Today we are discussing two of the six LSI factors: water temperature and Cyanuric Acid (CYA). There is a widespread belief in the industry that hotter weather means pools need more CYA to protect chlorine from sunlight degradation. Is it true? Let's find out.
Covered in this article:
- Hotter water means faster chemical reactions
- Using chlorine vs. Losing chlorine
- Does water temperature directly impact cyanuric acid?
- The pH impacts how well chlorine stays bound to CYA
- Conclusion
Hotter water means faster chemical reactions
Let's start with some basic chemistry. Most chemical reactions occur faster at higher temperatures. This is true in swimming pool water chemistry too.
With that in mind, chlorine gets used up faster in warmer water because it's working faster. It's that simple.
This article aims to show the difference between using chlorine and losing chlorine.
Using chlorine vs. Losing chlorine
When chlorine does its job, it gets used up. For example, let's look at how chlorine kills algae.
Hypochlorous acid (HOCl), the active form of chlorine in water, kills germs and algae quickly and easily. It does so by breaking through the cell wall or membrane and destroying the inside. The technical explanation of this is complex, so we'll simplify it. HOCl steals an electron (e-) from a contaminant (oxidation) and exchanges its Oxygen for it. The negatively charged electron (e-) then reduces HOCl down to HCl (hydrochloric acid). It can no longer kill or oxidize.1,2
Warmer water not only increases chlorine speed, it also increases the likelihood of contaminants for chlorine to deal with. More bathers, more sunscreen and cosmetics, more pollen, more bugs, more germs, and more algae. These contaminants are much less present (if at all) when the water is cold.
Losing chlorine, however, is about UV degradation (also called photodecomposition or photolysis).3,4,5 In other words, how fast does chlorine get burned and destroyed out by the sun?
Without CYA in the water, chlorine degrades quickly. Depending on factors like water depth and temperature, unprotected chlorine's half-life is only about 20-45 minutes (depending on the source cited), meaning about 75% of chlorine will be gone in under two hours. After three hours, all the chlorine may be gone.
So the water either uses chlorine killing and oxidizing contaminants, or it loses chlorine as it gets broken down by UV light (sunlight). The reaction is:
2HOCl + UV → 2HCl + O2
2 Hypochlorous acid + UV → Hydrochloric acid + Oxygen
Either way, it's more difficult to hold free chlorine levels in warmer water.
Does water temperature directly impact cyanuric acid?
The short answer is no, water temperature does not impact cyanuric acid or its ability to protect chlorine from UV decomposition.
When we began to research this question, we struggled to find good information online. So we asked water chemistry guru Richard Falk. And according to Richard, CYA's sunlight protection is independent of temperature. You do not need more CYA when the temperature goes up.
That being said, summer days are longer, with more hours of direct sunlight. The time and amount of sunlight exposure might justify higher levels of CYA, but not the water or air temperature. Hotter days do not mean CYA protects chlorine any less.
This myth is believed by so many because chlorine loss coincides with hot days. But coincidence is not the same as causality. Pool pros will notice that higher levels of CYA can allow them to hold chlorine better over seven days (for a once-a-week pool route), but that is not so much because of sunlight protection. It's because CYA slows chlorine down.4,6 Slower chlorine means it might still be there after a week. The question is how effective that chlorine is, and if it's keeping up with the rate of algae growth.
Related: Minimal CYA | Pillar 4
The pH impacts how well chlorine stays bound to CYA
Take a look at the chart below, showing the comparison of chlorine in a non-stabilized pool vs. a pool with 30 ppm of cyanuric acid:
On the left chart, the pH directly determines the "strength" of chlorine, as defined by the percentage of the active chlorine, Hypochlorous acid (HOCl) relative to its slower and weaker conjugate base, Hypochlorite Ion (OCl-). These two are in equilibrium, and the pH determines the Hydrogen concentration. As the pH rises, Hydrogen (H+) dissociates from HOCl, leaving OCl-. The equilibrium is as follows:
HOCl ⇄ H+ + OCl-
Hypochlorous Acid ⇄ Hydrogen ion + Hypochlorite ion
But again, that's in a pool with zero cyanuric acid. If there is any CYA in the water, that chart does not apply. The chemistry is fundamentally different and looks more like the chart on the right. The chart on the right is with 30 ppm CYA. Where is the red line now?
Related: Chlorine, pH, and Cyanuric Acid Relationships
As you can see, the pH has very little impact at all on the percentage of HOCl, because most of the chlorine in the water is bound to CYA and protected from sunlight (chloroisocyanurates). Can you see a difference in chlorine strength between 7.0 and 8.5 pH? It's not much, and we consider it negligible.
