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A standard solution is a solution whose concentration of the main component or other quantitative value has been determined. Different situations require different standard solutions; one must never make generalizations. Let’s take a look below~ Table 1: Standard solutions commonly used in chemical analysis. Name of standard solution, method of expressing concentration, common units of concentration, number of significant figures, method of preparation, and purpose. Standard solutions for titrimetric analysis (standard titrant solutions): Molar concentration (CB), in mol/L; 4 significant figures. 1. Reference substances can be prepared directly. 2. Non-reference substances are prepared using calibration methods. Primarily used for the analysis of major components and constant elements in samples. Standard solutions for determining impurities (including elemental standard solutions and standard comparison solutions): Mass concentration (ρB), in mg/mL or μg/mL; 2 significant figures, up to 3. 1. Prepared directly using reagents of high purity. 2. Prepared directly using highly pure reagents or specialized reagents. Primarily used for the quantitative analysis of trace, semi-trace, or limited amounts of components in samples. Standard buffers for pH measurement: Molar concentration (CB); 3 significant figures. These have accurate values and are prepared from pH reference reagents. Used for calibrating (setting the scale) pH meters. Preparation of standard solutions for titrimetric analysis: 1. General requirements (GB/T 601 “Preparation of Standard Solutions for Titrimetric Analysis”). GB/T 601-2016 specifies the general requirements for preparing standard solutions for titrimetric analysis: Serial number, item, specific requirements. 1. Purity of reagents/experimental water: Analytical grade or higher; should meet the specifications for grade 3 water as defined in GB/T 6682. 2. 1. Temperature at which standard titrant solutions are prepared. 2. Analytical balances, weights, burettes, volumetric flasks, single-mark pipettes. 1. The concentration of standard titrant solutions refers to the concentration at 20°C. If there is a temperature difference during use, corrections should be made according to Appendix A of GB/T 601. 2. All of these instruments must be calibrated regularly. 3. During calibration and use, the titration rate should generally be maintained between 6 mL/min and 8 mL/min. 4. When weighing reference reagents, the mass should be less than or equal to 0.5 g; precision should be to 0.01 mg ； Greater than 0.5g: Weigh to an accuracy of 0.1mg. 5 During preparation, the concentration value should be within ±5% of the specified value. 6 The concentration of the standard titration solution must be determined by two persons, with each performing four replicates ； For the two individuals, the relative value of the extreme difference among the eight parallel measurements shall not exceed 0.18% as the critical relative value for extreme differences (the requirement for four parallel measurements per individual is no more than 0.15%). The average of the eight parallel measurements for the two individuals is taken as the measurement result. Five significant figures are retained during the calculation, while the reported concentration value is given with four significant figures. 7. The expanded uncertainty of the average concentration should generally not be greater than 0.2%. 8. Preparation of standard solutions with low concentrations: When preparing standard solutions with a concentration of ≤0.02 mol/L: 1. Dilute them with a concentrated standard solution using freshly boiled and cooled water just before use; 2. Re-calibrate if necessary. 9. The material of the containers used for storing standard titration solutions should not react chemically or physically with the solution, and the thinnest part of the container wall should be at least 0.5 mm thick. 10. Storage period: At room temperature (15°C–25°C), the storage period generally should not exceed 2 months; if the solution becomes turbid, it should be prepared again. 2. Preparation and calibration methods: (1) Direct preparation method: Reference substance → Drying treatment → Weighing on an analytical balance → Dissolving in pure water → Transferring to a volumetric flask (which has been calibrated) → Diluting with pure water to the mark → Stirring well. (2) Calibration by standardization: ① Non-reference substances are first prepared as solutions with a concentration that is approximately (slightly higher than) the desired concentration ； ②The accurate concentration is then determined using a reference substance; this process is called calibration. Three methods: including direct titration, indirect titration, and comparison method. 