In the preparation of 1., in order to prevent its acidic decomposition, copper ions contained in water were removed and a small amount of Na2CO3 was added to make the solution weakly alkaline (under this condition, the microbial activity was low) and the solution concentration was stable. The prepared na2so 3 solution was stored in brown reagent bottle and calibrated after two weeks. Sodium thiosulfate standard solution should not be stored for a long time, and should be recalibrated after a period of use. If it is found that the solution becomes turbid or sulfur is precipitated, it should be recalibrated after filtration, or it should be discarded and reconstituted. 2. When preparing Na2S2O3 solution, it is necessary to use newly boiled distilled water (to remove CO2 and kill bacteria) and cool it, or dissolve Na2S2O3 reagent in distilled water, boil it for 10min, then cool it, and add a small amount of Na2CO3 to make the solution alkaline to inhibit the growth of bacteria. There are three reasons why sodium thiosulfate solution is unstable: (1) reacts with CO2 dissolved in water: Na2SO4+CO2+H2O = nahco3+nahso3+SO2 (2) reacts with O2 in air: Na2S2O3+O2 =2Na2SO4+2S↓⑶+2S↓ (3) reacts with microorganisms in water: Na2SO3+S.