Does alkalinity impact chlorine?

Surprisingly, total alkalinity indirectly impacts chlorine, and vice versa.

It is known that in water without any cyanuric acid (CYA), the pH of the water controls the strength of chlorine (%HOCl). And in pools with CYA, the amount of CYA determines the strength (and speed) of chlorine.

Less known, however, is alkalinity's indirect relationship(s) with chlorine. We discuss them in passing in some other articles, but we'll expand on them here.

Total alkalinity can be increased by certain chlorine types (when acid is not used often), and high enough alkalinity in outdoor pools can lead to chlorine loss due to sunlight.


Chlorine can increase Total Alkalinity over time

If your swimming pool uses acid as its primary pH adjuster, this phenomenon likely goes unnoticed (if it happens at all). But for pools that hardly use any acid––or they use CO2 in lieu of acid––alkalinity tends to rise. But why?

We discuss it further in this article: Why does alkalinity rise in pools using CO2

Disclaimer: this chemistry is simplified intentionally. We plan on having a more detailed and thorough explanation in a blog article in the near future.

In short, the two most popular non-stabilized chlorine types are sodium hypochlorite (aka liquid chlorine or bleach), and calcium hypochlorite (aka cal hypo). Both of these leave behind hydroxide byproducts.  

  • Sodium hypochlorite leaves behind sodium hydroxide (NaOH)
  • Calcium hypochlorite leaves behind calcium hydroxide (Ca(OH)2)

All types of chlorine, when dissolved in water, create Hypochlorous Acid (HOCl), which is the strong, killing form of chlorine. The opposite anion of this is Hypochlorite ion (OCl-), which is over 100x slower and weaker than HOCl.  So let's focus just on HOCl for a moment.

When HOCl kills or oxidizes, it trades its Oxygen, so HOCl creates some HCl (hydrochloric acid, aka muriatic acid). This acid almost completely neutralizes the high pH of the chlorine product itself, but not quite. There is a small amount of excess hydroxide left behind. Those hydroxides accumulate and contribute to total alkalinity over time.

It is these excess hydroxides that are not neutralized by acid that increase the TA on a pool using CO2, or a pool rarely using acid.

In this sense, a pool hardly ever using acid can see relatively stable (or even increasing) TA levels simply by using liquid chlorine over time. We have seen this first-hand.

High TA leads to chlorine dissociating from CYA

This is also an indirect relationship, but worth mentioning. When TA is high enough, it causes the pH ceiling to be higher than desired. When the pH rises higher, more OCl- leaves CYA, and therefore it is exposed to sunlight and can be destroyed.

Higher TA > higher pH ceiling > higher pH after a few days > OCl- leaves CYA > OCl- is destroyed by sunlight > chlorine demand/consumption is increased

graphs showing pH impact on free chlorine percentage, with and without cyanuric acid, Orenda

Focus on the chart on the right side. At the bottom corner, you can see the yellow and green lines curve up after about 7.5 pH. This increase corresponds directly with the decrease in the purple line at the top. What you're seeing is chlorine breaking away from CYA, and is therefore exposed to sunlight, which can destroy it quickly.

In our experience, this is not noticeable until you are over 8.0 pH.  It's impossible for us to know exact thresholds for every pool, but we have a general understanding of this. At about 8.3 pH, enough chlorine is leaving CYA that chlorine demand noticeably increases during the week on outdoor pools. This is yet another reason why we encourage pool owners and operators to keep their pH ceiling below 8.3, so that their pH does not get this high in the first place.