How to test chlorine residual

The DPD test, step by step

The DPD colorimetric method is the one most small systems use. It's approved in federal rules for free and combined chlorine, with test kits allowed if your state approves them (40 CFR 141.74). DPD turns pink in proportion to the chlorine present; a colorimeter reads that color as mg/L.

1. Pick the tap. Use the sampling point your state or sample siting plan specifies: a cold-water tap, not a hose or a treatment device.
2. Flush. Run the tap long enough to draw water from the main, not water that has sat in the line.
3. Rinse and fill the sample cell. Rinse the clean cell with the sample water, then fill it to the mark.
4. Zero the meter. With a colorimeter, zero on a blank of the same sample water.
5. Test free chlorine. Add the free chlorine DPD reagent, mix, and read within one minute.
6. Test total chlorine if needed. On a fresh sample in a separate cell, add the total chlorine reagent and read after the reaction time, typically 3 to 6 minutes.
7. Record it. Write the date, time, location, free and total results, method and your initials straight away.

Test right away: chlorine starts to break down as soon as the sample leaves the pipe, and Hach's method notes the samples can't be preserved for later analysis. It also warns against using the same cell for free and total tests, because reagent carried over from a total test can inflate the next free reading.

Free vs total chlorine

Definitions follow Hach's explanation of free and total chlorine (Hach). For compliance with the federal chlorine maximum, free or total chlorine may be measured (40 CFR 141.131).

What to write down

A residual reading is a record, and federal rules say what a sample record must include (40 CFR 141.33):

• Date, place and time of sampling, and who collected it
• What kind of sample it was: routine distribution, check sample, raw water or special purpose
• Date of analysis, and who did it
• The method used (for example, “DPD colorimeter”)
• The result, as read: 0.70 is not the same record as 0.7

The chlorine residual log sheet has a column or header line for each of these.

Mistakes that give false readings

• Reading free chlorine late. Waiting past a minute lets combined chlorine show up in the color and overstates free chlorine.
• Old reagents, or a faded comparator. Texas requires current reagents and an unfaded, clear comparator (30 TAC §290.110). Check expiry dates.
• Dirty or scratched cells. Fingerprints and scratches change the light path in a colorimeter.
• Sampling from a hose or a softener or filter outlet. Those don't represent the water in your mains.
• Carrying the sample in a plastic bottle. Hach recommends clean glass, because some plastics use up chlorine.
• Rounding. Write what the instrument shows. Pennsylvania's operator training, for example, says results should not be rounded.

When the reading is low

Retest first, from the same tap, to rule out a bad sample. If it's still below your minimum, check the chlorinator: solution level, feed pump running and primed, injection point not clogged. Record both readings and what you found. If your system provides 4-log treatment, federal rules call for follow-up samples every four hours until the residual is restored, and notice to the state if it isn't restored within four hours (40 CFR 141.403, 141.405). Your state's rules say what applies to you: see chlorine residual requirements.