​Key Factors Affecting Residual Chlorine Test Results

Analysis of Influencing Factors on Residual Chlorine Detection Results

As a key artificially added indicator in tap water quality standards, residual chlorine plays an important role in preventing secondary bacterial contamination of water bodies — insufficient content fails to achieve bacteriostatic effect, while excessive content may exert adverse impacts on human health. Therefore, the accuracy of its detection results is directly related to the judgment of tap water quality and drinking safety. However, in actual water quality testing, residual chlorine detection results are often interfered by various factors, which need to be focused on pertinently to ensure data reliability.

I. Water Sample Characteristics: Impacts of Temperature, pH Value and Redox Status

The basic physical and chemical properties of water samples are the primary factors affecting residual chlorine detection results. On the one hand, temperature can alter the chemical activity of residual chlorine: elevated temperature may accelerate the decomposition of residual chlorine, leading to detected values lower than the actual content; excessively low temperature may slow down the reaction rate, and failure to fully balance the temperature during detection is likely to cause result deviation. On the other hand, pH value has a significant impact on the form and stability of residual chlorine: in acidic environments, residual chlorine is more likely to exist in the form of hypochlorous acid (HClO) with high stability; in alkaline environments, it tends to convert into hypochlorite ions (ClO⁻) and may further decompose, resulting in a decrease in the detected total residual chlorine content. In addition, if the water sample contains antioxidants (such as sulfites, vitamin C) or reducing agents (such as ferrous ions), they will undergo redox reactions with residual chlorine, directly consuming residual chlorine and causing lower-than-actual detection results that fail to reflect the true residual chlorine level.

II. Differences in Detection Methods: Variations in Sensitivity and Accuracy Across Technical Approaches

The selection of residual chlorine detection methods directly determines the reliability of results. Currently, commonly used detection methods exhibit distinct differences in sensitivity and accuracy due to varying principles. For instance, the DPD method (N,N-diethyl-p-phenylenediamine spectrophotometry) features simple operation and rapid reaction, with high sensitivity for free chlorine detection, but it is susceptible to interference from water color, turbidity or other oxidants, which may lead to overestimated results. In contrast, the electrode method directly measures residual chlorine concentration via dedicated electrodes, enabling real-time monitoring, yet the electrode response speed, calibration status and service life will affect data accuracy. Moreover, it is highly sensitive to oil contaminants and heavy metal ions in water samples, and electrode fouling is prone to causing detection deviation. In addition, although the traditional o-tolidine colorimetric method has low cost, it is greatly affected by temperature and pH value, with poor color development stability, making it difficult to precisely control detection results.

III. Reagents and Instruments: Quality Stability Determines the Foundation of Detection

The reagents and instruments used in the detection process serve as the "hardware foundation" for ensuring the accuracy of residual chlorine detection results. In terms of reagents, insufficient purity (e.g., DPD reagents containing impurities), inaccurate preparation concentration or expiration will directly affect the reaction efficiency with residual chlorine — for example, low reagent concentration may lead to insufficient color development, resulting in underestimated detected values; expired reagents may lose reaction activity, causing result invalidation. In terms of instruments, the precision, calibration status and maintenance of detection instruments are crucial: wavelength calibration deviation or cuvette contamination of spectrophotometers will affect the accuracy of absorbance measurement; failure to regularly clean, activate or calibrate residual chlorine electrodes will cause electrode response value drift, failing to accurately reflect residual chlorine concentration. In addition, instrument stability also impacts results — for example, insufficient battery power of portable detectors may lead to data fluctuations.

IV. Detection Timing: Impacts of Residual Chlorine Time Sensitivity

Residual chlorine is somewhat unstable and will undergo natural decomposition (e.g., affected by light and temperature) or be consumed through reactions with organic substances in water over time. Therefore, the selection of detection time points has a significant impact on results. If the water sample is not tested immediately after collection, the residual chlorine content will gradually decrease during storage, leading to detected values lower than the actual residual chlorine level at the time of collection. For example, when tap water is collected from the pipe network and placed at room temperature for 1 hour, the residual chlorine loss may reach 10%-20% (the specific loss varies with water sample characteristics). Therefore, standard detection procedures usually require that water samples be tested as soon as possible after collection. If immediate testing is not feasible, preservation measures such as light shielding and low-temperature storage (e.g., refrigeration at 4℃) should be adopted, and the storage time should be strictly controlled to reduce deviations caused by residual chlorine loss.

V. Interfering Substances in Water: Interference from Chlorides, Oxidants and Reducing Agents

In addition to residual chlorine, the target analyte, other coexisting substances in water samples may also interfere with detection results. First, other chlorides: if the water contains high concentrations of chloride ions (e.g., industrial wastewater mixed into tap water), some detection methods (such as the electrode method) may misidentify them as residual chlorine, leading to overestimated detected values. Second, other oxidants (such as ozone, chlorine dioxide, potassium permanganate): these substances will react similarly with residual chlorine detection reagents (e.g., developing color with DPD reagents), causing detection results to include non-residual chlorine oxidizing substances and resulting in "false high values". Third, reducing agents (such as sulfides, organic substances): in addition to directly consuming residual chlorine, some reducing agents may also react with detection reagents, inhibiting color development or electrode response, leading to underestimated residual chlorine detection values. For example, if the water contains a large amount of humus (a natural organic substance), it will react slowly with residual chlorine and may also adsorb onto the electrode surface, affecting the electrode's response to residual chlorine.

In summary, the accuracy of residual chlorine detection results is affected by multiple factors including water sample characteristics, detection methods, reagents and instruments, detection timing, and interfering substances. In practical testing, standardized operating procedures should be formulated for these factors — such as strictly controlling water sample preservation conditions, selecting appropriate detection methods, regularly calibrating instruments and verifying reagent quality, and eliminating the impact of interfering substances — to ensure that residual chlorine detection results are true and reliable, providing a scientific basis for tap water quality assessment and drinking safety.

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