Residual chlorine, as a key disinfection indicator in drinking water treatment, its detection accuracy directly affects water quality safety and disinfection effectiveness evaluation. The detection results of residual chlorine water quality detector are influenced by various factors, including instrument performance, operating standards, environmental conditions, sample characteristics, and chemical interference. The following systematically analyzes these influencing factors from multiple dimensions.

1、 Instrument performance and technical principles

1. Sensor type and selectivity

The core of residual chlorine detector is the sensor, and its type determines the detection accuracy and scope of application:

-Electrochemical sensor: Based on polarography or potentiostatic method, residual chlorine concentration is calculated by measuring the current change of chlorine oxidation-reduction reaction. This type of sensor is susceptible to water temperature, turbidity, and interfering ions such as bromine and ozone, and requires regular calibration.

-Colorimetric sensor: The residual chlorine reacts with DPD (N, N-diethyl-phenylenediamine) to determine absorbance using a spectrophotometer. This method has high specificity for free chlorine, but strict control of color development time, pH, and reagent quality is required.

-Fluorescence sensor: detects residual chlorine through fluorescence labeling reaction, with high sensitivity but high cost, suitable for laboratory analysis.

2. Calibration and maintenance status

-Calibration frequency: Failure to calibrate regularly (such as daily or weekly) can lead to baseline drift, especially in environments with high humidity or large temperature differences.

-Accuracy of standard solution: Improper preparation of standard solution for calibration (such as using expired reagents or not considering temperature correction) can introduce systematic errors.

-Electrode aging: After long-term use, the sensitive film of electrochemical sensors may be contaminated or passivated, and it needs to be polished or replaced in a timely manner.

3. Detection limit and resolution

-The detection of low concentration residual chlorine (such as 0.01 mg/L) requires instruments with high resolution (such as 0.001 mg/L), otherwise data distortion may occur due to insufficient resolution.

-When high concentration samples exceed the detection range, dilution treatment is required, but the dilution process may cause errors due to improper operation.

2、 Operating standards and sample processing

1. Sampling and storage conditions

-Sampling container: Failure to use clean, chlorine free containers (such as brown glass bottles) may result in sample contamination or adsorption loss.

-Storage time and method: Residual chlorine is easily volatile and decomposes under light. Failure to store in the dark or delayed detection (>2 hours) can result in lower results. Suggest rapid on-site testing or adding stabilizers (such as sodium thiosulfate).

-Temperature control: High temperature accelerates the decomposition of residual chlorine, while low temperature may cause changes in solubility. The sample temperature needs to be recorded and corrected.

2. Standardization of operating procedures

-Colorimetric operation: Failure to add DPD reagent or buffer solution according to regulations or insufficient mixing can result in incomplete color development or local concentration deviation.

-Electrochemical stirring: If the stirring speed is too fast, it may introduce bubble interference, while if it is too slow, it may lead to incomplete reaction.

-Reading timing: The colorimetric method requires strict adherence to the color development time (such as 5 minutes), as reading too early or too late can affect the linear relationship.

3. Cross contamination and cleaning

-After continuous detection of high concentration samples, if the sensor is not cleaned or the sample pool is not replaced, residual chlorine may contaminate subsequent low concentration samples.

-Organic substances (such as algae and oil stains) adhering to the surface of the colorimetric dish or electrode can hinder the reaction, and should be washed with chlorine free water and dried.

3、 Environmental factors and water quality characteristics

1. Temperature impact

-Chemical reaction rate: The rate of color reaction between residual chlorine and DPD increases with temperature, but excessive temperature may lead to unstable color development.

-Electrode signal: The current output of electrochemical sensors is significantly affected by temperature, and temperature compensation function (such as Nernst equation correction) needs to be integrated.

2. pH interference

-The form of residual chlorine varies with pH: Cl ₂ is predominant under acidic conditions, HOCl/OCl ⁻ under neutral conditions, and chloramine (NH ₂ Cl/NHCl ₂) is easily generated under alkaline conditions. The activity of chlorine varies greatly among different forms, and the detection method needs to be adjusted according to pH.

-Colorimetric methods typically require a pH of 6.5-7.5, and deviations from this range may result in color fading or side reactions.

3. Turbidity and suspended solids

-The sediment and microorganisms in high turbidity water samples (such as river water and sewage) can adsorb residual chlorine or block the light path, resulting in lower colorimetric results. Pre filtration (0.45 μ m filter membrane) or centrifugation is required.

-Colloidal particles may encapsulate residual chlorine, resulting in a "false" low concentration measured by the electrode method and insufficient actual disinfection efficacy.

4. Complexity of water quality background

-Oxidant interference: Ozone, peroxides, permanganate salts, and other substances in water may compete with residual chlorine for reaction, leading to misjudgment of electrochemical sensors.

-Reductive substances such as sulfides and ferrous ions will consume residual chlorine and need to be removed through pretreatment (such as acidification and blowing).

-Organic interference: Humic acid, tannins, and other substances react with residual chlorine to produce inactive chlorinated organic compounds, masking the true level of residual chlorine.

4、 Chemical reagents and color reactions

1. Stability of DPD reagent

-DPD solution is prone to oxidation and deterioration, and needs to be refrigerated in the dark (4 ℃) and used for a limited period of time (usually 1 month). Invalid reagents can cause pale or colorless color development.

-The pH of buffer solutions (such as phosphate and glycine NaOH) needs to be precisely controlled, otherwise it will affect the color sensitivity.

2. Chromogenic side reactions

-High concentration chloramines (such as NH ₂ Cl) react with DPD to produce red products, which may be mistaken for free chlorine. It needs to be distinguished by dilution or prolonging the color development time.

-Metal ions such as copper and iron may catalyze the decomposition of DPD, leading to an abnormal increase in absorbance in the blank control group.

5、 Data processing and instrument errors

1. Linear range of standard curve

-The colorimetric method requires regular updates of the standard curve, especially when the reagent batch number changes or the instrument light source ages. The poor linear correlation in the low concentration range (0-0.5 mg/L) will amplify the error.

-Electrochemical methods require multiple calibration points (at least 5 gradients) to avoid nonlinear deviations caused by a single calibration point.

2. Unit conversion and recording errors

-Confusion of residual chlorine concentration units (mg/L, μ g/L, ppm) may lead to misunderstandings in the results and should be uniformly labeled.

-Failure to consider significant figures or rounding rules during data recording may mask true fluctuations.