8.3 Sample Collection & Lab Analysis (Chlorine Residual, pH, Turbidity)

8.3 Sample Collection & Lab Analysis (Chlorine Residual, pH, Turbidity)

Quick Answer: Good data start with a good sample: a clean, sterile bottle (with a sodium thiosulfate tablet for bacteriological samples so residual chlorine does not keep killing bacteria in transit), a flushed tap, a complete label, ice, on-time delivery, and chain-of-custody. In the field you measure chlorine residual (DPD), pH (meter), and turbidity (nephelometer) right away because all three drift within minutes of collection.

Sample Collection

A laboratory result is only as good as the sample that reaches the lab. Four habits produce a representative sample:

1. Right location — Sites in the distribution grid are chosen to represent different pressures, materials, ages of water, and dead-ends, not just the convenient tap at city hall.
2. Clean, sterile container — For bacteriological (coliform) work the bottle is sterile and contains a small sodium thiosulfate tablet. Thiosulfate reduces any chlorine residual to chloride so that disinfection does not continue inside the bottle during transport — without it, a low-level coliform sample would falsely read as absent.
3. Flushed tap — Let the tap run 2–3 minutes (or per your system's sampling plan) so you sample water from the main, not water that has stagnated in the service line and premise plumbing. Do not flame or over-treat the tap unless your procedure calls for it.
4. No headspace for volatile analyses — Fill to the brim and cap (for THM or VOC samples), leaving no air gap.

Labeling, Preservation, Chain-of-Custody

Every sample bottle is labeled with:

• Location (site ID and description)
• Date and time of collection
• Sampler's name
• Sample type (routine distribution, repeat, special, etc.)

Preservation is method-specific. Most distribution samples are iced at 4°C and kept in the dark until analysis. Each method has a holding time — for total coliform it is typically 30 hours from collection to analysis; many chemistry parameters have 48 hours or less. Missing the holding time invalidates the result.

A chain-of-custody form tracks who collected the sample, who took possession of it, and when, from the tap to the lab bench. Legal samples (those that may trigger a violation) require unbroken custody.

Field Measurements: Why They Cannot Wait

Three parameters are unstable after collection and are measured in the field, immediately:

If you collect a sample and drive it back to the lab, the chlorine residual will read lower than at the tap, the pH may shift by tenths, and turbidity can change as particles settle. Field instruments prevent that error.

Chlorine Residual: The DPD Method

The most common field and lab test for chlorine is N,N-diethyl-p-phenylenediamine (DPD) colorimetric.

• Free chlorine — DPD indicator is added to the sample; free chlorine oxidizes DPD to a pink compound. The intensity of the pink color is proportional to the free chlorine concentration and is read on a colorimeter or by visual comparison to standards.
• Total chlorine — A second portion is treated with DPD plus potassium iodide; combined chlorine is then also measured, giving total chlorine. Subtract free from total to obtain combined chlorine.
• Titrimetric DPD — For higher residuals (such as the plant effluent), ferrous ammonium sulfate (FAS) titrant is added dropwise until the pink color disappears; the titrant volume gives the concentration directly.

A colorimeter read in the field at the sample tap is standard practice for distribution residual monitoring.

pH by Meter

pH is measured electrometrically with a glass electrode and a calibrated pH meter. The meter is calibrated with two buffer solutions (commonly pH 4 and pH 7, or pH 7 and pH 10) bracketing the expected sample pH before each use. Field pH meters must be calibrated at least daily and checked with a standard between samples.

Turbidity: Nephelometric Measurement

Turbidity is a measure of the light-scattering property of water caused by suspended matter. A nephelometric turbidimeter (turbidimeter) measures the light scattered at 90° to the incident beam and reports in Nephelometric Turbidity Units (NTU). Higher NTU = cloudier water.

Turbidity in the distribution system is a clue to problems: sediment stirred up by a flow reversal or hydrant use, iron or manganese precipitation, or biofilm sloughing.

Typical Distribution Ranges

Field versus Lab Roles

Field work gives the operator an immediate decision-quality number: Is the residual holding? Is the pH in range? Is turbidity acceptable? The certified laboratory produces the legally defensible numbers that go into compliance reports — total coliform, DBPs, metals, nitrate — under strict holding times and methods. The Class I operator uses both, but the daily operational signal comes from the field.