What does matter, however, is the amount of CYA in the water. Specifically, the ratio of free chlorine to cyanuric acid (FC:CYA). This ratio, as outlined by the CDC's Council for the Model Aquatic Health Code, matters more than pH for determining the %HOCl and contact times (CT) for disinfecting water.7
And while the pH has a negligible impact on chlorine performance in a stabilized pool, it still matters because it determines how well chlorine can stay bound to CYA.8 See the bottom right corner of the chart above.
As you can see, the %HOCl stays below 3% and gradually decreases as the pH rises. But the %OCl- climbs as the pH increases. Its increase is directly proportional to the decrease of the purple line, chloroisocyanurate. This means as the pH rises, more OCl- breaks away from CYA and becomes exposed to sunlight. Here's what Richard Falk said:
"Don't forget that the breakdown of chlorine in sunlight is VERY pH dependent because hypochlorite ion breaks down much faster than hypochlorous acid, and with CYA in the water, higher pH doesn't have HOCl drop very much, but it has hypochlorite ion rise a lot (since CL-CYA and OCl- are in equilibrium)." - Richard A. Falk (colors added by Orenda to match the graphs above)
This ties into the broader conversation about containing pH vs. trying to control it, thanks to CO2, Henry's Law, and the pH ceiling. In short, higher levels of TA can lead to a higher pH ceiling, and thus more chlorine loss due to chlorine leaving CYA and getting destroyed by the sun.
Conclusion
We wrote this article to debunk the notion that higher temperatures mean you need more CYA to protect chlorine. While chlorine does get reduced faster in warmer water, it has nothing to do with CYA's sunlight protection. Rather, this increased rate of chlorine consumption is due to an increased oxidant demand. Sunlight protection is independent of temperature. So no, you do not need higher CYA levels in the summer. The sun is the sun, regardless of temperature.
We should not underestimate the chlorine reaction rate either. Warmer water means chlorine works faster. There is a difference between using chlorine and losing chlorine. More CYA slows chlorine down, allowing it to potentially stay in your water longer. There is a benefit to that, but at some point there is too much CYA, and the killing speed of chlorine can be slower than the reproduction rate of contaminants. That threshold is known as overstabilization and should be avoided at all costs.
1 We expand on this concept in several other articles. Redox reactions are discussed at length in our article about ORP. We have several podcast episodes about chlorination too.
2 To stay on topic in this article, we will not go down the rabbit hole about HOCl becoming HCl, and instead elaborate here in the footnotes. The HCl (hydrochloric acid) created by chlorine as it kills or oxidizes also brings down the pH of the water. It is almost exactly proportional to the hydroxides introduced by hypochlorite chlorine types (liquid sodium hypochlorite, calcium hypochlorite, and lithium hypochlorite). These chlorine types only temporarily raise pH, because the HCl will bring it back down.
3 Nowell, Lisa N., Hoigné, Jürg. (1992). Photolysis of aqueous chlorine at sunlight and ultraviolet wavelengths––I. Degredation rates. Water Research. Vol. 26 (5), pp. 5993-598.
4 Wojtowicz, John. (2004). Effect of Cyanuric Acid on Swimming Pool Maintenance. Journal of the Swimming Pool and Spa Industry. Vol. 5 (1), pp. 15-19.
5 Williams, K.M. (2000). Cyanurics ~ Benefactor or bomb?
6 There are too many credible sources to cite in this article, but here are a few that demonstrate the impact that CYA has on chlorine disinfection speed:
Shields, J.M., Arrowood, M.J., Hill, V.R., & Beach, M.J. (2009). The effect of cyanuric acid on the disinfection rate of Cryptosporidium parvum in 20-ppm free chlorine. Journal of water and health, 7 1, 109-14.
US Environmental Protection Agency. (1992). Chlorinated Isocyanurates. R.E.D. Facts, US EPA Office of Prevention, Pesticides and Toxic Substances. EPA-738-F-92-010.
Pulsar (Retrieved 2024). The Effect of Cyanuric Acid on Disinfection. Pulsar Technical Bulletin.
7 Falk, R.A.; Blatchley, E.R., III; Kuechler, T.C.; Meyer, E.M.; Pickens, S.R.; Suppes, L.M. (2019). Assessing the Impact of Cyanuric Acid on Bather’s Risk of Gastrointestinal Illness at Swimming Pools. Water. (11), 1314.
8 Pickens, Stanley R. (retrieved 2024). Relative Effects of pH and Cyanurate on Disinfection. White paper.