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Physical & Chemical properties

Water solubility

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Determining the aqueous solubility of a test substance that is a hydrophobic chemical is inherently difficult. This test substance is a liquid at room temperature and has a density similar to that of water. Using a standard OECD 105 shake flask water solubility method promotes the formation of micelles and suspensions, which can be stable and difficult to discern from dissolved chemical. Also, trace amounts of particulate matter can serve as adsorptive sites for the test substance effectively increasing its apparent solubility. The presences of undissolved or adsorbed test substance in these forms can bias the results high, an effect that has been evident in much of the earlier solubility data on chemicals with these characteristics. Researchers have sucessfully demonstrated the use of a slow-stirring technique to measure the aqueous solubility of substances of this type. The slow-stir technique has been incorporated into OECD Method 123 for determining octanol-water partitioning coefficient. OECD 105 column elution and nephelometric methods can also be utilized.

The solubility of bis(2-ethylhexyl)terephthalate was determined in two experiments using the slow-stir water solubility method. Sample aliquots were extracted using solid phase extraction (SPE) and the resulting extracts were analyzed using gas chromatography with mass spectrometric detection (GC/MS). Due to the length of the tests (up to 25 days), if bacteria were present, the test substance could biodegrade yielding inaccurate water solubility results. The use of a biocide in the second experiment ensured that the results of experiment #1 were not affected by biodegradation. The mean measured solubility detennined in Experiment #1 was 0.6 ug/L, and in Experiment #2 was 0.2 ug/L. The result from the first experiment appears to be higher. This could be due to the presence of a very small amount of micelle, suspended or adsorbed test chemical stable in solution. It is more likely however that the apparent difference between the results of the two tests is due to the day-to-day and test-to-test analytical variability (at least a factor of 2X). Thus, the average of the results of both experiments provides the most reliable value for the water solubility of bis(2-ethylhexyl)terephthalate,

0.4 ug/L.