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Water solubility

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water solubility
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2001-07-20 through 2001-08-14 (Experiment 1); 2001-10-05 through 2001-10-29 (Experiment 2)
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study
equivalent or similar to guideline
other: Method Reference: Ellington, JJ, 1999, J. Chem. Eng. Data, 44, 1414-1418.
Principles of method if other than guideline:
The slow-stir technique has been used to measure the aqueous solubility of phthalate esters, a class of chemicals similar to DOTP. Restricting the agitation of the test solution, and allowing several days for equilibration achieved reproducibility and accurate results. Even though the duration of the published tests was often several weeks, steady-state equilibration was observed within a few days.
GLP compliance:
Type of method:
other: Slow-stir technique
Key result
Water solubility:
ca. 0.4 µg/L
22.5 °C
ca. 5
Details on results:
A relative response factor (RRF) was determined for each test substance standard analyzed by multiplying the nominal concentration (6.024 mg/L) of internal standard by the area of the test substance peak and dividing that by the area of the internal standard peak and the nominal concentration of the test substance. The mean RRF based on the test substance standards was calculated on each day of analysis.

The concentration of test substance in each sample was calculated by multiplying the area of the test substance peak by the nominal concentration (approx. 3 µg/L) of internal standard spiked into the sample and dividing that by the area of the internal standard peak and the mean RRF.

The potential for biodegradation during these long term tests (>20 d) was evaluated in Experiment #2 with one of the test solutions containing HgCl2. The data from Experiment #2 show that there was no difference in the results for the test solutions with and without HgCl2.

The mean measured solubility determined in Experiment #1 was 0.6 µg/L, and in Experiment #2 was 0.2 µg/L. The average of the solubility values from both experiments resulted in an estimated aqueous solubility value of 0.4 µg/L.

Results from the Water Solubility Test of DOTP Using the Slow Stir Method
Concentration of DOTP (µg/L)
Experiment #1
Sample ID
Day 10 Day 20 Day 25 Mean Value
Blank A ND (<0.2) ND(<0.2) ND(<0.3) ND(<0.2)
Blank B ND (<0.2) ND(<0.2) ND(<0.2) ND(<0.2)
Tank A 0.43 0.69 0.69 0.6
Tank B 0.36 0.68 0.50 0.51
Experiment #2
Sample ID
Day 3 Day 20 Day 24 Mean Value
Blank ND(<0.1) ND(<0.1) ND(<0.1) ND(<0.1)
Tank A* 0.14 0.32 0.19 0.22
Tank B 0.16 0.15 0.14 0.15
*contains mercuric chloride to inhibit microbial growth  
Interpretation of results: insoluble (< 0.1 mg/L)
DOTP had extremely low solubility in water.
Executive summary:

Two experiments were performed to detemine aqueous solubility of DOTP in distilled water using the slow-stir method. The solubility of DOTP in distilled deionized water was determined to be approximately 0.4 µg/L at 22.5±1.5 °C. 

Description of key information

Key value for chemical safety assessment

Water solubility:
0.4 µg/L
at the temperature of:
20 °C

Additional information

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.