What is the Right Combination of Numbers

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
Jack Sparrow
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What is the Right Combination of Numbers

Postby Jack Sparrow » Tue 14 Aug, 2007 22:02

I've been doing a lot of reading and there seems to be a ton conflicting info on "proper" pH and TA levels.

I hear that 80-120 are good numbers for TA, but that if you have a vinyl liner your target should be lower-- down around 80-100.

I see that 7.5 seems to be a popular target for pH. But why? If the pH level of the human eye is 7.2 wouldn't it make sense to keep it at 7.2 to eliminate irritation? Plus, there seems to be a higher rate of outgassing at lower pH levels helping to keep climbing TA levels under control.

I'd love to hear some input on this.

Thanks,
Joe


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Postby chem geek » Tue 14 Aug, 2007 22:34

The actual pH of most human tears is 7.5 (see this post for one example study) and that is why the recommendation for pool water is typically around 7.5. Low ph below 7.0 can become too corrosive to metals and vinyl, but higher pH is usually not a problem (adjusting for other parameters that I'll get to in a minute) and some pools have operated in the 8.2-8.6 range, but these days that's rare as it's hard to open the eyes in that water.

For TA, you want enough for decent pH buffering capacity if you use an acidic source of chlorine, so that would include Trichlor (and Dichlor which is actually acidic, not pH neutral, when you take into account the usage of chlorine, not just its addition). So a TA of around 120 is typical in a Trichlor pool and there is a "natural" TA you will get to if you just use pH Up / Arm & Hammer Super Washing Soda / Sodium Carbonate to make the pH rise from Trichlor's lowering of pH. This natural TA is different for every pool as it depends on the amount of carbon dioxide outgassing which depends on aeration and whether a pool cover is used, etc.

If you use a pH neutral source of chlorine such as bleach or chlorinating liquid -- which are actually pH neutral when one accounts for the usage of chlorine, not just its addition -- then a lower TA around 80 or even lower is usually appropriate. A higher TA will lead to a greater tendency for the pH to rise because a pool is intentionally over-carbonated (that's what most of TA is) to provide a pH buffer and to have carbonates in the water for calcium carbonate saturation.

In plaster/gunite pools or pools with exposed grout from tile, you need to saturate the water with calcium carbonate so that it does not dissolve such plaster/gunite/grout surfaces (since they contain calcium carbonate). So there is a balance between pH, Total Alkalinity (TA) and Calcium Hardness (CH) along with lesser effects from Total Dissolved Solids (TDS) and Temperarature. These combine into what is called the Calcite (or Langelier) Saturation Index. When the index is near 0, then the pool is properly balanced, but there's a lot of leeway before you get into trouble and shouldn't worry about it until the index gets to +/- 0.5 or higher. This online Pool Calculator will calculate the CSI, among other things.

In vinyl and fiberglass pools, calcium carbonate saturation is not needed, but there is some speculation that some calcium is needed to help fiberglass keep its gelcoat intact while some calcium even in vinyl pools will help prevent foaming in the water, but 100-150 ppm CH is usually sufficient for these purposes and is far lower than typical saturation levels (250-350 ppm).

You refer to "climbing TA levels", but TA usually doesn't climb unless you use fill water from a well high in TA. That's typically how CH climbs as well -- from fill water. Usually it isn't TA that climbs, but pH if you use a pH neutral source of chlorine (i.e. bleach or chlorinating liquid). Lower pH, higher TA, and more aeration all cause pH to rise (or if using an acidic source of chlorine, to not drop as fast or to be just balanced). This chart shows the relative rate of carbon dioxide outgassing as a function of pH and TA.

Richard

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