3. Substances that can be used to directly prepare or standardize standard titration solutions are called primary standards. Reference materials must meet the following requirements: ① The material must have sufficient purity, with a purity level of over 99.9% ； And the impurity content should be below the error limit permitted by titrimetric analysis ； ②The composition of the substance (including its crystal water content) should be constant and consistent with the chemical formula ； ③The reagent has stable properties and is not prone to absorbing moisture or carbon dioxide from the air, nor to undergoing other chemical changes ； ④It has a relatively high molecular weight. Substances that can meet the above requirements are called reference substances, such as anhydrous sodium carbonate, potassium hydrogen phthalate, potassium dichromate, zinc oxide, calcium carbonate, etc. For reagents such as hydrochloric acid, sodium hydroxide, potassium permanganate, and sodium thiosulfate, which do not meet the requirements of reference materials, the direct method cannot be used to prepare standard titration solutions; instead, the indirect method must be employed. 4. Precautions for preparing standard titration solutions When preparing and calibrating titration solutions, it is necessary to minimize errors during the procedure. The most important points are: (1) The mass of the sample should not be too small in order to ensure the accuracy of the analysis results. The weighing error of a typical analytical balance is ±0.0002 g; therefore, the sample mass must be greater than 0.0002 g. The reading of the burette often has an error of ±0.02 mL, so the volume of titrant consumed must be over 20 mL. (2) Calibrated instruments should be used. Generally, the equipment and measuring instruments used, such as burettes, volumetric flasks, pipettes, etc., should be relatively calibrated. (3) The experimental conditions for standardizing the standard titrant solution when determining the components of the sample should be kept as consistent as possible in order to offset systematic errors during the experiment. Preparation of standard solutions for impurity determination GB/T602 – General requirements for the preparation of standard solutions used for impurity determination. Sequence Number | Item | Specific Requirements: 1. The water used for preparing standards shall meet at least the specifications for grade 2 water as defined in GB/T6682. 2. The purity of the reagents used should be of ultra-pure grade. 3. The volume transferred by the pipette each time should be such that 0.05 mL ≤ V < 2.00 mL. 4. The shelf life under normal temperature conditions (15–25°C) is generally 2 months ； Reprepare the solution when turbidity, precipitation, or color changes occur. Precautions for preparing solutions: 1. The solutions used in analytical experiments should be prepared with pure water; the containers must be washed with pure water at least three times. For solutions with special requirements, a blank test using pure water should be conducted first. 2. The solution should be stored in a reagent bottle with a stopper ； Solutions that decompose easily when exposed to light should be stored in brown bottles ； For solutions prepared with volatile reagents (such as organic reagents), the bottle caps must be tightly sealed ； Solutions that deteriorate easily when exposed to air and release corrosive gases should also be kept tightly covered; for long-term storage, they should be sealed with wax ； Concentrated alkali solutions should be stored in plastic bottles; if stored in glass bottles, they must be sealed with rubber stoppers, not glass stoppers. 3. Each reagent bottle must have a label indicating the name, specifications, concentration, and preparation date. 4. When preparing solutions of sulfuric acid, phosphoric acid, nitric acid, hydrochloric acid, etc., the acid should always be poured into water. Reagents that release a large amount of heat upon dissolution should not be prepared in reagent bottles to avoid explosion. When preparing a sulfuric acid solution, concentrated sulfuric acid should be added to water in small amounts slowly, while stirring continuously; if necessary, the outer wall of the beaker should be cooled with cold water. 5. When preparing solutions using organic solvents (for example, when making indicator solutions), the organic substances may dissolve slowly; in such cases, stirring from time to time is necessary. The solution can also be warmed in a hot water bath, but direct heating should be avoided. Flammable solvents must be kept away from open flames when in use. Almost all organic solvents are toxic and should be handled in a fume hood. Unnecessary evaporation of organic solvents should be avoided, and the beaker should be covered. 6. It is necessary to be familiar with the preparation methods of some common solutions. For iodine solutions, iodine should be dissolved in a concentrated aqueous solution of potassium iodide before dilution. To prepare an aqueous solution of salts that are prone to hydrolysis, it is necessary to first dissolve them in acid and then dilute the solution with a dilute acid of appropriate concentration. For example, when preparing a SnCl2 solution, if hydrolysis has already occurred due to improper handling, adding a large amount of acid still fails to dissolve the precipitate. 7. Do not touch corrosive and highly toxic solutions with your hands. Highly toxic waste liquids must be detoxified before being discharged directly into the sewer system.