How to determine the total iron content in water

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Reagents and materials (1) Sulfuric acid； (2) Ammonium ferric sulfate ； (3) Sulfuric acid: 1+35 solution ； (4) Ammonia water: 1+3 solution ； (5) Acetic acid-sodium acetate buffer solution (pH=4.5): Dissolve 82 g of sodium acetate in 300 mL of water, add 42 mL of acetic acid, and prepare a 500 mL solution ； (6) Ascorbic acid (20 g/L): Dissolve 10.0 g of ascorbic acid in 200 mL of water, add 0.2 g of disodium ethylenediaminetetraacetate (EDTA) and 8.0 mL of formic acid, dilute to 500 mL with water, mix well, and store in a brown bottle (validity period: one month) ； (7) O-phenanthroline solution (2.0 g/L): Weigh 0.20 g of o-phenanthroline and dissolve it in 100 mL of water; store it in a brown bottle in a dark place, away from light ； (8) Potassium persulfate solution (40.0 g/L): Dissolve 4.0 g of potassium persulfate in water and dilute to 100 mL; store in a brown bottle at room temperature (shelf life: one month) ； (9) Iron standard solution I (1 mL contains 0.100 mg of Fe): Weigh 0.863 g of ammonium ferric sulfate, to an accuracy of 0.001 g, and place it in a 200 mL beaker. Add 100 mL of water and 10.0 mL of sulfuric acid; after dissolution, transfer the entire mixture to a 1000 mL volumetric flask and dilute to the mark with water, then mix well. (Valid for two months) ； (10) Iron standard solution II (1 mL contains 0.010 mg Fe): Take 10 mL of iron standard solution I containing 0.100 mg Fe and dilute it to 100 mL; for use only on the same day. Instruments and equipment: Spectrophotometer: equipped with an absorption cell with a thickness of 1 cm. Analysis steps: (1) Preparation of the working curve. Take 0 mL (blank), 1.00 mL, 2.00 mL, 4.00 mL, 6.00 mL, 8.00 mL, and 10.00 mL of Iron Standard Solution II respectively and place them in 7 50 mL volumetric flasks. Add water up to approximately 20 mL, then add 0.50 mL of (1+35) sulfuric acid to adjust the pH to around 2. Add 3.0 mL of ascorbic acid solution, 10.0 mL of acetic acid-sodium acetate buffer solution, and 5.0 mL of o-phenanthroline solution. Dilute with water to the mark and shake well. Leave it at room temperature for 15 minutes, then measure the absorbance using a spectrophotometer at 510 nm, with the reagent blank used for zeroing. A working curve is plotted with the measured absorbance on the vertical axis and the corresponding amount of Fe2+ ions (in μg) on the horizontal axis. (2) Determination of total iron: a. Sample decomposition: Take 10 mL of the water sample into a 100 mL conical flask (if the volume is less than 50 mL, add water to bring it up to 50 mL), add 2.0 mL of (1+35) sulfuric acid solution, and 5.0 mL of potassium persulfate solution. Place the flask on an electric stove and heat it slowly until boiling for 15 minutes, ensuring that the volume remains at least 20 mL. After that, remove the flask from the heat and allow it to cool to room temperature. Adjust the pH to around 2 using either (1+3) ammonia solution or (1+35) sulfuric acid solution, and set it aside for use. b. Determination of absorbance: Transfer all the samples with the adjusted pH value into a 50 mL volumetric flask. Add 3.0 mL of ascorbic acid solution, 5 mL of acetic acid-sodium acetate buffer solution, and 2 mL of o-phenanthroline solution; dilute to the mark with water. Allow the mixture to stand at room temperature for 15 minutes. Then, use a spectrophotometer to measure the absorbance at 510 nm, adjusting the reading with the reagent blank, and record the data. Expression of analysis results: The total iron content (X) in the sample, expressed in mg/L of Fe2+, is calculated using the following formula: X = m/V. Here, m represents the amount of Fe2+ in μg, as determined from the calibration curve ； V — Volume of the test solution taken, in mL.