recommendations. The remaining hold-out, unfortunately, is widely read and refuses to change – not based on science, but because they claim that service techs in the field think the acid column works!It is time to get this nonsense laid to rest, and stick to what is scientifically sound and that we all know works. There are proven ways to preferentially affect pH and alkalinity, and there are proven treatment strategies for pools that don’t want to follow the pH and alkalinity norms. Let’s stick to them, and quit risking damage to pools in our care.Here is a photograph sequence of an acid column in action. The pictures were taken with an underwater camera, and the acid was colored with phenol red powder... the same chemical used to test pH. Unless the circulation system is running, or the unless the resultant puddle of acid on the bottom of the pool is brushed or moved around some way, the puddle will stay visible and intact for hours – with a pH of about 2.5 right on the plaster. Ouch!
There is nothing that complicated about this. pH and TA move together when one adds a pure acid or pure base to water. So having someone add an acid to lower the TA and then add a base to raise the pH is something no one should do, especially since soda ash increases TA more than a pure base so it just ends up making the problem even worse (i.e. the TA even higher than before).
The way to reduce TA is ultimately through removal of carbonates from the water and the way to do that efficiently is to follow the procedure outlined in this post: http://x.havuz.org/viewtopic.php?f=16&t=390 . This procedure was first described (as far as I can tell) from Ben Powell at The Pool Forum, but it's based on known chemistry. Basically, acid addition decreases both pH and TA while aeration of the water causes the pH to rise with no change in TA (due to carbon dioxide outgassing -- technical details about this are in the post at this link: http://www.troublefreepool.com/post116131.html#p116131 ). The rate of pH rise is accelerated with aeration and at lower pH which is why the procedure maximizes this effect (see the table at the following link to see the excess of carbon dioxide in pool water at various pH and TA: http://richardfalk.home.comcast.net/~richardfalk/pool/CO2.htm ).
I agree that if the TA is high AND the pH is fairly stable and not rising, then a high TA isn't a problem unless the saturation index is high. Scaling isn't just an issue on pool surfaces, but in heaters where scaling is more likely due to the higher temperatures and in saltwater chlorine generator (SWG) cells at the hydrogen gas generation plate since the pH is much higher (though the use of 50 ppm Borates as an additional pH buffer in the pool helps mitigate that).
It needs to be understood that pools are intentionally made or kept as over-carbonated. The equilibrium amount of TA (with no CYA and 77F temp) in water with a pH of 7.5 is only 8 ppm, but this rises to an equilibrium TA of 25 ppm at a pH of 8.0 and a TA of 85 ppm at a pH of 8.5. Natural waters often exceed these equilibrium values because they dissolve calcium carbonate in rock and are not fully exposed to air and the outgassing becomes very slow as one gets closer to equilibrium. The bottom line is that a pool that doesn't have any acid to it (including Trichor or Dichlor chlorine sources) will tend to have its pH rise. This occurs more quickly at higher TA levels. This is useful if using Trichlor, for example, but is not good if using hypochlorite sources of chlorine (which are net pH neutral when accounting for chlorine usage/consumption -- technical details about that are here: http://www.troublefreepool.com/post4367.html#p4367 ).
When it comes to adjusting the TA, pool owners can be played like a yo yo. They add acid and then they add soda ash. The TA ends up too high, so they add more acid and the cycle repeats. Nature doesn't always allow for a pH and TA in range. Who says a TA of 180 is a bad thing, if the pH is 7.2-7.6 and there is no sign of scaling? Vinyl pools don't need 250 PPM of hardness, but the computer sells it anyway. For years I have been telling people, through my website, that there is no compelling reason to have to lower the TA, if the pH is in range, the water is clear and there is no signs of scaling. I have had my doubts about adding acid in one spot and have said so. However, your stating, of the subject, was much more comprehensive and makes complete sense. Making pool ownership unnecessarily expensive does not help the situation, in this time of depressed pool sales. Neighbors do compare notes and dealer's wonder why pool owner's buy chemicals in big box stores. How many people have been deterred from buying a pool, because of fears of the chemical costs?
Comments
The way to reduce TA is ultimately through removal of carbonates from the water and the way to do that efficiently is to follow the procedure outlined in this post: http://x.havuz.org/viewtopic.php?f=16&t=390 . This procedure was first described (as far as I can tell) from Ben Powell at The Pool Forum, but it's based on known chemistry. Basically, acid addition decreases both pH and TA while aeration of the water causes the pH to rise with no change in TA (due to carbon dioxide outgassing -- technical details about this are in the post at this link: http://www.troublefreepool.com/post116131.html#p116131 ). The rate of pH rise is accelerated with aeration and at lower pH which is why the procedure maximizes this effect (see the table at the following link to see the excess of carbon dioxide in pool water at various pH and TA: http://richardfalk.home.comcast.net/~richardfalk/pool/CO2.htm ).
I agree that if the TA is high AND the pH is fairly stable and not rising, then a high TA isn't a problem unless the saturation index is high. Scaling isn't just an issue on pool surfaces, but in heaters where scaling is more likely due to the higher temperatures and in saltwater chlorine generator (SWG) cells at the hydrogen gas generation plate since the pH is much higher (though the use of 50 ppm Borates as an additional pH buffer in the pool helps mitigate that).
It needs to be understood that pools are intentionally made or kept as over-carbonated. The equilibrium amount of TA (with no CYA and 77F temp) in water with a pH of 7.5 is only 8 ppm, but this rises to an equilibrium TA of 25 ppm at a pH of 8.0 and a TA of 85 ppm at a pH of 8.5. Natural waters often exceed these equilibrium values because they dissolve calcium carbonate in rock and are not fully exposed to air and the outgassing becomes very slow as one gets closer to equilibrium. The bottom line is that a pool that doesn't have any acid to it (including Trichor or Dichlor chlorine sources) will tend to have its pH rise. This occurs more quickly at higher TA levels. This is useful if using Trichlor, for example, but is not good if using hypochlorite sources of chlorine (which are net pH neutral when accounting for chlorine usage/consumption -- technical details about that are here: http://www.troublefreepool.com/post4367.html#p4367 ).