I have been working with 4 arm PEG-thiol, dissolved in phosphate buffer at 20%. I have been told the solution should be clear and colorless, however my solution turned yellowish-green. I checked the pH of both the buffer and the buffer with PEG at they are both about 7.4 (which was my desired pH). I checked the solution under a fluorescent microscope (in case the PEG was attached to fluorescin or something like it) but it was not fluorescent.
Does anyone know why this would happen?