1. Slowly add phosphorus trichloride dropwise to water while stirring; control the rate of addition to prevent the reaction from becoming too vigorous, and cool the reactor using an ice bath. Upon completion of the reaction, filter the mixture. Heat the filtrate at a temperature below 140°C to evaporate it down to one-third of its original volume, thereby expelling hydrogen chloride gas. Then, cool the solution to 60–85°C, add a small amount of crystalline phosphorous acid to serve as seed crystals, and continue cooling and stirring to induce crystallization, yielding phosphorous acid.
2. Take a 1000 mL three-necked round-bottom flask and equip it with a mechanical stirrer, a dropping funnel, and a condenser, connecting the outlet to an HCl absorption device. Place distilled water into the flask and cool it with ice. While stirring, slowly add 100 mL of freshly distilled PCl3 (boiling range: 74–76°C) from the dropping funnel. Filter the mixture, then evaporate the filtrate at a temperature below 140°C until its volume is reduced to one-third of the original. Cool the solution to 60–65°C, sprinkle in H3PO3 crystals as seed crystals, and continue cooling and stirring to induce crystallization. The yield is approximately 63 g. For further purification, dissolve 10 g of the product in 3.5 g of water; then add the remainder of the product to this solution, stir thoroughly, and collect the crystals by suction filtration. The yield is 40 g. Store the product in a ground-glass stoppered bottle.
3. Use the same apparatus as described above. Place 300 mL of CCl4 and 100 g of freshly distilled PCl3 into the flask, cool the mixture with ice, and slowly add 75 g of distilled water dropwise from the dropping funnel while stirring. Once all the water has been added, remove the ice bath and continue stirring vigorously for 1 hour. Transfer the reaction mixture to a separatory funnel and wash it with four times its volume of CCl4; if any crystals precipitate during this process, add 2–3 mL of water. Transfer the washed acidic solution to a flask via suction filtration using a glass funnel. Under reduced pressure, evaporate the solution in a water bath (at a temperature not exceeding 60°C) for 3 hours to remove the HCl. Finally, place the product in a desiccator to cool and crystallize.
