1 Preparation method of standard solution and reference substance Standard solution refers to a solution with known accurate concentration, which is one of the basis for quantitative calculation in titration analysis. No matter what titration method is used, the standard solution is inseparable. Therefore, correctly preparing the standard solution, determining its accurate concentration, and properly storing the standard solution are all related to the accuracy of the titration analysis results. There are generally two methods for preparing standard solutions: 1.1 Direct preparation method Accurately weigh a certain amount of substance with an analytical balance, dissolve it in an appropriate amount of water and transfer it to a volumetric flask quantitatively, dilute to the mark, dilute to volume and shake well. Calculate the exact concentration of the solution based on the mass of the solute and the volume of the volumetric flask. A substance that can be used to directly prepare a standard solution is called a reference substance or a reference reagent. It is also a reference substance used to determine the exact concentration of a solution. As a reference substance, it must meet the following requirements: (1) The reagent must have a sufficiently high purity, generally requires its purity to be above 99.9%, and the impurities contained should not affect the accuracy of the titration reaction. (2) The actual composition of the substance is completely consistent with its chemical formula. If it contains crystal water (such as borax Na2B4O7.10H2O), the number of crystal water should also be completely consistent with the chemical formula. (3) The reagent should be stable. For example, it is not easy to absorb moisture and carbon dioxide in the air, it is not easy to be oxidized by air, and it is not easy to decompose when heated and dried. (4) The reagents should preferably have a larger molar mass, which can reduce the weighing error. Commonly used reference materials are pure metals and certain pure compounds, such as Cu, Zn, Al, Fe and K2Cr2O7, Na2CO3, MgO, KBrO3, etc. Their content is generally above 99.9%, even up to 99.99%. It should be noted that although some high-purity reagents and spectrally pure reagents have high purity, they can only show that the impurity content is very low. Because it may contain water and gas impurities of uncertain composition, its composition and chemical formula may not exactly match, so that the content of the main component may not reach 99.9%, and it cannot be used as a reference substance at this time. Some commonly used reference substances and their application range are listed in Table 2.1. Table 2.1 Drying conditions and applications of common reference materials Reference substance Composition after drying Drying condition ℃ Calibration object name Chemical formula Sodium bicarbonate NaHCO3 Na2CO3 270─300 acid Sodium carbonate decahydrate Na2CO3 · 10H2O Na2CO3 · 10H2O 270─300 acid Borax Na2B4O7 · 10H2O Na2B4O7 · 10H2O Placed with NaCl And sucrose saturated solution In a closed vessel acid Oxalic acid dihydrate H2C2O4 · 2H2O H2C2O4 · 2H2O Air drying at room temperature Alkali or KMnO4 Potassium hydrogen phthalate KHC8H4O4 KHC8H4O4 110─120 Alkali Potassium dichromate K2Cr2O7 K2Cr2O7 140─150 reducing agent Potassium bromate KBrO3 KBrO3 130 reducing agent Sodium oxalate Na2C2O4 Na2C2O4 130 KMnO4 Calcium carbonate CaCO3 CaCO3 110 EDTA Zinc Zn Zn Store in a room temperature dryer EDTA Sodium chloride NaCl NaCl 500-600 AgNO3 Silver nitrate AgNO3 AgNO3 220-250 chloride 1.2 Indirect preparation method (calibration method) Many reagents required to prepare standard solutions cannot fully meet the necessary conditions for the above reference materials, for example: NaOH is very easy to absorb carbon dioxide and moisture in the air, and the purity is not high; the accurate content of HCl in commercially available hydrochloric acid is difficult to determine and easy Volatilization; KMnO4 and Na2S2O3 are not easy to purify, and decompose in light, unstable in air, etc. Therefore, this kind of reagent cannot be prepared by direct method, but only by indirect method, that is, it is first prepared into a solution close to the required concentration, and then the reference substance (or the standard solution of another substance) is used to determine its accurate concentration. . This operation to determine its exact concentration is called calibration. For example, if you want to prepare a 0.1mol.L-1HCl standard solution, first dilute it with a certain amount of concentrated