Every teacher engaged in environmental monitoring work knows that according to the "Technical Regulations for the Preservation and Management of Water Quality Sampling Samples" issued by the National Environmental Protection Standard HJ 493-2009 of the People's Republic of China, natural water, domestic sewage, and industrial wastewater are included. When the collected water samples (instantaneous or mixed) cannot be immediately analyzed on site and must be sent to the laboratory for testing. Samples of various water qualities are stored from collection to analysis. Due to the varying degrees of changes caused by physical, chemical, and biological processes, the samples used for analysis are no longer the same as those used for sampling. In order to minimize this change, protective measures must be taken for the sample during sampling. Different substances need to be added to the collected sample to adjust its pH value to the ideal state according to different sample categories and detection items. After returning to the laboratory, some water samples had to be manually adjusted back to neutral. Before Changsha Sichen Instrument developed the pH automatic control liquid addition system, this work was done manually, which was time-consuming, inefficient, had large errors, and sometimes wasted materials, affecting the detection data results. Since the launch of the SC-100A/200A/300A series products by the company in 2013, these issues have been completely resolved. Here are some ideal pH values that need to be adjusted for different testing items specified in the standards compiled by the author, for reference only by teachers

The commonly used method for measuring metal ions in water samples is to acidify them with nitric acid to pH 1-2. This can prevent the hydrolysis and precipitation of heavy metals, as well as the adsorption of metals on the wall surface. At the same time, it can also inhibit biological activity in acidic media with pH 1-2. By using this method for storage, most metals can remain stable for several weeks or months. The water sample for cyanide determination needs to be adjusted to pH 12 with sodium hydroxide. The water sample for measuring hexavalent chromium should be adjusted to pH 8 with sodium hydroxide, as hexavalent chromium has a high oxidation potential in acidic media and is easily reduced. To preserve the water sample of total chromium, nitric acid or sulfuric acid should be added to pH 1-2. To inhibit biological effects, inhibitors can be added to the sample. In the 3HJ 493-2009 water sample for measuring ammonia nitrogen, nitrate nitrogen, and COD, mercuric chloride or trichloromethane and toluene are added as protective agents to inhibit the redox reactions of organisms to nitrite, nitrate, and ammonium salts. Adjust the pH value of the solution with phosphoric acid in the phenol water sample, and add copper sulfate to control the activity of phenol decomposing bacteria.