2.3 Chemical testing & alternative sanitizers

Why and How We Test

Testing converts guesswork into numbers. An operator who cannot measure free chlorine, pH, and combined chlorine cannot defend water quality to a health inspector or protect swimmers from an unsanitized pool. The CPO program stresses knowing which test measures what, its useful range, and its limits. Two families of tests dominate day-to-day work: sanitizer tests and pH tests, and each has a right tool for the job.

Chlorine and Sanitizer Tests

DPD — the workhorse

DPD (N,N-diethyl-p-phenylenediamine) is a colorimetric reagent that turns pink to red in the presence of chlorine; the deeper the color, the more chlorine present. Its key strength is that it can read free chlorine and combined chlorine separately — DPD reagents 1 and 2 develop the free-chlorine color, then reagent 3 develops the combined fraction, so the operator gets FAC, CC, and TC. Its weakness is that at very high chlorine (above roughly 10 ppm) DPD can "bleach out," reading falsely low.

FAS-DPD titration — the accurate method

For precise results, especially at high chlorine or shock levels, use a FAS-DPD titration. Instead of matching a color, the operator counts drops of ferrous ammonium sulfate (FAS) until the pink color disappears; each drop equals a known increment of ppm. FAS-DPD resists bleach-out and is the preferred method for measuring combined chlorine and confirming that a pool has reached breakpoint.

OTO — the older test

OTO (orthotolidine) turns yellow in the presence of chlorine and is cheap and simple, but it measures total chlorine only — it cannot separate free from combined. Darker yellow means more total chlorine. Many older test kits pair OTO (yellow) with phenol red (pH) in a two-well comparator.

Test strips

Test strips are dip-and-read pads that estimate several parameters at once. They are fast, portable, and convenient for quick checks, but they are the least precise method and are affected by strip age, moisture, and technique; they do not replace a good reagent kit for compliance testing.

pH Testing

Phenol red is the standard pH indicator, turning yellow (low) through red to purple (high) across a useful range of about 6.8-8.4. Very high chlorine can bleach phenol red or turn it purple, giving a false reading, so many kits include a chlorine neutralizer to add before the pH test.

Testing Frequency

Frequency scales with bather load and local code, but a workable commercial schedule looks like this:

The two fast-moving numbers — free chlorine and pH — are tested most often because they change hour to hour with sun, heat, and swimmer load.

Taking a Good Sample

A test is only as good as the water in the vial. Collect the sample about elbow-deep (roughly 12-18 inches down), well away from return inlets, skimmers, and dead corners, and test it promptly before chlorine and pH drift. Rinse the comparator with pool water first, fill to the line and read at eye level, add reagents in the correct order, and compare against a white background in good light. Sloppy sampling — testing surface water next to a return, or letting a filled tube bake in the sun — produces numbers that do not match the pool the swimmers are actually in, and every logged result should record the time so trends are meaningful.

Alternative and Supplemental Sanitizers

The exam is firm on one point: the systems below are supplemental oxidizers or on-site generators, not stand-alone replacements for a chlorine or bromine residual. A measurable halogen residual must still be maintained in the water at all times.

• UV (ultraviolet): water passes a UV lamp that inactivates bacteria, viruses, and — importantly — chlorine-resistant Cryptosporidium. UV also helps destroy chloramines, but it leaves no residual, so it protects only the water passing through the chamber; a chlorine residual is still required in the pool.
• Ozone: a powerful oxidizer generated on site (by corona discharge or UV). It oxidizes contaminants and chloramines but leaves no lasting residual and must be off-gassed before water returns to the pool. Supplemental only.
• Salt chlorine generators (electrolytic): dissolved salt (roughly 2,700-3,500 ppm) is electrolyzed into chlorine at the cell. This is still chlorine — the pool is chlorinated, not "chlorine-free" — and it still needs cyanuric acid and pH management. The cell tends to raise pH.
• Mineral systems (silver/copper): release trace metals that are bacteriostatic and algaestatic. They reduce chlorine demand but are supplemental; a low chlorine residual is still required, and excess metals can stain surfaces.

A defensible routine uses a reagent kit (DPD or FAS-DPD) for chlorine and phenol red for pH, logs results on a schedule, and treats UV, ozone, salt, and mineral systems as helpers that lower workload — never as reasons to let the sanitizer residual fall to zero.