HCl and water to prepare a dilute solution with a concentration of about 0.1mol? L-1, and then use this solution to titrate accurately weighed anhydrous Na2CO3 Reference substance, until the quantitative reaction between the two is complete, then calculate the exact concentration of the HCl solution based on the volume of the HCl solution consumed in the titration and the mass of anhydrous Na2CO3. The exact concentration of most standard solutions is determined by the calibration method. In the determination of constant components, the concentration of the standard solution is roughly in the range of 0.01 mol.L-1 to 1 mol.L-1, and the concentration of the standard solution is usually selected according to the level of the component to be measured. In order to improve the accuracy of calibration, the following points should be noted during calibration: ⑴ Calibration should be measured in parallel 3 to 4 times, repeated at least three times, and the relative deviation of the measurement results should not exceed 0.2%. ⑵ In order to reduce the measurement error, the amount of the reference substance should not be too small, at least 0.2g should be weighed; the volume of the standard solution consumed at the end of the titration should also not be too small, preferably more than 20mL. (3) The measuring instruments used in the preparation and calibration of solutions, such as burettes, volumetric flasks and pipettes, etc., should be corrected in volume when necessary, and the influence of temperature should be considered. ⑷ The calibrated standard solution should be stored properly to avoid changes in the concentration of the solution due to water evaporation; some are not stable enough. For example, the standard solutions such as AgNO3 and KMnO4 that are easily decomposed by light should be stored in brown bottles and kept in a dark place. ; A strong alkaline solution that can absorb carbon dioxide in the air and have a corrosive effect on the glass, it is best to install it in a plastic bottle, and install a soda lime tube at the mouth of the bottle to absorb carbon dioxide and water in the air. For unstable standard solutions, after a long period of storage, the concentration must be recalibrated before use. 2 Standard solution concentration method 2.1 The amount of substance concentration (c, referred to as concentration) The mass concentration of a substance refers to the amount of a substance containing solute B in a unit volume of solution, which is represented by the symbol cB. which is cB = nB / VB (2.1) Because nB = mB / MB (2.2) So mB = cBVBMB (2.3) In formula (2-1), B represents the chemical formula of solute, nB is the amount of substance of solute B, its SI unit is mol, VB represents the volume of the solution, and its SI unit is m3; so the amount of substance concentration cB SI The unit is mol.m-3, and the commonly used unit in analytical chemistry is mol.L-1 or mol.dm-3. In formula (2-2), MB is the mass of substance B, the common unit is g, MB is the molar mass of substance B, its SI unit is Kg.mol-1, and the commonly used unit in analytical chemistry is g.mol -1. In this unit, the molar mass of any atom, molecule or ion is numerically equal to its relative atomic mass, molecular mass or relative ion mass. 2.2 Titer (T) In industrial production, because the measurement object is relatively fixed, the same standard solution is often used to measure the same substance, so the titer is often used to indicate the concentration of the standard solution, making the calculation simple and fast. Titer refers to the mass of 1mL standard solution equivalent to the substance to be tested (unit is g.mL-1 or mg.mL-1), which is represented by the symbol TA / B. Where A is the substance to be tested and B is the titrant. For example, the 1.00mL K2Cr2O7 standard solution happens to completely react with 0.005682g Fe, then the titer of this K2Cr2O7 solution to Fe TFe / K2Cr2O7 = 0.005682g / mL-1. In the actual production process, it is often necessary to determine the same component in a large number of samples. If the concentration of the standard solution is expressed by titer, it is very simple to calculate the content of the measured component. As in the above example, if the volume V (mL) of the K2Cr2O7 standard solution is known to be consumed during the titration, the mass mFe of the measured component is: mFe = TFe / K2Cr2O7 / V ZENGHUI PAPER PACKAGE is a good Paper Bags manufacturers and
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Standard solution preparation method and reference substance