The molecular formula of sodium hypochlorite is NaClO or NaOCl, and the appearance is a slightly yellow solution with a chlorine-like odor. Commonly used for disinfection, sterilization and water treatment, there are also products that are only suitable for general industrial use.

　　The concentration of sodium hypochlorite solution is generally determined by the iodometric method. The principle is that sodium hypochlorite reacts with potassium iodide in an acid solution to release a certain amount of iodine, which is then titrated with a sodium thiosulfate standard solution.

　　The principle of its chemical formula is:

　　2KI+2CH3COOH→2CH3COOK+2Hl，2HI+NaClO→I2+NaCl+H2O，I2+2NaS2O3→2NaI+Na2S4O6

　　The specific operation steps are as follows: take 100mL of water sample with 0.1 (or take an appropriate amount of sample and dilute to 100mL) in a 250mL iodine measuring flask, add excess potassium iodide and 50mL water, add 5mL acetate buffer solution, close the plug and mix, set Leave it in the dark for 5 minutes, titrate with the sodium thiosulfate solution to be calibrated until the solution is light yellow, add 1 mL of starch indicator solution, continue titration, and the blue color just disappears, and record the standard solution dosage.

　　The calculation formula is: where C represents the concentration of sodium hypochlorite solution in mg/L; C0 represents the concentration of sodium thiosulfate standard solution; V1 represents the amount of sodium thiosulfate standard solution when titrating the sample in ml; V represents the volume of the water sample in ml; 37.221 It is the molar mass of 1/2NaClO in g/mol.

　　Precautions:

　　1. After adding acetate buffer solution to the sample, the pH should be 3.5-4.2. If it is greater than some pH value, you should continue to adjust the pH to 4,

　　Then carry out the titration.

　　2. The titration must be carried out in a neutral or weakly acidic solution.

　　3. The added KI must be excessive, generally 2 to 3 times the excess.

　　4. Light can catalyze I-oxidation by air and increase the activity of bacteria in Na2S2O3 solution. Therefore, the reaction of precipitation I2,

　　To be placed in a dark place for approximation, immediately after titration with Na2S2O3.

　　5. Do not shake the solution vigorously during titration, and the titration speed should be appropriately faster, so as to reduce the volatilization of I2 and the oxidation of I- by O2.