Regarding standard solutions

Please describe the problem in more detail

1. Standard solution: A solution whose mass concentration (or molar concentration) is known with accuracy. 2. Purpose of a standard solution: To be used for titrating the concentration of other solutions； 3. Preparation methods: direct method and indirect method ； 4. Direct method: Based on the desired mass concentration (or molar concentration), a precise amount of the substance is weighed. After it is dissolved, it is transferred quantitatively into a volumetric flask and diluted to the mark. The accurate mass concentration (or molar concentration) of the standard solution can then be determined through calculation. This solution is also known as a standard solution. The substance used to prepare this solution is called a Standard Substance. The requirements for reference materials are: ① high purity, with the mass fraction of impurities below 0.02%, and they should be easy to prepare and purify ； ② The composition (including crystal water) exactly matches the chemical formula ； ③ It has stable properties, does not decompose, does not absorb moisture, does not absorb CO2 from the atmosphere, and does not lose its crystalline water ； ④ It has a higher molar mass in order to reduce the relative error in weighing. 5. Indirect method (also known as calibration method): If the reagent used to prepare a standard solution is not a primary standard, the direct method cannot be used for its preparation. (Note: Although reagents of grade reagent or analytical grade have high purity, their composition may not correspond to the chemical formula, and they may not be suitable for use as references.) The indirect preparation method involves first preparing a solution with a mass concentration (or concentration) that is approximately equal to the desired value, and then using a standard substance to determine the exact mass concentration (or concentration) of the prepared solution through titration; this process is known as calibration.