Determination of the mass fraction of acidity

Thread Content

This post was last edited by liaifeng on 2018-8-15 at 18:36. Chapter 6: Determination of the mass fraction of acidity 1 Principle: Acidic substances in sulfur are extracted using a water-isopropyl alcohol mixture, and then titrated with a standard sodium hydroxide solution, using phenolphthalein as an indicator. 2 Reagents: The distilled water used in this experiment should also be boiled and cooled before use. (1) Isopropyl alcohol: analytical grade. (2) Sodium hydroxide standard titration solution: (NaOH)=0.05 moL/L. A standard titration solution with (NaOH) = 0.5 moL/L was prepared and standardized in accordance with GB/T 601-2002, and this standard titration solution was then diluted 10 times to obtain the desired concentration. (3) Phenolphthalein indicator: 10 g/L. 3 Analysis steps: Weigh approximately 25 g of the sample, with an accuracy of 0.01 g, and place it in a 250 mL conical flask equipped with a ground glass stopper. Add 25 mL of isopropanol, seal the flask so that the sulfur is completely moistened, then add 50 mL of water. Seal the flask again and shake it for 2 minutes; after that, let it stand for 20 minutes, shaking it from time to time. Add 3 drops of phenolphthalein indicator, and titrate with standard sodium hydroxide solution until a pink color appears, which should be maintained for 30 seconds without fading. Conduct a blank test at the same time. 4 Result Calculation The mass fraction of acidity is calculated based on the mass fraction of sulfuric acid (H2SO4), expressed as a percentage, using formula (8-5): = (8-5) Where: —— is the volume of the standard sodium hydroxide titrant solution used in the measurement, in mL ； ——Volume of sodium hydroxide standard titrant solution used in the blank test, mL ； ——The exact value of the sodium hydroxide standard titration solution, in mol/L ； ——Mass of the sample, g ； ——Measured mass fraction of moisture, % ； ——The molar mass of sulfuric acid is (98.08), g/mol. The arithmetic mean of the parallel measurement results is taken as the measurement result. Table 8-4: The absolute difference between the results of parallel measurements shall meet the specified requirements. Mass fraction of acidity/% Absolute difference between parallel measurement results/% ≤0.0020 ≤0.0003 >0.0020～≤0.0060 ≤0.0004 >0.0060～≤0.020 ≤0.002 >0.020 ≤0.003