The Science of Spa Water Chemistry: Jets, Heat, and the Carbonate Equilibrium Dynamics In the world of spa water chemistry, a couple of questions sometimes bubble up: Why does scale seem to favor spas over pools? And why does the pH in a hot tub seem to want to rise compared to a pool? The answers lie in the coordination of heat, aeration, and the chemistry of calcium carbonate — the culprits behind scale.
Jets
Jets in a hot tub shake things up. Beyond their massaging action, jets significantly impact the carbon dioxide levels in the water. Picture this: you crack open a soda can, give it a good stir, and watch it fizz. The jets do the same thing in a hot tub, causing carbon dioxide to leave. It's the end of the spa's 'fizz.'
This accelerated outgassing messes with the equilibrium, the delicate balance between reactants and products in the carbonate reaction: CO¬2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ H+ + CO32 In simple terms, carbon dioxide reacts with water, creates carbonic acid, which then breaks down into hydrogen ions and bicarbonate, further splitting into carbonate. While this equilibrium is standard pool fare, the jets in a hot tub disrupt the balance. The result? The pH goes up.
Heat
Now, what about heat? Beyond the cozy factor, the temperature also has a role in chemistry. The reaction between calcium and carbonate — the recipe for scale — is influenced by the game-changer that is heat. Unlike common wisdom suggesting substances dissolve with warmth, calcium carbonate goes against the tide. It loves to form scale, and it does it more efficiently when things are hot.
Imagine adding salt to water — it dissolves better with heat. Undissolved calcium carbonate, on the other hand, thrives in higher temperatures.
pH Rise and Scale
Now, let's zoom in on the carbonate equilibrium, the source of scale. In both pools and hot tubs, this equilibrium involves a delicate balance of carbon dioxide, carbonic acid, and carbonate. But in a hot tub, the jets accelerate the outgassing of carbon dioxide. And carbon dioxide escapes, the equilibrium gets unbalanced.
To regain balance, carbonates combine to produce more carbon dioxide. But here's the kicker — hydrogen ions get used up in this chemical shuffle, leading to a rise in pH. Why? Because pH measures the hydrogen ion concentration, and when those ions go missing, the pH goes up.
Calcium Content
But don’t forget about calcium. Its existence in the water is a gamechanger. High calcium? Get ready for scale. But why does it seem to favor spas over pools?
High calcium is a ticket to scale formation, but in the hot tub, the temperature makes it inevitable. The thermodynamics of the calciumcarbonate reaction mean that higher temperatures make it even more likely for scale.
Calcium, carbonate, heat, and jets engage in a coordinated dance. This intricate choreography is the science behind the scenes, determining the fate of hot-tub water.
Real-Life Scenarios: Pools vs. Spas
Let's look at some real-world scenarios to highlight the differences in water chemistry between pools and spas.
Imagine a pool with moderate temperatures, no wild jets, and balanced water chemistry. In this scenario, equilibrium reactions involving carbonate putter along at a steady pace. Without jets causing a CO2 frenzy, concentrations remain stable, leading to a consistent pH.
Moderate pool temperatures don't incite rapid outgassing, keeping equilibrium disruptions in check. The potential for scale formation is relatively low compared to a spa.
Now, shift gears to a spa scene — jets on full throttle, water temperature cranked up, and maybe a slightly ignored chemical balance. Jets expel carbon dioxide at breakneck speed, disrupting equilibrium.
The elevated temperature, playing its supporting role, not only adds to the equilibrium turmoil but also fuels the endothermic calcium carbonate precipitation. The result? A hot tub chemistry where scale formation is probable.
In the end, the intricate science of spa water chemistry explains the quirks of hot tub behavior. It's not just about warm water and bubbling jets; it's about the molecular reactions happening beneath the surface. And understanding these nuances isn't just for the scientists — it's for anyone who wants to keep their spa inviting and scale-free.
Why use Bromine in a hot tub?
Why do people like bromine for spa disinfection?
Bromine, in the form of hypobromous acid, is a strong oxidant and disinfectant, but it is not as strong as chlorine.
Bromine is a less effective sanitizer that chlorine on a ppm basis. For example, 0.3 ppm free available chlorine provides 99.99 percent inactivation of both S. faecalis and P. aeruginosa after two minutes at 25°C and pH 7.5. By comparison, 5 ppm electro-generated bromine provides 92.8 and 85.5 percent inactivation under the same conditions.
And unlike chlorine, bromine cannot be effectively stabilized with cyanuric acid to protect against ultraviolet degradation from the sun.
Not only is bromine less effective and cannot be stabilized — it is also more expensive.
But some people don’t like the smell of chlorine, and they feel bromine is less harsh on the skin and eyes.
Perhaps one big selling point of bromine is that after it is used up in the various chemical reactions it performs, it can be regenerated with an oxidizing agent. When either chlorine or bromine react with the contaminants in a pool, chloride or bromide ions are formed. But bromide ions are easily oxidized back to hypobromous acid with the simple addition of chlorine or potassium monopersulfate (nonchlorine shock). For chloride ions to regenerate back to hypochlorous acid, a salt water chlorine generator is needed.
At the end of the day, the main reason to use bromine in spa applications has to do with convenience. Just as using trichlor tabs for swimming pools simplifies water maintenance, it is easy to use bromine tabs to maintain a sanitizer residual.
