Description of common problems in the detection process of water quality COD detector

- Aug 18, 2020-

Water sample collection


1. The sampling device and container should be prepared before sampling. The sampling devices currently in use mainly include buckets and various water collectors. If the sampling device is plastic or glass, it can be cleaned according to the general cleaning method of the container; if it is iron, it needs to be cleaned thoroughly with detergent, then rinsed with tap water and dried for later use. However, the water samples taken cannot be used for the analysis of metals such as iron and nickel.


2. The number of samples depends on the measurement items. Different cod detectors have different requirements for the consumption and storage conditions of water samples. Therefore, the required amount should be calculated according to the actual situation of the measurement item, and the actual sampling amount should be increased by 20~30%. If the volume of the sampling container is limited, and one sampling cannot meet the required sampling volume, you can take multiple samples, mix them in a larger container, and then load the sample container.


3. Sampling method The method of water sample collection has a lot to do with the analysis result. The water sample must be representative to truly reflect the water quality of the entire water body.


4. Before testing, the collected water samples should be shaken or mixed with other methods to avoid problems such as precipitation caused by long-term storage.

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5. Collection of water sample COD detectors during water system pollution investigations. When conducting pollution investigations on a water system, it is necessary to consider the entire water area, set up sampling points reasonably, and establish a water pollution monitoring network in order to report the pollution situation and the water area in a timely and accurate manner. The dynamic changes of various harmful substances. Second, the transportation and storage of water samples of various water quality, from collection to analysis and determination will pass a certain time, the time difference may be from a few minutes to a few hours, and sometimes even shelving for several days, due to changes in environmental conditions, microorganisms The influence of metabolic activity and chemical action will cause changes in some physical parameters and chemical components of the water sample, which will lead to inaccurate water quality testing results of the COD detector. In order to minimize these changes, it is necessary to shorten the transportation time as much as possible, analyze and determine as soon as possible, and take necessary protective measures; some items must be measured at the sampling site.


  (1) Transportation of water samples Every water sample collected should be recorded, and the sampling bottle should be labeled and transported to the laboratory. During transportation, the following points should be noted:


 1. Tighten the stopper of the sampling container, and seal it with sealing glue or paraffin if necessary (water samples for oil testing cannot be sealed with paraffin.


 2. In order to avoid loss or contamination of the water sample due to vibration or collision during transportation, it is best to pack the sample bottle and squeeze it tightly with foam or paper strips.

COD tester  

 3. Samples that need to be refrigerated should be equipped with a special insulated container, put refrigerant in, and place the sample bottle in it.


4. In winter, heat preservation measures should be taken to avoid freezing and cracking the sample bottle.


  (2) Storage of water samples The container for storing water samples may absorb the components to be tested, or the COD detector for sewage samples, so choose containers made of materials with stable performance and low impurity content. Commonly used container materials are borosilicate glass, quartz, polyethylene and polytetrafluoroethylene. Among them, quartz and PTFE contain little impurities, but they are expensive. Generally, containers made of polyethylene and borosilicate glass are widely used in routine monitoring. For water samples that cannot be transported in time or analyzed as soon as possible, appropriate storage methods should be adopted according to the requirements of different monitoring items. The transportation time of water samples is usually 24 hours as the maximum allowable time.


(3) Laboratory analysis stage Different types of water quality and different monitoring items have different sample pretreatment processes. Many monitoring items of groundwater and surface water can be directly measured without pretreatment. For the more polluted industrial wastewater and domestic sewage, different monitoring items of the COD detector may require pretreatment operations such as filtration, precipitation, digestion, distillation, and extraction, which must not be omitted. Otherwise, the analysis process and analysis results will have large errors. When analyzing each batch of samples, there is a standard sample from the National Environmental Monitoring Center or the Provincial Environmental Monitoring Center. Generally speaking, as long as the standard sample analysis result is within the allowable error range, the sample analysis is qualified, and the calibration of the meter is ignored. Generally speaking , The larger the capacity of the measure, the smaller the relative error of its capacity. Therefore, large-capacity containers should be used as much as possible. In addition, when choosing glassware, you should try to use A-grade glass measurers to avoid the use of different levels of measurers.

COD analyzer 

(4) Conclusion

As one of the important links of environmental protection, water quality testing should be paid attention to, not just to meet environmental protection requirements. We need to cherish this cause from the bottom of our hearts, strengthen our own operational capabilities and theoretical knowledge, and make water quality testing COD detectors more accurate.