Thread Content
1.0 Referenced Standards 1.1 GB/T601-88: Preparation of standard solutions for titrimetric analysis (volumetric analysis) of chemical reagents; 1.2 GB/T4701.7-1994: Chemical analysis methods for ferrophosphorus; 1.3 GB/T 3885.4-1983: Chemical methods for lithium mica and spodumene concentrates – Determination of lithium content by flame atomic absorption spectrometry. 2.0 Determination of iron content 2.1 Method summary: In hydrochloric acid solution, iron(III) is reduced to iron(II) using tin(II) chloride, after which mercuric chloride is added to oxidize any excess tin(II) chloride. Sodium diphenylamine sulfonate is used as an indicator, and a potassium dichromate standard solution is used for titration. The reaction equations are as follows: 2Fe3+ + Sn2+ + 6Cl- → SnCl62- + 2Fe2+; 4Cl- + Sn2+ + 2HgCl2 → SnCl62- + Hg2Cl2; 2Fe2+ + Cr2O72- + 14H+ → 6Fe3+ + 2Cr3+ + 7H2O. 2.2 Reagents 2.2.1 Hydrochloric acid: (1+1); 2.2.2 Sulfuric acid–phosphoric acid mixture: 150 ml of sulfuric acid is slowly added to 500 ml of water; after cooling, 150 ml of phosphoric acid is added, and the mixture is diluted to 1 L with water; 2.2.3 Tin(II) chloride solution (100 g/L): 10 g of tin chloride is dissolved in 10 ml of (1+1) hydrochloric acid, and the solution is then diluted to 100 ml with water (if the solution becomes cloudy, it needs to be filtered)； 2.2.4 Sodium diphenylamine sulfonate indicator (0.5%): 2.2.5 Saturated solution of mercuric chloride [C(1/6K2Cr2O7) = 0.0500 mol/L]: Weigh 2.4518 g of potassium dichromate (a primary standard), which has been dried at 150 degrees for 1 hour, and place it in a 250 ml beaker. Dissolve it in a small amount of water and then transfer the solution to a 1 L volumetric flask, filling it to the mark with water. 2.3 Analytical procedure: Weigh 0.2000 g of the sample into a 250-ml Erlenmeyer flask. Add 10 ml of hydrochloric acid solution, then heat on a low-temperature electric furnace until complete dissolution. Remove from heat and allow it to cool slightly. Add 30 ml of water and bring to a boil. While hot, add tin(II) chloride solution dropwise until the yellow color disappears; then add 1–2 more drops in excess. Cool under running water to room temperature. Add 10 ml of saturated mercuric chloride solution, mix well, and let it stand for 2 minutes. Dilute with water to 80 ml. Add 20 ml of mixed sulfuric-phosphoric acid solution and 4–5 drops of sodium diphenylamine sulfonate indicator. Titrate with standard potassium dichromate solution; the endpoint is reached when the solution changes color from green to blue-violet. 2.4 Calculation: The percentage content of iron is calculated using the following formula: Fe(%) = C × V × 0.05584 × 100. Where: C is the concentration of the potassium dichromate standard solution, in mol/L; V is the volume of the potassium dichromate standard solution used in the titration, in ml; m is the mass of the sample taken, in g. The coefficient of 0.05584 is the molar mass of iron, expressed in g/mol. Precautions: (1) A sufficient amount of hydrochloric acid must be present for reduction; to ensure that all ferric ions are reduced to iron and to prevent ferrous ions from being oxidized again, stannous chloride must be used in slight excess. (2) The oxidation bymercuric dichloride is slow; after adding it, continuous stirring or shaking is required, along with leaving it for 2 to 3 minutes. Only after the reaction is complete can it be titrated with potassium dichromate. (3) Adding phosphorus sulfide mixed acid before titration is to prevent the yellow color of trivalent iron from masking the blue-violet endpoint. (4) If there is carbon powder in the sample, after dissolving the sample in hydrochloric acid, it must be filtered through filter paper, and the carbon powder must be washed with water 8–10 times. 