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Water quality – Determination of sulfates – Flame atomic absorption spectrometry 1 Subject matter and scope 1.1 This standard specifies the flame atomic absorption spectrometric method for the indirect determination of soluble sulfates in water. 1.2 This standard is applicable to the determination of soluble sulfates in surface water, groundwater, and drinking water. 1.3 The minimum detectable concentration of this standard is 0.4 mg/L, and the upper limit of determination is 30 mg/L when the sample volume is 10 mL. When the sample volume is 1 mL, it is 300 mg/L. By appropriately diluting the water sample, the measurement range can be expanded further. 1.4 Pb2+ and PO43- interfere with the determination, but Pb2+ or PO43- at levels below 10 μg can be tolerated. 2 Principle In an ammoniacal water-ethanol medium, sulfates react with a barium chromate suspension. The reaction is as follows: SO42‑ + BaCrO4 → BaSO4↓ + CrO42‑. By measuring the chromate ions released in this reaction using atomic absorption spectroscopy, the content of sulfate can be calculated indirectly. The flame used. It is a yellow flame resulting from the rich combustion of air and acetylene, with a measurement wavelength of 359.3 nm. 3 Reagents Unless otherwise specified, analytical grade reagents that meet **standards or professional standards, as well as deionized water or water of equivalent purity, shall be used in the analysis. 3.1 Hydrochloric acid (HCl): ρ=1.19 g/mL. 3.2 Glacial acetic acid (CH3COOH): ρ = 1.05 g/mL. 3.3 Ammonium hydroxide (NH4OH): ρ = 0.880 g/mL. 3.4 Anhydrous ethanol (CH3CH2OH). 3.5 Ammonium hydroxide solution: 1+1. Prepared using ammonium hydroxide (3.3). Prepare it just before use. 3.6 Mixed acid solution: 0.42 mL of hydrochloric acid (3.1) and 14.7 mL of glacial acetic acid (3.2) are mixed, and then diluted to 200 mL with water. 3.7 Calcium solution: 1 mg/mL. Dissolve 0.28 g of calcium chloride (CaCl2) in 100 mL of water and stir well. 3.8 Barium chromate suspension: Weigh 0.5 g of barium chromate (BaCrO4) and dissolve it in 200 mL. The mixed acid solution (3.6) is stored in a polyethylene bottle. Shake before use. 3.9 Sulfate standard solution, SO42‑: 100 mg/L. Weigh 0.0740 g of anhydrous sodium sulfate (Na2SO4, dried at 105°C for 2 hours) accurately, dissolve it in an appropriate amount of water, transfer the solution to a 500 mL volumetric flask, dilute to the mark with water, and mix well. 4 Instruments: General laboratory instruments and 4.1 Atomic absorption spectrophotometer. 4.2 Chromium hollow cathode lamp. 4.3 Acetylene supply device. 4.4 Air compressor, with oil, water, and impurity removal devices. 4.5 Filters 5 Sampling and Samples: After water samples are collected, suspended solids are immediately removed by filtration using 0.45μm filter membranes, and the samples are stored in polyethylene bottles. Step 6.1 Sample: Take 10 mL of the water sample and place it in a 25 mL colorimetric tube. If the sulfate content is greater than 30 mg/L, a smaller amount of sample can be used, after which water should be added to bring the volume to 10 mL. 6.2 Measurement 6.2.1 Pretreatment: To the sample (6.1), add sequentially 2 mL of barium chromate suspension (3.7), 1 mL of ammonium hydroxide solution (3.5), 1 mL of calcium solution (3.6), and 8 mL of anhydrous ethanol (3.4); then add water to the mark and mix well. After being left for 30 minutes, it is filtered by vacuum using a 0.45 μm filter membrane (the setup is shown in Figure 1) and transferred into a 10 mL dry cuvette for subsequent analysis. 6.2.2 Measurement: Adjust the instrument to its optimal operating conditions in accordance with the instrument’s user manual, and determine the absorbance of the filtrate. 6.3 Preparation of the calibration curve: Add 0, 0.50, 1.00, 1.50, 2.00, 2.50, or 3.00 mL of sulfate standard solution to a set of 25 mL beakers; then proceed with the pre-treatment as described in step (6.2.1), and determine their absorbance under the conditions specified in (6.2.2). A calibration curve is plotted using the absorbance after subtracting the blank value against the corresponding sulfate concentration (mg/L). 7 Presentation of analysis results 7.1 Sulfate content is given by the following formula: Where: c —— concentration of sulfates in the sample, mg/L ； c/——Concentration obtained from the standard curve, mg/L ； V —— Volume of the sample taken, mL ； 25 —— Volume of the colorimetric tube, mL. 7.2 Sulfate content, calculated using a regression equation. 8 Precision and accuracy: Eight laboratories analyzed the same sample at three different concentration levels, with sulfate contents of 4.83, 10.5, and 25.7 mg/L respectively. 8.1 Repeatability The relative standard deviations for repeatability were 3.69%, 3.65%, and 2.65%, respectively. 8.2 Reproducibility The relative standard deviations for reproducibility were 7.98%, 3.84%, and 4.07%, respectively. 8.3 Accuracy: The relative errors are +1.45%, –2.86%, and –1.56%, respectively.