Reagent: 1. 30% (g/g) chromium trioxide solution 2. Sodium hydroxide titration (0.02 mol/L) 3. Phenol phthalate indicator 4. Sodium bicarbonate 5. Dilute sulfate 6. Potassium iodide 7. Sodium thiosulfate titration solution (0.02 mol/L) 8. Starch indicator instrument equipment: specimen preparation: 1. Sodium hydroxide titration (0.02 mol/L) Formulation: Hydrogen clarification Sodium oxide Saturated solution is 5.6mL, and freshly boiled cold water is added to 1000mL. Calibration: Take the benchmark potassium hydrogen phthalate dried at 105℃ to constant weight about 6g, accurately weigh, add 50mL of freshly boiled cold water, shake to make it dissolve as much as possible; add 2 drops of phenolphthalic indicator liquid, titrated with this liquid. When approaching the end point, potassium hydrogen phthalate should be completely dissolved, titrated until the solution is pink. Each 1 mL of sodium hydroxide titrant (1 mol/L) is equivalent to 20.42 mg of potassium hydrogen phthalate. The concentration of the liquid is calculated based on the consumption of the liquid and the amount of potassium hydrogen phthalate taken. Quantitatively diluted 5 times to bring the concentration to 0.02 mol/L. Storage: In a polyethylene plastic bottle, sealed and stored; there are 2 holes in the plug, 1 glass tube is inserted into each hole, 1 tube is connected with a sodium lime tube, and 1 tube is used to suck out the liquid. 2. Phenol phthalate indicator liquid take 1 g of phenol phthalate, add 100 mL of ethanol to dissolve 3. Sodium thiosulfate titration (0.02 mol/L) Formulation: take 26 g of sodium thiosulfate and 0.20 g of anhydrous sodium carbonate, add an appropriate amount of freshly boiled cold water to dissolve into 1000mL, shake well, leave for 1 month and filter.