3.0 Determination of phosphorus content 3.1 Method summary In a nitric acid medium, phosphorus reacts with ammonium molybdate to form a yellow precipitate of ammonium phosphomolybdate; after filtration, this precipitate is dissolved using a standard sodium hydroxide solution. Excess sodium hydroxide is then titrated with a standard nitric acid solution, using phenolphthalein as an indicator. The reaction is as follows: H3PO4 + 12(NH4)2MoO4 + 21HNO3 → (NH4)3PO4•12MoO3↓ + 21NH4NO3 + 12H2O; 2(NH4)3PO4•12MoO3 + 46NaOH → 2(NH4)2HPO4 + (NH4)2MoO4 + 23NaMoO4 + 22H2O; NaOH + HNO3 → NaNO3 + H2O. 3.2 Reagent preparation 3.2.1 Potassium nitrate solution: 20 g/L – Dissolve 20 g of potassium nitrate in 1 liter of boiled and cooled water, then stir well. 3.2.2 Ammonium molybdate solution: Solution A (prepared by dissolving 70 g of ammonium molybdate in 53 ml of ammonia and 267 ml of water) is slowly poured into Solution B (prepared by mixing 267 ml of nitric acid with 400 ml of water), then cooled and allowed to stand overnight. 3.2.3 Standard nitric acid solution C (HNO3) ≈0.1 mol/L: Take 7 ml of nitric acid and transfer it to a 1 L volumetric flask, then dilute to the mark with boiled and cooled water. 3.2.4 Sodium hydroxide standard solution C (NaOH) ≈ 0.1 mol/L: Weigh 4 g of sodium hydroxide (extra pure grade) and dissolve it in boiled and cooled water, then make up the volume to 1 liter. Calibration: Weigh 0.1000 to 0.2000 g of high-purity potassium hydrogen phthalate (KHC8H4O4), which has been dried at 120 degrees for 2 hours, and place it in a 250 ml conical flask. Add 50 ml of boiled and cooled water to dissolve the substance; then add 2 to 3 drops of 1% phenolphthalein indicator. Titrate with 0.1 mol/L NaOH standard solution until a pink color is reached, at which point the titration is complete. C(NaOH) = m × 1000 / (V1 × 204.2); f = C(NaOH) × 1.347 / 1000. Where: m – mass of potassium hydrogen phthalate, in grams; C(NaOH) – concentration of the standard sodium hydroxide solution, in mol/L; f – mass of phosphorus, in grams, equivalent to 1.00 ml of the standard sodium hydroxide solution ； 204.2 —— Molar mass of potassium hydrogen phthalate, M(KHC8H4O4), g/mol ； 1.347 —— molar mass of phosphorus, M(1/23P), g/mol. 3.3 Analytical procedure: Transfer 0.1000 g of the sample into a 250-ml beaker, moisten it with a small amount of water, add 15 ml of hydrochloric acid, cover with a watch glass, and heat on a hot plate until the sample is completely dissolved. Evaporate to near dryness, add 5 to 10 ml of nitric acid, evaporate to 4 to 5 ml, then dilute with a little water. Filter through medium-speed filter paper into a 500 ml conical flask. Wash the beaker 3 to 4 times with hot water, and wash the precipitate 8 to 10 times. The volume of the filtrate should remain around 100 ml at this point. Neutralize the filtrate with ammonia until a hydroxide precipitate appears, then neutralize it with nitric acid until the hydroxide precipitate disappears completely. Add 5 ml of excess nitric acid, and slowly add 60 to 100 ml of ammonium molybdate solution while shaking the conical flask. Shake for 2 to 3 minutes, and let the precipitate stand for more than 4 hours. Filter the pulp using filter paper. First, wash the conical flask and the precipitate 2 to 3 times with a 2% (V/V) nitric acid solution, then wash both the conical flask and the precipitate until they reach neutrality using a 20 g/L potassium nitrate solution. Transfer the precipitate together with the filter paper back into the original conical flask, add 30 ml of boiled and cooled water, and gently shake the flask to break up the filter paper into a paste. Add sodium hydroxide standard solution drop by drop until the yellow precipitate dissolves. Add 5 drops of 10 g/L phenolphthalein solution, then add 5 to 10 ml of excess sodium hydroxide standard solution. Wait for a short while, then add another 5 to 10 ml of excess sodium hydroxide standard solution. After another short wait, titrate the solution with 0.1 mol/L nitric acid standard solution; stop when the solution becomes colorless. A blank test should be conducted simultaneously with the analysis of the sample. 