Pretreatment of Turbid Water Samples for Total Chlorine Determination

Turbidity in water samples interferes with total chlorine analysis, particularly in colorimetric and amperometric methods. This short note presents simple pretreatment procedures—centrifugation, membrane filtration, and coagulation-sedimentation—that effectively clarify turbid samples without altering total chlorine levels. Key precautions to avoid chlorine loss are emphasized.

1. Introduction

Total chlorine (free + combined) is routinely measured in drinking water, wastewater, and industrial effluents. Turbid samples, caused by suspended particles, lead to overestimation (light scattering in colorimetry) or electrode fouling. Direct analysis is often unreliable, necessitating proper pretreatment that removes particles while preserving chlorine species.

2. Pretreatment Methods

The following methods are suitable for routine laboratories:

Centrifugation (recommended for moderate turbidity)

Centrifuge the sample at 2000–3000 rpm for 5–10 minutes. Carefully decant the clear supernatant for analysis. This method minimizes chlorine loss because it is fast and closed to air. Avoid prolonged centrifugation (>15 min) which may induce chlorine volatilization.

Membrane filtration (for low turbidity)

Filter through a 0.45 μm pore-size membrane filter (e.g., cellulose acetate). Pre-rinse the filter with deionized water. Discard the first 10–20 mL of filtrate to avoid adsorption effects. Use immediately after filtration.

Coagulation-sedimentation (for high turbidity)

Add 1–2 mL of alum solution (10 g/L) per 100 mL sample, stir gently for 1 min, let settle for 10 min. Alternatively, use polymeric coagulants (e.g., polyDADMAC) at low dosage (0.5–1 mg/L). Caution: Overdosing or acidic coagulants may alter pH and decompose chlorine; always verify with spike recovery tests.

3. Critical Precautions

No air stripping: Avoid vigorous shaking, vacuum filtration under strong suction, or heating.

No chemical reductants: Do not use traditional clarifying agents like sodium thiosulfate or activated carbon – they consume chlorine.

Rapid analysis: Perform chlorine determination immediately after pretreatment (within 5 minutes) to prevent decay.

Validation: For any new sample matrix, conduct a recovery test by adding known total chlorine (e.g., chlorinated standard) to turbid sample, pretreat, and measure.