Trichlor tabs dissolve slowly in swimming pool water but rather quickly in hot water. And trichlor is also very acidic, which can be extremely corrosive to the spa’s surfaces and equipment, so trichlor is not recommended in spas.
With a relatively neutral pH, dichlor is often used in spas, but it tends to result in an overaccumulation of cyanuric acid, which can make it difficult to keep up with demand, and for that reason can increase the incidence of hot tub rash.
But bromine tabs dissolve slowly, and after bromine has performed its oxidation duties, can be very simply regenerated with an oxidizing agent.
However, according to scientific studies, from a health standpoint, combined bromines, otherwise known as disinfection byproducts (DBPs), are worse than combined chlorines.
A 2022 article appearing in the Journal of Environmental Sciences investigated the mutagenicity (the property of a chemical that induces genetic mutation) of both chlorine and bromine disinfection by-products. (Some types of genetic mutations change proteins in ways that cause healthy cells to become cancerous.)
It has been found that swimming in disinfected pools with high levels of disinfection byproducts is associated with increased risks for asthma. In addition, other deleterious health effects, such as an elevated risk for bladder cancer, have been linked to swimming with high levels of bromine disinfection byproducts.
Disinfection byproduct formation in pools and spas results from the reaction of disinfectants such as chlorine or bromine with organic matter, such as those found in source water, as well as human inputs, such as sweat, urine, pharmaceuticals, and personal-care products. Nearly all of the disinfection byproducts that have been studied thus far are genotoxic (damaging to DNA).
And in general, bromine disinfection byproducts are more cytotoxic, genotoxic, and mutagenic than chlorine disinfection byproducts.
The study found that while chlorinated waters generally had higher concentrations of disinfection byproducts than brominated waters, bromine’s disinfection byproducts are generally more harmful to people.
Furthermore, the concentrations of harmful disinfection byproducts increase when one compares spas to
pools: highertemperaturesleadtomore.
The study identified two classes of disinfection byproducts that were formed at significantly higher concentrations if the water was treated with bromine compared to chlorine. These were trihalomethanes and nitrogen-containing byproducts. Trihalomethanes — particularly tribromomethanes — are linked to bladder cancer.
So if the choice is between chlorine and bromine and a person is concerned about health effects, then chlorine would be the better choice.
How to Use Bromine
For those considering switching to bromine for spa use, there are three different ways that it can be administered to the water.
NaBr
One way is to add sodium bromide, which is a simple salt. Alone, this compound does nothing at all. When sanitation and oxidation are required, an oxidizer is added, which oxidizes the bromide ion to form hypobromous acid.
Some people use chlorine as the oxidizer. Others, preferring a non-chlorine spa, use potassium monopersulfate as the oxidizer.
The shock is added only after bathers have used the spa. After bathers exit the spa, they administer the required oxidizer dose and put the cover in place. Those endorsing this method enjoy the fact that bathers are not exposed to sanitizer while in the spa.
Initially, the sodium salt is added to the water after the spa has been filled. Common instructions say to add 1 ounce of sodium bromide per 200 gallons of water to establish a bromide reserve. The filtration system is used to mix the salt. Then add either chlorine or non-chlorine shock, following manufacturer’s instructions on dosage.
After this point, shock is added after each bather use.
BCDMH
Another way is to add bromine is with tablets or sticks in a floating feeder or some other feeder system. The tablets contain a mixed bromine sanitation compound, bromo-chlorodimethyl- hydantoin (BCDMH), which provides a continuous bromine residual for extended times.
Users should be aware that this tablet dissolves faster at elevated temperatures, which can be a problem for spas. Therefore, use a spa bromine feeder to avoid over-bromination. To use this product, begin by adding sodium bromide at 1 ounce per 200 gallons of water to create a bromide bank. Theoretically, this product does not require a separate oxidizer for use because it already contains chlorine for this function. But in practice, an oxidizer is used for periodic shocking. When added to water, bromine tablets dissolve and react to produce hypobromous acid, hypochlorous acid and dimethylhydantoin (DMH) residue. Hypobromous acid is the primary sanitizer in this system. As the hypobromous acid reacts and generates bromide ions, in the presence of chlorine or other oxidizers, hypobromous acid is regenerated as the predominant sanitizer. Thus, even with chlorine present in the bromine tablets, bromine is the predominant sanitizer in the water.
DBDMH
DBDMH is 1,3-Dibromo-5,5dimethylhydantoin and comes as a tablet or briquette. This product does not contain any chlorine, so it is preferable for those wishing to eliminate chlorine use entirely. It is still necessary to create a bromide bank with the addition of sodium bromide and add shock initially because you need to have enough bromine early on as the tabs dissolve slowly. Like all bromine systems, used bromine converts to bromide ions and can be re-oxidized with non-chlorine shock to become active again. One of the main advantages of DBDMH is that it is chlorine free, so it cannot form chlorinated disinfection by-products. Chloramines, which are associated with a poorly maintained pool smell, cannot form with DBDMH, and some people prefer that.
Testing Bromine
According to ANSI/APSP/ICC-11 Standard for Water Quality in Public Pools, the ideal range for bromine is 3 to 4 ppm for pools and 4 to 6 for spas. Test kits designed for measuring bromine are widely available. When testing for bromine with a chlorine test kit, the reading should be multiplied by 2.25 to obtain the bromine concentration. Additionally, Oxidation-Reduction Potential (ORP) can be used to test bromine treated water; however, the reading will be lower than that obtained for chlorine.
