Testing Free and Combined Chlorine

This procedure produces a trustworthy chlorine record. It does not select a shock treatment, calculate a dose, add a product, or release a restricted pool or spa.

Free chlorine (FC) is the chlorine measured by the approved method as available disinfectant. Total chlorine (TC) includes free chlorine plus the combined chlorine measured by that method. When the method supports the calculation:

Combined chlorine (CC) = total chlorine − free chlorine

A result is useful only when the sample is representative, the method is valid, and the value is interpreted with pH, CYA, pool or spa characteristics, temperature, treatment history, demand, and equipment state.

Why this matters

Free chlorine is the disinfectant measurement used to judge the immediate chlorine residual, while combined chlorine can reveal reaction products and unresolved demand. Confusing FC with TC can make water with little available disinfectant appear adequately treated. Missing a bleached-out DPD result can produce the opposite error: adding chlorine to water that is already far above the test range.

The CDC identifies properly maintained chlorine and pH as the first defense against germs in residential pools and hot tubs. A clear appearance, strong odor, salt-system display, feeder setting, or last week's result cannot replace a valid current measurement.

Before you begin

Review the property and visit context:

• correct pool, spa, or connected-system mode;
• sanitizer source: liquid chlorine, cal-hypo, trichlor, dichlor, salt chlorine generator, proprietary cartridge, or another identified product;
• CYA use and most recent valid result;
• feeder setting, salt-system output, automation schedule, cover, heater, jets, spillway, and features;
• last FC, TC, CC, pH, treatment, and retest;
• recent homeowner dosing, non-chlorine shock, heavy use, rain, fill, overflow, contamination, algae, or service interruption;
• current circulation state and time since any addition.

Do not assume “salt pool” means a different disinfectant. A salt chlorine generator produces chlorine, and its water still requires a valid FC test. Do not assume an ozone, UV, mineral, or proprietary supplemental system replaces the residual required by the approved plan.

Inspect the test kit before collecting water. Confirm the exact current instructions, correct cells and caps, readable reagent labels, usable dates, storage condition, valid range, sample volumes, conversion factors, and any standard check required by the method.

Use Test Accuracy, Sample Handling, and Common Interferences when any part of the measurement chain is questionable.

Choose the approved chlorine method

FAS-DPD titration

FAS-DPD is the preferred field method when your company has approved the kit and the technician is trained to use it. It measures free and combined chlorine with a distinct titration endpoint and a useful higher range without requiring a visual shade match.

Taylor's representative FAS-DPD materials describe two common resolutions: a 10 mL sample in which each drop represents 0.5 ppm, or a 25 mL sample in which each drop represents 0.2 ppm. These numbers apply only when the exact Taylor method says so. Never import them into another kit, sample volume, reagent, or instruction.

Visual DPD comparator

A visual DPD comparator can be used when it is the approved method, the expected result fits the scale, the sample is clear and uncolored, and suitable lighting is available. It requires prompt, method-specific color matching.

The CDC recommends DPD testing over strips for more accurate residential chlorine and pH results. CDC also warns that chlorine above 10 ppm may partly or completely bleach a DPD result to a false low or zero.

Photometer or colorimeter

Use only an approved program, cell, blank, reagent, range, and timing. A digital display does not remove the need to detect bleach-out, turbidity, colored water, bubbles, dirty cells, poor blanking, or chemical interference.

Test strip

An approved strip may support a screening or backup decision within its labeled range, but it should not replace the approved drop or instrument method for a consequential adjustment when that method is available. Follow the exact dip and read timing and confirm surprising or decision-changing results.

Detailed procedure

1. Collect a representative sample

Use the complete water-sampling procedure.

For routine chlorine testing:

• sample the correct pool or spa in its known active mode;
• collect fresh water from the mixed body, below the immediate surface;
• avoid a return, skimmer, feeder, tablet plume, chemical injection point, salt-cell outlet, spillway, autofill, hose, surface film, and stagnant step or corner;
• wait for the product- and procedure-required mixing interval after treatment;
• test promptly on a stable, clean work surface.

If comparing two locations for diagnosis, label them separately. Do not use a high reading at a return or a low reading in an isolated feature as the routine representative pool or spa result.

2. Prepare the chlorine cell

Use the clean cell assigned or approved for the method. Follow its rinse instruction. Set the exact sample volume at eye level and do not estimate between unrelated markings.

Prevent these cross-contamination failures:

• total-chlorine reagent or a total-test cap entering the free-test cell;
• fingers, sunscreen, cleaning product, treatment chemical, or deck water contacting the sample;
• a dropper tip touching the cell or another bottle;
• leftover reacted sample being topped up or reused;
• a sample or reagent being returned to its original container.

3. Measure free chlorine

Follow the approved method card from beginning to end.

For a FAS-DPD method, this generally means:

1. use the exact sample volume;
2. add the specified amount of DPD indicator and mix as instructed;
3. observe whether the sample develops the expected color and whether it flashes, fades, turns abnormal, or remains colorless;
4. add the specified titrant dropwise, with the bottle held and mixed exactly as instructed;
5. count full drops to the defined colorless endpoint;
6. multiply by the conversion factor for that exact sample volume;
7. record FC immediately with units and method.

For a visual DPD method, add the exact free-chlorine reagents, mix and time the reaction, then compare within the stated window and lighting. Hold the comparator as directed. Do not extrapolate beyond its highest block or choose the desired target when the shade is uncertain.

