The following is a comprehensive analysis of the influencing factors of COD monitoring instruments, covering key dimensions and mechanisms of action:
1、 Sample properties and composition
1. Suspended solids and particulate matter: Suspended particles in water samples may adsorb organic matter or obstruct light, resulting in insufficient digestion or measurement errors in photometry. When unfiltered sewage is directly measured, large organic particles may not be oxidized, resulting in low COD; The scattering effect of colloidal particles can interfere with the absorbance measurement of a spectrophotometer.
2. Chloride ion: In the potassium dichromate oxidation method, Cl ⁻ will be oxidized to Cl ₂ under acidic conditions, consuming oxidants and leading to a false increase in COD. Usually, it is necessary to add mercuric sulfate to mask Cl ⁻, but excessive mercuric sulfate can introduce toxicity risks and cannot completely eliminate the interference of high concentrations of Cl ⁻ (>2000 mg/L).
3. Chromaticity and turbidity: Dark water samples (such as coking wastewater) or high turbidity water samples will absorb or scatter light, affecting the accuracy of photometry. It is necessary to reduce interference through dilution, activated carbon decolorization, or centrifugal pretreatment, otherwise it may lead to high or low COD results.
4. Volatile organic compounds: If water samples containing volatile organic compounds (such as methanol and acetone) are not sealed and stored before digestion, it may cause the components to evaporate, resulting in lower COD measurement values. In addition, prolonged storage of water samples may result in distorted results due to microbial degradation or precipitation.
2、 Instrument performance and calibration
1. Sensors and optical systems: Sensors are affected by the concentration of electrolyte solution, electrode materials, and environmental temperature and humidity, which may lead to a decrease in sensitivity; The fluctuation of light source intensity or wavelength shift in a spectrophotometer can affect absorbance measurement, directly affecting the accuracy of detection results.
2. Temperature control system: The digestion reaction requires precise temperature control, and deviations exceeding ± 2 ℃ may cause changes in reaction rate, thereby affecting COD results.
3. Colorimeter and detector: The cleanliness and material differences of the colorimetric dish can affect the optical path and transmittance; Excessive detector noise levels can mask weak signals and increase errors in low concentration samples.
3、 Operation process and human error
1. Control of digestion conditions: Insufficient reaction time can lead to incomplete oxidation, while prolonged time may cause secondary oxidation of inorganic substances; Insufficient mixing can result in absorbance deviation due to local temperature gradients or precipitation stratification.
2. Reagent preparation and dosage: The concentration deviation of potassium dichromate directly affects the calculation results; Improper ratio of sulfuric acid silver sulfate catalyst may catalyze side reactions and interfere with the measurement.
3. Sampling and pipetting operations: Volume errors in pipetting or the introduction of bubbles may cause actual concentration deviations, affecting the accuracy of the digestion system.
4、 Environmental conditions and external disturbances
1. Temperature and humidity fluctuations: Laboratory temperature fluctuations can affect the kinetics of digestion reactions, and high humidity may cause reagents to absorb moisture, change sulfuric acid concentration, or precipitate potassium dichromate crystals.
2. Electromagnetic interference: The unstable power supply voltage of the instrument or the strong electromagnetic field nearby may interfere with the signal processing circuit, leading to data drift.
3. Cross contamination: If the high concentration samples are not cleaned thoroughly after continuous measurement, residual organic matter may contaminate subsequent low concentration samples, resulting in a "memory effect".
5、 Limitations of Chemical Reagents and Methods
1. Purity and expiration date of reagents: Expired reagents or impurities can change the oxidation ability or blank value; The standard curve needs to be validated regularly, and if it exceeds the linear range, the sample needs to be diluted.
2. Inherent defects of the method: The potassium dichromate method may oxidize some inorganic substances, leading to false elevation; The rapid detection method has weak anti-interference ability for complex water quality.
The accuracy of COD monitoring instruments is affected by multiple factors, and comprehensive measures such as standardized operation, regular calibration, optimized preprocessing, and environmental control are needed to improve data reliability.