2.1 Test Methods, Kits, and Sampling

Clear water is not a test result. Free chlorine is consumed by bather waste, outdoor ultraviolet light, and organic dirt; pH drifts as carbon dioxide leaves the water and as sanitizer products add acid or base. Domain 1 of the Aquatic Facility Operator (AFO) exam treats testing methods as the first chemistry skill: if the sample is taken in the wrong place, the reagent is expired, or the color is bleached, every later dosage decision is fiction. Independent operator teaching in this section uses public-health practice from the Centers for Disease Control and Prevention (CDC)—including the Model Aquatic Health Code (MAHC) as a voluntary model many health departments consult when they write local rules—plus standard DPD kit chemistry. Your authority having jurisdiction (AHJ) (state or local health code) always wins when it is stricter or more specific than a national model.

Why frequency and location matter

Bather load is the largest short-term chlorine demand. Sweat, urine, lotions, and skin soils consume free chlorine (FC)—the hypochlorous acid (HOCl) and hypochlorite ion (OCl-) residual that still has disinfecting work left. A quiet weekday morning and a Saturday lesson block are not the same pool. Test more often when occupancy, water temperature, or organic load rises. Spas, therapy pools, and splash pads have a high bather-to-volume ratio and usually need a tighter watch than a large lap pool.

Sunlight photolyzes unstabilized chlorine at the surface of outdoor venues, so afternoon free chlorine can collapse even when the morning log looked comfortable. Indoor pools lose less chlorine to ultraviolet light but accumulate combined chlorine (CC) (chloramines) in water and air when oxidation cannot keep up. An indoor reading taken only at a controller probe can miss a stagnant corner; an outdoor reading skimmed off a sun-warmed surface film can miss the bulk water below.

The 2024 MAHC illustrates a common commercial cadence that agencies may adopt in whole or in part: test DPD free chlorine (or bromine) and pH before opening every day; retest every 2 hours while open if disinfectant is fed manually without an automated controller; retest every 4 hours while open if an automated feed system is in use. Total alkalinity is commonly a weekly test, calcium hardness monthly, and cyanuric acid monthly (weekly if stabilized chlorine is the primary sanitizer). Many AHJs require more frequent logs than the model. Controllers and oxidation-reduction potential (ORP) probes supplement wet chemistry; they do not replace it. Record the wet test at the same clock time as the probe reading so you can see when the two disagree.

Location is as important as the clock. Return inlets dump freshly treated water. A vial filled over a return can show high FC and "perfect" pH while the shallow end where children stand is under-sanitized. Skimmers and gutters collect surface film, not mixed basin water. Chemical feed points, splash-out, and recently dumped product clouds are equally misleading.

Sample collection that represents the basin

A bulk-water sample is water taken from the pool itself, not only from a pipe. MAHC bulk-sample practice that operators should know for public-health work is:

1. At least 18 inches (about 45 cm) below the surface—roughly elbow depth on an adult.
2. A water depth of about 3–4 feet when that depth exists (typical shallow-end standing water).
3. A spot between inlets, not on top of a return jet.

Rotate sites around the shallow end so you are not always blessing the same corner. Include a deep-end grab in the weekly rotation. If the venue has an in-line sample port on the recirculation piping, use it for the circulating stream—and still collect at least one midday bulk sample so pipe water and basin water can be compared. Multiple recirculation systems mean a separate sample from each port.

Vial technique is part of the method, not a nicety:

• Rinse the sample cell two or three times with the water you are about to test so leftover reagent, dust, and yesterday's sample do not dose the new test.
• Fill to the etched line. Read the meniscus the way the kit instructions require.
• Cap and invert to mix; do not seal the vial with a finger (oils and extra chlorine from skin).
• Run chlorine and pH immediately. A standing sample off-gasses chlorine, changes temperature, and picks up carbon dioxide from room air. Do not fill a pitcher at opening and retest that same water all afternoon.
• Work in the shade. Direct sun bleaches developed DPD color and fools a comparator.

DPD: free chlorine and total chlorine

DPD is N,N-diethyl-p-phenylenediamine (CDC also writes N,N-diethyl-1,4-phenylenediamine sulfate). In a measured sample it forms a pink to magenta color with free chlorine. That DPD free-chlorine result (often labeled DPD-FC) is the number commercial codes use for "is there sanitizer working?" A chemistry nuance in MAHC definitions matters for outdoor, stabilized water: when cyanuric acid (CYA) is present, DPD-FC includes cyanurate-bound available chlorine as well as HOCl/OCl-. Operators still log DPD-FC as the code residual; they do not pretend the kit isolated only HOCl.

