How are common standard solutions prepared?

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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. Table 1: Standard Solutions Commonly Used in Chemical Analysis
| Name of Standard Solution | Concentration Representation Method | Common Concentration Units | Number of Significant Digits | Preparation Method | Purpose |
|--------------------------|------------------------------------|----------------------------|------------------------------|--------------------|---------|
| Standard solution for titrimetric analysis (standard titrant) | Molar concentration (C), in mol/L | 4 digits | 4 | 1. Can be prepared directly from reference substances; 2. For non-reference substances, calibration is used | Mainly used for the analysis of major components and constant elements in samples |
| Standard solution for determining impurities (including elemental standard solutions and standard comparison solutions) | Mass concentration (ρ), in mg/mL or μg/mL | 2 digits, up to 3 digits | 2–3 | 1. Prepared directly from reagents of high purity; 2. Prepared directly from specially designed reagents | Mainly used for the quantitative, semi-quantitative, or limit analysis of trace components in samples |
| Standard buffer for pH measurement | Molar concentration (C) | 3 digits | 3 | Has accurate values; prepared from pH reference reagents | Used for calibrating (setting the zero point) pH meters |

1. General provisions for the preparation of standard solutions (GB/T 601 “Preparation of Standard Solutions for Titrimetric Analysis”)
GB/T 601-2016 specifies the general requirements for preparing standard solutions used in 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 | Temperature for preparing standard titrants | |
| 3 | Analytical balance, weights, burettes, volumetric flasks, single-mark pipettes | |
| 4 | Concentration of standard titrants | Refers to the concentration at 20°C; if there is a temperature difference during use, corrections shall be made according to Appendix A of GB/T 601 | All of these instruments must be calibrated regularly |
| 5 | Rate of titration during calibration and use | Should generally be maintained between 6 mL/min and 8 mL/min |
| 6 | Mass of reference reagents used for weighing | Less than or equal to 0.5 g; measurements should be accurate 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 ； The relative value of the extreme difference among the eight parallel measurements for the two individuals shall not be greater than 0.18% as the critical relative value of extreme difference (the requirement for four parallel measurements per individual is not greater 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 container used to store standard titration solutions should not react chemically or physically with the solution, and the thinnest part of the 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 → Dissolution in pure water → Transfer to a volumetric flask (already calibrated) → Dilution with pure water to the mark → Stir well. (2) Calibration method: Calibration: ① First, prepare a solution of the non-reference substance at a concentration that is approximately (slightly higher than) the desired concentration; ② Then use a reference substance to determine its exact concentration – this process is known as 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. The reference substance must meet the following requirements: ① The substance must have sufficient purity, with a purity level of over 99.9%; the content of impurities should be below the error limits permitted by titrimetric analysis; ② The composition of the substance (including its water of crystallization content) must be constant and consistent with its chemical formula; ③ The reagent should have stable properties, and should not absorb moisture or carbon dioxide from the air or undergo any other chemical changes; ④ It should have a high molar mass. 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 substances, 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 essential to minimize errors during the process. 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 burette reading often has an error of ±0.02 mL, so the volume of titrant consumed must be above 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 titration 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. Solutions should be stored in reagent bottles with stoppers; solutions that decompose easily when exposed to light must be kept in brown bottles; for solutions prepared from volatile reagents (such as organic reagents), the stoppers must be airtight; solutions that deteriorate easily when in contact with air or release corrosive gases also need to be tightly covered, and should be sealed with wax for long-term storage; concentrated alkali solutions should be stored in plastic bottles; if they are 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 outside 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 should 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 methods for preparing some common solutions. For iodine solutions, iodine should be dissolved in a concentrated potassium iodide aqueous solution before dilution. When preparing an aqueous solution of a salt prone to hydrolysis, the salt should first be dissolved in acid, and then diluted with a dilute acid of a certain concentration. For instance, when preparing a SnCl2 solution, if hydrolysis occurs due to improper handling, it becomes difficult to dissolve the precipitate even after adding a considerable amount of acid. 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.