Although it has been fourteen years since onBalance (Que Hales, Doug Latta, Kim Skinner) first debunked the Acid Column Myth in the Journal of the Swimming Pool and Spa Industry, we still regularly get asked in our trade show chemistry classes about its validity. As recently as this year’s shows we found the need to re-debunk the concept, while another instructor in another class continued to teach it as valid… without any supporting facts. Why has this incorrect and potentially pool-damaging practice so persistently managed to survive? We would like to recount what the myth is, why it is wrong, why following the practice could damage your pools (costing you money), and why the idea still seems to refuse to lay down and die.First, the myth. As we mentioned in the 1995 research journal article, at that time many pool industry publications promoted this (faulty) method of preferentially lowering pH and/or alkalinity:– That if acid is added by walking it around a pool and evenly distributing it throughout the water, the pH will be preferentially lowered, with only a minor decrease of alkalinity.– And that if acid is poured in a concentrated area, variously referred to as a column, slug, well or cloud, the alkalinity will be drastically reduced, but the pH will not drop as much as it otherwise would.Various attempts have been made to develop a chemical rationale for why this is supposed to work. As can be expected (since the whole idea is a myth...), none of the attempted explanations are valid science, nor are they what actually happens in a swimming pool.Most of the rationales center around the (mistaken) assumption that lowering pH decreases carbon dioxide (CO2) solubility – in other words, that lowering the pH enough will make more CO2 escape into the air. The fact of the matter is that pH is not a solubility factor for CO2. The actual things that govern CO2 solubility are temperature, pressure, and agitation (exposure to air).The easiest way for the non–chemist to remember these factors is to think of a can of carbonated beverage, such as soda pop or beer. The colder the can, the more fizz (CO2). The higher the pressure, the more fizz... opening the can and leaving it sit loses fizz. And shaking the can decreases fizz, by releasing CO2 gas from the liquid into the air. If anything, lowering the pH of “flat” or “stale” soda by adding dry ice (frozen CO2) increases the CO2 content – just ask anyone who has made home-made root beer!The plain and simple fact of the matter is that a given amount (or “dose”) of acid added to a fixed volume of water (the pool) will result in an identical reduction of both pH and alkalinity. Every time. No matter how it is added. That’s the rule, that’s science, and it can easily be demonstrated at poolside by anyone with a test kit. The only real chemical difference between the two addition methods is the time required for the acid to blend throughout the entire pool.
There have been several reports of residential pool services who used chlorine gas improperly and damaged the plaster surfaces of pools so I would dispute that carbon dioxide cannot cause damage. It can readily lower the pH to be too low -- the fact that there are carbonates in the water to buffer it can't completely compensate for this. If one introduces carbon dioxide to a pool surface without sufficient mixing, the pH can get very low -- there is no real limit to how low until carbon dioxide bubbles would no longer be able to dissolve in the water (i.e. the water is at equilibrium with carbon dioxide bubbles at 1 atmosphere or so). This occurs (in water with a TA of 100 ppm) at around a pH of 5. Of course, the carbon dioxide gets harder to dissolve as this pH is approached, but the point is that one can get to a pH well below 7 and cause damage with CO2.
Of course, the aforementioned problems can be avoided by proper application and good pool services do this by directing CO2 upward from the deep end at a rate that just has the CO2 fully dissolve before it reaches the surface. I don't know how this is typically injected into commercial/public pools but would be hope that it is done as part of the circulation system to ensure thorough mixing.
While I totally agree with the concept that no matter how you add Hydrochloric acid to the pool, eventually the chemical result will be the same. The acid will both reduce both the pH and the Total Alkalinity. However there is a difference between dosing by dilution in a bucket and running it around the edge of the pool - with the pump running; or dumping it down the skimmer or in one "column" in a stagnant pool. The difference is, that in the first case, the dispersal and the end result will be smoother and quicker without causing harm to the body of the pool or the pool equipment (particularly metals); while the second method will result in a greater delay of the arrival of the final balance and may indeed cause damage in particular instances: eg to a copper heat exchange of a gas heater; or even a stainless steel rail in the vicinity of the "dump".
The practice of adding Hydrochloric or Sulphuric Acid to reduce pH is highly undesirable since fluctuatng Akalinity will lead to fluctuating Saturation Indexes. Hence the pool surfaces may suffer and degrade - particularly painted pools or tiled surfaces which have been applied with limed mortar and grout (rather than epoxy mortars).
In the case of pH reduction with CO2 (creating Carbonic Acid), this will not happen because reducing the pH in this fashion does not reduce the Alkalinity and hence the need for buffering is reduced to the occaional need to replenish the Alkalinity after acid rain, or excessive backwashing and refilling with supply water. However the logistics of CO2 dosing in domestic pools are difficult to surmount - and that is a different discussion.
Comments
Of course, the aforementioned problems can be avoided by proper application and good pool services do this by directing CO2 upward from the deep end at a rate that just has the CO2 fully dissolve before it reaches the surface. I don't know how this is typically injected into commercial/public pools but would be hope that it is done as part of the circulation system to ensure thorough mixing.
The practice of adding Hydrochloric or Sulphuric Acid to reduce pH is highly undesirable since fluctuatng Akalinity will lead to fluctuating Saturation Indexes. Hence the pool surfaces may suffer and degrade - particularly painted pools or tiled surfaces which have been applied with limed mortar and grout (rather than epoxy mortars).
In the case of pH reduction with CO2 (creating Carbonic Acid), this will not happen because reducing the pH in this fashion does not reduce the Alkalinity and hence the need for buffering is reduced to the occaional need to replenish the Alkalinity after acid rain, or excessive backwashing and refilling with supply water. However the logistics of CO2 dosing in domestic pools are difficult to surmount - and that is a different discussion.