To obtain total chlorine (TC), the kit then adds an iodide reagent (commonly called DPD #3). Combined chlorine species become measurable, and the color deepens if CC is present. Combined chlorine = total chlorine − free chlorine. Color-block (comparator) DPD kits are the everyday commercial method when the residual sits in the printed range (often about 0.5–5 ppm, kit-specific). Read inside the manufacturer's time window; color can keep developing or start to fade.

FAS-DPD titration uses the same DPD pink, then ferrous ammonium sulfate (FAS) drops until the sample goes colorless. Each drop equals a stated ppm increment for the sample volume (commonly 0.2 ppm or 0.5 ppm). After the free-chlorine endpoint, DPD #3 is added; any returning pink is titrated as combined chlorine. FAS-DPD is the tool to prefer when you need CC to a tenth of a ppm, when FC is above the color-block scale, or when deck lighting or color vision makes a comparator a guess. Many public-health inspectors use FAS-DPD as their field kit for that reason.

Methods that do not belong on a free-chlorine commercial log

OTO (orthotolidine) turns yellow to orange with oxidizers. It does not separate free chlorine from combined chlorine; it reports a total oxidizer stain. It is the historic backyard test and is widely not allowed for public-pool free-chlorine control. If an older kit in the pump room is OTO-only, it cannot answer "is the residual free?"

Test strips are dip-and-read pads compared to a bottle chart. They are useful as a screen between official tests, or as a high-range check during hyperchlorination when a standard DPD kit maxes out—but they are not a substitute for DPD or FAS-DPD on a commercial operating log. Humidity, age, and lighting widen the error. CDC notes strips are less accurate than a DPD kit even for home pools.

Photometers, comparators, and how tests lie

A comparator is a plastic viewer with printed color blocks. You match the sample by eye. A photometer (colorimeter) shines a light through the cell and reports a digital ppm. Photometers reduce lighting bias and help operators who cannot discriminate pinks; they still fail if the cell is dirty, scratched, or overfilled, or if the reagent lot is dead.

Expired reagents under-report (weak pink) or fail to develop. Date the bottles; discard them when expired; keep caps tight, cool, and dark.

Sunlight bleaches the pink after it forms. Develop and read in shade, promptly.

High FC bleaching is a classic false-zero trap. CDC states that if the sampled chlorine is more than about 10 ppm, DPD may partially or totally bleach, looking like 0 ppm when the water is actually heavily chlorinated. A flash of pink that vanishes is a clue. Dilute with chlorine-free distilled or deionized water (for example 1:1, then multiply the reading by 2), or use FAS-DPD / a high-range method. During fecal hyperchlorination (often 20–40 ppm FC), CDC notes that ordinary kits cannot read that high unless you dilute or use high-range strips.

Phenol red pH tests can also go purple (false-high) when chlorine in the sample is extreme; good kits include a chlorine-neutralizer drop before the pH reagent. Follow the manufacturer.

pH, alkalinity, hardness, and cyanuric acid

pH uses phenol red (phenolsulfonephthalein). The sample runs yellow on the acid side, through orange-red near typical swimming water, toward purple when high. The useful kit window is commonly about 6.8–8.4. Commercial target bands are often 7.2–7.8; CDC Healthy Swimming discusses keeping pH about 7.0–7.8 for germ kill, equipment life, and comfort. Local code wins.

Total alkalinity (TA) is an acid titration (indicator such as bromcresol green–methyl red) to a defined color change. It is the buffer that slows pH swings. MAHC's model range is 60–180 ppm (mg/L); many operators aim near 80–120 ppm depending on sanitizer type.

Calcium hardness (CH) is typically an ethylenediaminetetraacetic acid (EDTA) titration. Common commercial comfort bands are often 200–400 ppm; MAHC sets an upper model cap of 2500 ppm. Hardness is a water-balance factor, not a daily sanitizer test.

Cyanuric acid (stabilizer) is a melamine turbidity test: the reagent clouds in proportion to CYA, read against a black-dot tube or a standard. Test outdoor venues that use trichlor, dichlor, or added CYA. Indoor pools often use little or no CYA. MAHC model practice: test CYA monthly when used, 24 hours after adding CYA, and weekly if stabilized chlorine is the primary sanitizer. Do not skip CYA when you interpret why outdoor FC "disappears" by afternoon.

Treat the kit as a measuring instrument: shade, fresh reagents, a rinsed cell, a mixed sample from elbow depth, and a number written down before you walk back to the pump room.