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Moravian Chemistry Professors Test the Waters

Maintaining Proper Water Quality in Swimming Pools is Two-pronged Problem

Bethlehem, Pa., August 2, 2005——Everyone knows swimming pools are disinfected with chlorine. Right?

Two Moravian College chemistry professors, who have submitted their findings to the Journal of Chemical Education, believe that pool owners and users should understand that swimming-pool chemistry is a two-pronged problem. According to Carl Salter and David L. Langhus, associate professors of chemistry, disinfecting a swimming pool is more exacting process than backyard pool-owners and community pool managers may understand.

As an application of practical chemistry, says their article, “The Chemistry of Swimming Pool Maintenance,” the factors that affect pool water sanitation and chemical balance can hardly be bettered for study in college classrooms.

Langhus finds the chemistry practical on a daily basis during the summer, when he works at Haycock Camping Ministries in Kintnersville, which has a junior Olympic-sized pool: 25 meters long, holding more than 750,000 gallons of water.

Here’s the chemistry involved: Water for human consumption and swimming is disinfected with hypochlorous acid (HOCl), the aqueous version of chlorine. Because sunlight causes hypochlorous acid to degrade, freeing the chlorine atoms, sodium carbonate (commonly called soda ash) is added to the water to boost alkalinity and counter the acidity. Difficulties may arise if calcium is present in the water—as it would be in swimming pools because the cement or plaster lining of most pools is made up largely of calcium (limestone) and silicon (clay or sand).

Homeowners deal with “hard water”—water with enough calcium to create a grainy deposit called “scale” and eventually to clog the pipes—by attaching a water softener to their inbound water lines. The water softener is a sodium filter-compound called zeolite.

But the combination of “chlorine” and calcium in a swimming pool can create problems too large for a water softener to solve. No household water softener could keep up with the volume of water needed to fill and circulate in even a backyard pool.

“You need enough acidity [in the water] to keep the hypochlorous acid intact,” says Salter, “but not too much or you break down the calcium compounds and then your pool starts to disintegrate.” If too much soda ash is added to correct the pH, it can peel the hydrogen atoms off any unstable acid molecules, forming sodium hydroxide, the active ingredient in lye or caustic soda. Sodium hydroxide interacts with hypochlorous acid to form a toxic cloud of ammonia and water vapor.

Creating the proper balance of chemical additives is the province of the 1,200-page Hach Water Analysis Handbook, a general catalogue of water tests. Though far too detailed for pool owners, it contains all the advice and formulas for adjusting the content of water to make it clean, balanced, and fit for human use.

In most pools, chemicals are not present in sufficient quantities that imbalances would sicken the swimmers.

But early in July, in Canal Winchester, near Columbus, Ohio, what was termed a “chlorine surge” at a municipal pool caused a bad scare in the community. For most of the swimmers, a quick hosing-off was enough to take care of acid on their skin or swimsuits. But some children, after breathing the chlorine fumes, were sent to the hospital for respiratory therapy.