The sewage COD detector plays an important role in water quality monitoring due to its diverse working principles and advantages, providing strong technical support for environmental protection and pollution control.

1. Potassium dichromate oxidation method: In a strongly acidic solution, potassium dichromate is used as the oxidant, silver sulfate as the catalyst, and mercury sulfate as the chloride ion masking agent to heat and digest the water sample. During this process, organic matter in the water sample is oxidized by potassium dichromate, while hexavalent chromium in potassium dichromate is reduced to trivalent chromium. Afterwards, the content of trivalent chromium was determined by colorimetric method, and the COD value was calculated based on this. This method has a wide measurement range and high accuracy, but it has the disadvantages of long digestion time and the use of reagents with certain toxicity.

2. Electrochemical method: using electrodes to undergo oxidation-reduction reactions with organic matter in water samples, resulting in changes in current or potential. Determine the COD content by measuring the changes in these electrical signals. This method has fast response speed, easy operation, and can achieve real-time online monitoring. However, its measurement range is relatively narrow and is subject to many water quality interference factors.

3. UV absorption method: It belongs to physical detection method, does not require chemical reagents, and avoids secondary pollution. The hardware system extracts water samples to the measurement tank through an external pump, and uses photodiodes to convert signals of specific wavelengths (such as visible light 546nm and ultraviolet light 254nm) into electrical signals. After processing by an embedded computer, the data is displayed or transmitted. The instrument adopts a dual beam detection structure and achieves index conversion based on the correlation between UV absorbance and chemical oxygen demand.

Measurement steps of sewage COD detector:

1. Preparation and instrument inspection before startup

-Reagent preparation: Confirm that the reagents are complete and meet the requirements, including COD specific digestion agent, catalyst, blank solution (ultrapure water), COD standard solution (for calibration, if necessary). Verify the expiration date of the reagent to ensure no deterioration, stratification, or precipitation.

-Sample pretreatment: Prepare the water sample for testing. If the water sample is turbid, it needs to be filtered in advance (using a 0.45 μ m filter membrane) to remove suspended solids interference. For high turbidity water samples, further purification can be achieved through centrifugation or filtration; In the face of high chloride water samples (chloride ions>1000mg/L), sufficient amount of mercuric sulfate must be added as a masking agent to eliminate interference.

-Equipment status verification: Ensure that the instrument has been calibrated and zeroed, and operate according to the instructions. Check the smoothness of the pipeline, whether there are leaks, and keep the instrument clean.

2. Accurate measurement of samples and reagents

-Use clean pipettes to aspirate a specified volume (e.g. 5mL, according to the instrument manual) of blank solution and water sample, and inject them into dedicated digestion tubes in sequence. When pipetting, it is necessary to avoid liquid residue, and each pipette should only be used for a single liquid to prevent cross contamination.

3. Standardized operation of digestion reaction

-Place the digestion tubes containing blank solution and water sample into the instrument digestion module in sequence, ensuring that they are securely placed without tilting or leaking. According to the type of water sample and instrument requirements, set the digestion temperature (usually 165 ℃) and digestion time (usually 20 minutes), and start the digestion program. This process requires strict control of temperature fluctuations to ensure the oxidation reaction.

4. Instrument calibration and sample testing

-Blank calibration: Wipe the outer wall of the digestion tube containing cooled blank solution (to remove water droplets and fingerprints), and place it in a colorimetric bath to perform the blank calibration procedure.

-Span calibration: Replace the digestion tube containing COD standard solution, perform span calibration, and ensure that the calibration error is within the allowable range (usually ± 5). If it is for the first time or has not been calibrated for a long time, it is recommended to repeat it multiple times to improve accuracy.

-Sample measurement: After calibration, test the actual water sample in sequence and record signal values such as absorbance or potential difference.

5. Data recording and shutdown organization

-Fill in the testing information in the dedicated record form, including sample name, testing date, ambient temperature, reagent batch, blank value, standard solution concentration, sample testing value, and average value. Clean the colorimetric dish, digestion tube and other components in a timely manner after the detection is completed to prevent residual reagents from corroding the instrument; Disconnect the power supply and take measures to prevent dust and moisture.