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Science & Technology

June 18, 2007
Volume 85, Number 25
p. 70

What's That Stuff?

Pool Chemicals

A cornucopia of additives keeps pool water sparkling and clean

Ann Thayer

MAYBE YOUR MOTHER told you, "Don't drink the water," but most swimmers probably give little thought to how much stuff is added to swimming pools to keep the water "balanced," as the pool lingo goes. The water's chemical balance dictates a pool's physical condition, as well as swimmer comfort and safety.


If the water is too harsh, it can corrode equipment, dissolve concrete, or deposit mineral-based scale on pool tiles. Dissolved metals can cause staining; it's too much copper, not chlorine, that causes green hair. But without chemical sanitizers, green, yellow, and black algae can completely take over, making pool walls slimy and slippery.

A dizzying array of specialized pool products exists, but among pool chemicals, the protagonist is definitely chlorine, the traditional water sanitizer. A residual level of just a few parts per million of chlorine must be maintained to keep microorganisms at bay.

For many, chlorine comes to mind because of many pools' "chlorine smell." This odor is actually from chloramines, produced by hypochlorous acid reacting with nitrogen-containing contaminants introduced by swimmers. A pool with enough added chlorine to oxidize the chloramines would have no smell, explains a technical expert at Arch Chemicals. The chlorine smell is a tip-off to pool owners that they need to "shock," or increase the chlorine concentration.

But even chlorine isn't just chlorine. For a few decades, cyanuric acid (s-triazinetrione) has been added to chlorine formulations for outdoor pools as a stabilizer against ultraviolet light. Although chlorine needs constant replenishing, the stabilizer hangs around forever and can build up to unwanted levels.

For convenient use, trichloro-s-triazinetrione generally comes as slow-dissolving tablets or sticks. Whereas "trichlor" is very acidic and alters other aspects of pool chemistry, more expensive sodium dichloro-s-triazinetrione has a neutral pH, dissolves faster, and tends to be used in vinyl or fiberglass pools and spas.

Chlorine also comes in powder form, usually as the stabilized "dichlor" or unstablized calcium hypochlorite or the nearly twice as costly lithium hypochlorite. Pool chemical manufacturers continually develop new chlorine formulations for safer storage, better dissolution, lower use rates, or increased convenience. Other agents such as algicides or clarifiers may be incorporated for one-step application. Nonchlorine sanitizers also exist and are based on bromine or polyhexamethylene biguanide.

I learned pool-chemistry basics the hard way as a new pool owner. Confident in my own chemistry education and too cheap to pay a pool service, I set out with my test kit to do initial pool-chemistry assessments. The first target is a pH of 7.4-7.6. To lower the pH, you just add muriatic acid, an archaic name for 31% hydrochloric acid, or sodium bisulfate. To raise the pH, you add sodium carbonate.

Next, you measure the "total alkalinity," a weird pool-industry term to describe the water's ability to "buffer against pH changes." Acid reduces the "total alkalinity," whereas sodium bicarbonate increases it. Periodically, one measures the "calcium hardness," or calcium level. Intrepid pool chemists can add calcium chloride to increase it; or they can decrease it by dilution.

Besides the chemicals to do basic sanitation, pH tuning, and calcium adjustment, there are also biocides, oxidizers, scale inhibitors, clarifiers, flocculants, metal removers, surface cleaners, and defoamers, to name a few. For the spa or hot tub market, there are also softeners and fragrances.

When it comes to buying pool chemicals, the real shock comes at the pool store. Let's face it, many are basic commodity chemicals, but they aren't necessarily sold that way. Cheaper bulk quantities are available, but there are also pricey single-use packages. And something as simple as sodium bicarbonate can be called anything from the incomprehensible "Balance Pak 100," to a more understandable "Alkalinity Up," or simply "Total Alkalinity Increaser." One company manager notes, "Not everybody is a chemist, so we try to put everything in an easy-to-use, foolproof format."

Besides residential pools, these products go into municipal pools, water parks, and commercial pools at hotels, resorts, and sports facilities. The swimming pool chemicals market generates roughly $2 billion per year in wholesale sales and grows a few percent annually. Spa chemicals is a faster growing segment, at 7-10% per year.

Pool-chemical suppliers often serve much broader water treatment, personal care, or health and hygiene markets. Major players include Arch, Chemtura, PPG Industries, Clearon, Chem Lab, and DuPont.

MOST MAJOR suppliers offer product lines targeting different distribution channels and consumers. Some products are for pool owners who simply want clean and safe water. Others are aimed at those also looking for extra sparkle that comes from clarifiers or the silky feel that comes from softeners. Chemtura, for example, sells its products as BioGuard and Spa Guard through a service-oriented, authorized dealer-direct network; as Aqua Chem, Pool Time, and Spa Time via mass merchants; and as Omni, Guardex, and Hydrotech through the distributor and pool-store market.

Sorting out the reagents is just a small part of the backyard pool experiment. Other factors include pool volume and type; circulation, brushing, and vacuuming rates; climate, weather, and water temperature; nearby plants, critters, and bather load; and, last but not least, owner diligence.

Every pool season, the Centers for Disease Control & Prevention reminds owners and swimmers about healthy swimming practices. One of them is, "Don't drink the water."

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society

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