What is the yellow substance in phosphorous acid? How to remove

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1. Many years ago, I reacted organic acids with phosphorus trichloride (using a slight excess of phosphorus trichloride at a ratio of 3.05:1) to produce acyl chlorides and phosphorous acid; the mixture separated into layers after being left to stand, and everything seemed normal. So I tried heating the acyl chlorides and phosphorous acid together while stirring, and when the temperature reached 130 degrees, the liquid mixture suddenly polymerized and turned into a yellow solid, forcing me to discard the bottle. In the end, the production process was finalized as follows: first, the phosphorous acid was separated completely, and then the crude acyl chlorides were distilled to obtain the final product； 2. Our company has been engaged in this production for over a decade, but after the phosphorous acid is placed in plastic tanks, the crude phosphorous acid turns yellow over time, which creates problems during purification as the color is not suitable. 3. My question is: why does crude phosphorous acid turn yellow after being stored for a long time? What if it’s processed? Please discuss this! !