If no normal color develops, do not automatically record zero. Check for:

• an initial flash that quickly cleared;
• recent high-dose treatment or a feeder left high;
• expired or compromised reagent;
• wrong reagent or sample volume;
• another oxidizer or interfering product;
• a true low result confirmed by a valid alternate method.

4. Measure total or combined chlorine

Continue the same compatible method exactly as instructed. Some methods add another reagent to the FC sample and directly titrate combined chlorine; others produce a separate total-chlorine result.

Do not invent a hybrid. The result and arithmetic depend on the method design.

For a FAS-DPD method that titrates combined chlorine after FC, add the exact combined-chlorine reagent to the completed FC cell. If the expected color returns, titrate to the defined endpoint, count those drops separately, and apply the method's conversion. The second drop count represents CC for that method.

For a method that measures TC, record the valid TC result and calculate:

CC = TC − FC

Use values produced at compatible resolution from the same representative sample and timing. Do not subtract a coarse strip result from a precise titration or combine readings from different pool locations.

5. Check the arithmetic and resolution

Example using compatible valid measurements:

• FC = 3.5 ppm
• TC = 4.0 ppm
• CC = 4.0 − 3.5 = 0.5 ppm

If TC appears lower than FC, do not report negative CC. Small differences can result from resolution and endpoint uncertainty; larger differences suggest method, timing, contamination, arithmetic, or sample error. Repeat from a fresh sample and record no measurable CC only if supported at the method's resolution.

Do not add more decimal places than the method produces. A result counted at 0.5 ppm per drop should not be recorded as 3.47 ppm.

6. Validate the result

Compare the new result with:

• duplicate or alternate method when required;
• prior result and expected loss or production;
• exact products and amounts recently added;
• feeder, salt-cell, cover, automation, and circulation state;
• pH, CYA, temperature, demand, weather, use, contamination, and algae;
• odor, irritation report, water clarity, and visible condition;
• controller output or sensor value, recognizing the sample points may differ.

The goal is not to force agreement. The goal is to determine whether disagreement is explainable and which observation is valid.

7. Compare with the correct requirement

The CDC residential baseline is:

• pool without CYA: at least 1 ppm FC;
• pool using CYA or stabilized chlorine: at least 2 ppm FC;
• residential hot tub: at least 3 ppm FC;
• pH: 7.0–7.8;
• no CYA or stabilized chlorine in a hot tub.

These are health baselines, not complete weekly service targets. Use Water-Chemistry Targets and Decision Bands for the approved target structure. The stricter applicable product label, installed-equipment manual, surface instruction, property plan, or restriction controls.

Do not interpret FC without pH and CYA context. Do not reduce FC to a generic “normal” number when the pool's approved CYA-aware plan requires a different target.

8. Record and hand off

Record FC, TC, calculated or directly measured CC, units, method, sample volume, conversion, range, pool or spa, time, operating state, pH, CYA context, and any limitation.

Then choose the next approved workflow:

• within the property target and supported by trend: document and continue the service sequence;
• below target or falling unexpectedly: use the low/high chlorine diagnostic before calculating treatment;
• high, over range, or possible bleach-out: keep the pool or spa restricted and confirm before acting;
• persistent CC, odor, irritation, demand, algae, or contamination: diagnose the cause and use the applicable treatment procedure;
• invalid or unresolved: record unable to verify, preserve the restriction, replace or obtain the correct test system, and escalate.

Common problems

Flash then clear

High chlorine can consume the DPD color and appear low or zero. Record the flash, stop dosing, and use the exact approved high-range or dilution method. Never add chlorine merely because the final cell is clear.

Off-scale color

The result is greater than the method can quantify. Record greater than the highest valid value, not a guessed number. Confirm with an approved higher-range method.

Color develops after the read window

Delayed color can result from continued reaction or oxidation. Read only within the method's timing. Discard the late result and repeat.

Monopersulfate or other oxidizer interference

Non-chlorine shock and other oxidants can respond in some DPD methods and appear as total or combined chlorine. Identify recent products and use the method's approved interference-removal reagent or alternate method. Do not diagnose chloramines from the uncorrected number.

Free-test contamination from total-test reagent

Residual total-chlorine reagent in a cell or cap can add color to a later FC test. Keep components paired, clean them as directed, and repeat with a known-clean cell.

Strong chlorine odor but modest FC

Odor does not measure FC. Combined chlorine and other reaction products can contribute to odor and irritation. Confirm FC, TC, CC, pH, recent use, contamination, ventilation for an indoor installation, and product history. Do not tell a customer the pool simply has “too much chlorine” from smell alone.

Controller and field test disagree

Do not recalibrate either system from one unexplained difference. Confirm the manual sample and method, sample location, flow, probe cleanliness, calibration state, pH, CYA, temperature, recent feed, and controller manual.

Combined chlorine is a diagnostic signal

Combined chlorine is not backup sanitizer. It indicates chlorine reaction products measured by the method and should be interpreted with odor, irritation, contamination, use, demand, pH, CYA, treatment history, and repeatability.

The approved aim is no persistent measurable CC under a valid method. A small single result near the resolution of the test may be measurement uncertainty or a temporary condition. A repeatable result or worsening trend deserves diagnosis.

Do not automatically raise FC to ten times CC. CDC discusses that public-aquatics response in a public-venue context, but the selected treatment must account for the actual compounds, CYA, pH, product, surface, equipment, air environment, demand, and label. The correction belongs in an approved oxidation or contamination procedure, not in this measurement SOP.