3.4 Calculation The phosphorus content expressed as a mass percentage is calculated using the following formula: (P %) = (V2 – V3K) / f × m0 Where: V2 – the total volume of the sodium hydroxide standard solution added, in ml ； V3 – the volume of standard nitric acid solution consumed in titrating the excess standard sodium hydroxide solution, in ml; m0 – the mass of the sample taken, in g; K – the coefficient for converting the volume of standard nitric acid solution to the volume of standard sodium hydroxide solution. Determination of K value: Draw 25 ml of nitric acid solution, add 50 ml of boiled and cooled water, add 2 to 3 drops of phenolphthalein indicator, and titrate with sodium hydroxide solution until a pink color appears, which indicates the end point. K = Volume of sodium hydroxide standard solution consumed in titration / Volume of nitric acid standard solution taken. Precautions: (1) The precipitate must be washed with 20 g/L potassium nitrate until it reaches neutrality; this should be checked by taking 20 drops of the filtrate in a test tube, adding 1 to 2 drops of phenolphthalein indicator, and then adding 1 drop of sodium hydroxide solution – the solution should turn red. (2) If vanadium (V) is present in the sample, a small amount of hydroxylamine hydrochloride can be added to reduce vanadium (V) to vanadium (IV). 4. Determination of lithium content 4.1 Summary of the method The sample is dissolved in hydrochloric acid. In a 1% (V/V) hydrochloric acid medium, the absorbance of lithium is measured at a wavelength of 670.78 nm using an air-acetylene flame in an atomic absorption spectrophotometer. 4.2 Reagents and Instruments 4.2.1 Nitric acid: concentrated nitric acid 4.2.2 Hydrochloric acid: 1+1 volume ratio 4.2.3 Lithium standard storage solution: 1 mg/ml, stored in plastic bottles. 4.2.4 Lithium standard solution: 100 μg/ml; accurately transfer 10.00 ml of the lithium standard stock solution into a 100 ml volumetric flask, dilute to the mark with water, and mix well. 4.2.5 Atomic absorption spectroscopy, with lithium hollow cathode lamp. 4.3 Analysis Steps 4.3.1 Measurement: Weigh 0.5000 g of the sample and place it in a 250 ml beaker. Add a small amount of water to moisten the sample, then add 25 ml of hydrochloric acid (1+1). Cover the beaker with a watch glass and heat to dissolve the sample; once the sample has decomposed, remove the cover and allow the mixture to cool slightly. Add 10 ml of nitric acid and continue heating to dissolve everything. Evaporate the solution until its volume is around 5 ml, then remove it from heat and let it cool slightly. Add 20 ml of water, bring to a boil, then cool the mixture. Transfer it to a 250 ml volumetric flask and dilute with water to the mark, stirring well. Transfer 5.00 ml of the sample solution to a 250 ml volumetric flask, add 5 ml of hydrochloric acid (1+1), dilute to the mark with water, and mix well. Measure its absorbance at 670.78 nm using an atomic absorption spectrometer under the same conditions as those used for the standard solution; the lithium concentration is then determined from the calibration curve. 4.3.2 Preparation of the working curve: Transfer 0, 1.00, 2.00, 3.00, 4.00, and 5.00 milliliters of the lithium standard solution (100 ug/ml) into a set of 100 ml volumetric flasks, add 2 ml of hydrochloric acid (1+1), dilute to the mark with water, and mix well. The absorbance is measured at a wavelength of 670.78 nm in atomic absorption spectroscopy under the same conditions and simultaneously with the sample; lithium concentration is used as the abscissa, while absorbance serves as the ordinate ； Draw the working curve. 5. Determination of carbon content 5.1 Summary of the method: The LiFePO4/C composite material is dissolved in hydrochloric acid; carbon, however, is insoluble in hydrochloric acid, thereby allowing for the separation of carbon from LiFePO4. 5.2 Reagents and materials 5.2.1 Hydrochloric acid: (1+1); 5.2.2 Hydrochloric acid: (5+95); 5.2.3 4G glass sintered crucible. 5.3 Analytical procedure Weigh 5.0000 g of the sample into a 250 ml beaker, add 60 ml of (1+1) hydrochloric acid, and heat on an electric stove to dissolve it. After about 20 minutes, remove from heat and allow it to cool. Filter the solution through a 4G glass sintered crucible that has been dried at 105–110°C to constant weight. Wash the residue five times with (5+95) hydrochloric acid, then wash it 5 to 10 times with distilled water. Finally, dry it at 105–110°C until a constant weight is achieved. 7.4 Calculation of analysis results: The carbon content is expressed as a mass percentage and is calculated using the following formula. C(%) = (m1 - m2) × 100 / m, where: m1 — mass of the precipitate and the glass sintered crucible, in g; m2 — mass of the glass sintered crucible, in g; m — weight of the sample, in g. Note: The reagents used in the above analysis are of analytical grade, and the water used is distilled water.