How are sulfides in water measured?

What is sulfide in water? Some groundwater contains hydrogen sulfide (H2S), and when surface water is polluted by industrial wastewater, hydrogen sulfide can also be present. When hydrogen sulfide is contained in water, even if its concentration is only O. 5mg/L; can also be identified from the smell. There are free hydrogen sulfide and its salts in industrial wastewater such as man-made fibers, dyes, catalysis and oil refining; industrial wastewater containing protein will produce hydrogen sulfide due to the decomposition of protein; dissolved hydrogen sulfide and sulfide in water containing sulfate The relationship ratio between the total content of hydrogen compounds (H2S+HS-+S2-) is related to the pH value of the water. What is the determination principle of sulfide (methylene blue spectrophotometry)? After the water sample is acidified, the sulfide is converted into hydrogen sulfide and transferred in the zinc acetate-sodium acetate solution, and IV, IV-dimethyl-p-phenylenediamine and Ammonium ferric sulfate reacts to form a blue methylene blue complex, which can be quantitatively determined. If thiosulfate or sulfite in the water sample interferes with the determination, the zinc acetate precipitation-filtration-acidification-blowing method can be used at this time. How is sulfide (methylene blue spectrophotometry) determined? (1) Draw a standard curve ①Accurately draw a certain volume of sodium sulfide standard solution, inject a group of 100mL stoppers, and add 20mL of zinc acetate-sodium acetate solution. In the color tube, add about 60mL of water, slowly add 10mL of N, Ⅳ-dimethyl-p-phenylenediamine solution along the tube wall, seal it immediately, and slowly invert it once, add 1mL ammonium ferric sulfate solution, seal it immediately, and fully Shake well, let stand for 10min, dilute with water to the marked line, shake well. ② Use a 10mm cuvette, use water as a reference, measure the absorbance at a wavelength of 665nm, and do a blank test at the same time, take the absorbance of each standard solution minus the blank test as the vertical coordinate, and the corresponding standard solution The sulfur ion content in the horizontal axis is coordinates to draw a standard curve. (2) Determination of water samples ① Aeration should be prevented during sampling, and an appropriate amount of sodium hydroxide solution and zinc acetate-sodium acetate solution should be added to make the water sample alkaline and form zinc sulfide precipitates. The usual dosage is: add 1mL sodium hydroxide solution and 2mL zinc acetate-sodium acetate solution per liter of water, and add more as appropriate when the sulfide content is high. The water sample should fill the bottle with no air left under the cork. ②Colorless, transparent water samples without suspended solids can be determined by precipitation method. Take a certain volume of water sample into the separatory funnel and let it stand still. After the precipitation and the solution are separated, put the precipitation part into a 100mL stoppered colorimetric tube, add water to about 60mL, and follow the steps ① and ② in (1) below steps to measure. After subtracting the blank absorbance from the measured absorbance, find out the content of sulfide on the standard curve. ③ For water samples containing suspended solids, high turbidity, colored and opaque, the acidification-blowing-absorption method can be used for determination: 1) Take a certain volume of water sample, add 5mL of antioxidant solution, transfer the water sample into the reaction bottle, Add water to a total volume of about 200mL, turn on nitrogen, blow at a speed of 200-300mL/min for 2-3min, and then turn off the gas source. 2) Add 10mI to the nitrogen pipe. Phosphoric acid solution, connected to nitrogen, blowing at 300mL/min continuously for 30min. 3) Turn off the air source, rinse the joints of the absorption chromogenic tube with a small amount of water, and add water for about 60 minutes. 4) Follow steps ① and ② in (1) below for measurement. After subtracting the blank absorbance value from the measured absorbance value, check the sulfide content on the standard curve.