Alkalinity and pH - I am confused but happy

[chem geek]

Jim,

There are many over-simplifications or direct falsehoods the pool industry has with regards to pool water chemistry. One is that higher TA makes pH more stable. That is only half-true. Higher TA that comes from carbonates, which is the largest component in TA (another is a fraction of the CYA level and even less is the Borates level) is a source of rising pH itself. The reason is that pools are intentionally over-carbonated, just like a beverage, since they have more carbon dioxide dissolved in the water than would be naturally found from exposure to air. This over-saturation is done when you add baking soda or Alkalinity Up upon pool startup though even fill water has some TA.

The reason for having this over-saturation of carbonates is twofold. First, it does buffer pH FROM OTHER SOURCES of pH change, but it has the side effect of being a source of rising pH in its own right. Second, the carbonates portion of TA is necessary in plaster/gunite pools to prevent corrosion (technically dissolving) of such surfaces.

So a higher TA makes sense when using an acidic source of chlorine such as Trichlor (or even Dichlor which is technically acidic when one takes into account chlorine usage). It does not make as much sense to have high TA when using a pH neutral source of chlorine such as bleach or chlorinating liquid. I say "pH neutral" because though the initial addition of bleach or chlorinating liquid makes the pH rise, the usage of the chlorine (breakdown from sunlight, oxidation of organics, killing algae or bacteria, etc.) is acidic so the net result is very nearly pH neutral.

So it has been the recommendation on this forum for a while to lower the TA level if you are using bleach or chlorinating liquid and find a tendency for the pH to rise. The pH rises more quickly at higher TA and lower pH and with more aeration. This chart shows the relative rate of carbon dioxide outgassing at various pH and TA levels. This is a relative rate since the actual rate depends on the amount of aeration. These relative numbers would correspond to the amount of acid you needed to add each week. This chart shows the relative rise in pH and shows the fact that the lower TA is a smaller source of pH rise, but the lower TA also buffers the pH less so these two factors somewhat cancel each other out, but there is still a small net improvement at lower TA levels.

As for your question of side effects, a lower TA is more corrosive to plaster/gunite so those with such pools would need to either increase the Calcium Hardness level or the target pH or both to compensate. Since a higher pH also results in less of a tendency to rise further, most people lower the TA level and target a somewhat higher pH, such as 7.7 instead of 7.5.

As for how low to go, I wouldn't go below 50 ppm because you do want some pH buffering in the pool and also may need some carbonates if you have plaster/gunite. If you had much lower TA, then a relatively small amount of acid (say, from Trichlor or Muriatic Acid) could make the pH move more than expected and if the pH got low then that could corrode metal as well as plaster/gunite. Since you have a vinyl liner and don't need the calcium nor the carbonates to prevent corrosion, the only risk you are taking is if some acid or base is added to your pool. Even relatively soft water will have some TA in it and the CYA itself contributes to TA, but what you are seeing just confirms the theory that we've been saying for a while.

Pools with SWG systems (which isn't the case with your pool) have a particularly hard time with rising pH because the hydrogen gas bubbles generated in the SWG cell essentially pull the carbon dioxide out of the pool through aeration and that causes the pH to rise quickly. Lower TA helps such pools, but so does the use of Borates since they act as an additional pH buffer and are also an algaecide that let SWG users lower their SWG output which reduces the outgassing and therefore the tendency for the pH to rise.

So how low is your currently very low TA? With your CYA, it's got to be at least 8 or so and even soft water usually has a good amount of TA (technically, soft vs. hard water is about total hardness and is not directly related to TA though sometimes they track together in natural waters) so I suspect you are measuring at least 30 ppm or so. If you let me know your current TA (you already gave CYA and pH) and your pool water volume, then I can give you an idea of how much the pH would move with various amounts of acid or base.

Richard