Compared with ordinary reagents, pure water/ultrapure water, due to its high-purity characteristic, has many differences in its testing methods from the similar testing of ordinary reagents. During the process of laboratory pure water quality testing, various problems may be encountered. To address the many doubts that may arise when you test water quality by yourselves, we have now summarized the common water quality issues we frequently encounter for your reference. Q1: How to measure resistivity? Resistivity shall be tested in accordance with the requirements of water quality standards and testing methods; different water quality requirements correspond to different resistivity testing methods. During the ultrapure water testing process, attention should be paid to the following points: on-line testing is required, the temperature compensation function of the conductivity meter should be turned on, the accuracy of temperature compensation should reach 0.1℃, and sufficient flushing should be performed until the conductivity meter stabilizes before reading the data. Q2: Why does the data keep fluctuating when testing conductivity? Are there any requirements for the electrode? The contact container may have ion migration and dissolution, which affects the resistivity test; in addition, the dissolution of CO₂ in the air into water can also cause changes in resistivity. There are four parameters that affect the change of resistivity: temperature, ion concentration, ion charge-carrying capacity, and ion type. Q3: Do we have to test the conductivity immediately after collecting the water sample? Will the conductivity decrease as the storage time increases? An on-line conductivity meter should be used. Ultrapure water will deteriorate continuously after being stored; in particular, the dissolution of CO₂ will lead to the generation of carbonate, resulting in an increase in conductivity and a decrease in resistivity. Q4: Can a plastic cup be used to test conductivity? It can be used for water with a resistivity of below 10 MΩ·Cm, but it is recommended to minimize the time between water collection and testing to avoid environmental impacts; on-line testing is recommended for high-purity water. Q5: Is it necessary to send the prepared Grade 1 water to a third-party institution for verification? Laboratories with testing conditions and qualifications can conduct self-testing; laboratories that do not meet the testing conditions can choose to send the water sample for testing. Q6: How to conduct regular testing and acceptance for laboratory water in CMA laboratories? Testing and acceptance shall be carried out in accordance with the requirements of CL10. Q7: Can the parameters displayed on the ultrapure water monitoring interface be used as review evidence? Yes, but it is necessary to provide the calibration certificates of the relevant instruments. For the conductivity meter, you can choose the traceable conductivity calibration service provided by the manufacturer and keep the relevant calibration records. Q8: Is it necessary to verify the ultrapure water device? Since the consumables of the pure water device are gradually consumed during use, regular water quality testing is required to ensure the stability of water quality and the reliability of experimental results; the generally recommended testing cycle is one year. Q9: Which items need to be tested for ultrapure water used in laboratory testing? The widely applicable laboratory pure water standards are GB/T 6682-2008 and GB/T 33087-2016, which are only general standards. The core indicators or parameters shall ultimately refer to different applications or industries; there are more standards for pure water in various industries to meet the needs of certain applications within the industry. For specific application requirements, you can contact our application support team. Q10: Do we have to test the conductivity immediately after collecting the water sample? Will the conductivity decrease as the storage time increases? An on-line conductivity meter should be used. Ultrapure water will deteriorate continuously after being stored, leading to an increase in conductivity and a decrease in resistivity. Q11: What is the difference in results between on-line and off-line TOC testing methods? The impact of interference factors cannot be directly quantified. Environmental cleanliness and air contact time during the off-line process are important interference factors; on the other hand, the use of TOC meters with different accuracies and operational errors of laboratory personnel can also cause differences. On-line testing is recommended. Q12: Is distilled water classified as Grade 3 water? Distilled water is named according to the traditional purification process. Due to differences in the quality of raw water, the lack of clear water quality standards and preparation process requirements, there may be differences between different batches of distilled water. It is necessary to compare and determine distilled water with reference to the water quality standards of Grade 3 water.