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What are the methods for storing wastewater samples? Depending on the parameters to be measured, the containers used for collecting water samples are generally borosilicate glass bottles or polyethylene bottles. The method of storing water samples should be determined based on the different analysis requirements. (1) Fill the container or sample separately. During sampling, fill the sampling bottle with the sample so that there are no gaps above it, thereby reducing the shaking of the water sample during transportation. Sometimes, for certain special samples, it is necessary to take fixed-volume samples for storage; for example, this is done when measuring suspended solids. In this way, all of the samples can be used for analysis, preventing the samples from stratifying or adhering to the walls of the sampling bottles, which could affect the test results. (2) Refrigerate or freeze. Water samples should generally be stored refrigerated at 4°C; sometimes they can also be quickly frozen and kept in a dark place. Its function is to prevent biological activity, reduce physical volatilization, and slow down chemical reactions. For example, the refrigeration method can be used for preserving samples when measuring parameters such as conductivity, CODc, BOD5, ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, phosphates, sulfates, and microbial indicators. However, during the freezing process, solutes gradually accumulate toward the central solution and eventually concentrate at the center of freezing, resulting in stratification. Moreover, freezing can cause biological cells to rupture, allowing the chemical components within those cells to enter the aqueous solution and altering its composition; therefore, freezing should be avoided as much as possible when storing water samples. (3) Chemical protection. Adding certain chemical reagents to the water sample to stabilize or fix the properties of the components to be analyzed can ensure the accuracy of the results. However, it is important to ensure that the protective agent added does not interfere with subsequent measurements; corresponding blank tests should be conducted to correct the measurement results. If the protective agent is liquid, any resulting change in the volume of the water sample must be recorded.