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Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

Additional information


Terephthalic acid (TPA) is an organic solid. The high purity (>99.9%) form of TPA required for the manufacture of polyesters is a white, free-flowing, crystalline powder at ambient temperature and pressure. The commercial production process typically results in the formation of rounded crystals. Needles may be formed by slow crystallisation, but may alter flow characteristics (Sheehan, 2005; Sheehan 1986).

Melting point

Terephthalic acid sublimes at 402 - 404 degrees C at atmospheric pressure. Under normal conditions TPA will therefore undergo direct transition from the solid to the vapour phase without melting. Melting has been observed to occur at 427 degrees C under sealed tube conditions (Park & Sheehan, 1996; Sheehan, 1986; Lucchesi & Lewis, 1968).

Boiling point

Terephthalic acid is a solid. It sublimes at 402-404 degrees C at atmospheric pressure, without undergoing a transition to the liquid phase. Its boiling point cannot therefore be determined except under artificial physical conditions that have no practical relevance. A study to determine the boiling point of terephthalic acid is therefore scientifically unjustified.


The crystal density of terephthalic acid is 1.58 g/cubic cm at 25 degrees C. The specific gravity of terephthalic acid is 1.522 at 25 degrees C (Sheehan, 2005; Park & Sheehan, 1996).


The particle size distribution of a sample of terephthalic acid typical of the Lead Registrant's commercial production was characterised as follows, using the Coulter method: particles of diameter <4, <10 and <100 microns comprised 0.66%, 1.16% and 38.86%, respectively, of the sample by volume and 61.14% of the sample consisted of particles with a diameter >100 microns.

The data provided by the Lead Registrant are compatible with other, independently collated data contributed by members of the Terephthalic Acid Sub-Group of the Polyester Monomers Consortium (contributors anonymised, number of sources not known), whose collective particle size distribution data are described by the following envelope: 15% to 30% smaller than 50 microns, 75% to 90% in the range 40 to 350 microns.

Vapour pressure

The vapour pressure of terephthalic acid was estimated using the QSAR model MPBPVP v.1.43 of the US EPA. The definitive value selected by the model is the estimate of 0.00158 Pa calculated according to the Modified Grain method. Based on a mathematical relationship between temperature and vapour pressure for TPA, derived from published VP measurements at temperatures in the range 100 to 250 degrees C, the vapour pressure of TPA at 25 degrees C is estimated to be 0.0000000000259 mm Hg. (Sheehan, 1986).

Partition coefficient

The log octanol/water partition coefficient (log Kow) of terephthalic acid was estimated using the KOWWIN v1.67 QSAR model available from the US EPA. The estimated log Kow of terephthalic acid is 1.76. Additionally, KOWWIN provides a measured log Kow value of 2.00 for terephthalic acid, retrieved from the data base of experimental results used to construct the QSAR model.

Isophthalic acid is a structural analog of TPA that has a KOWWIN log Kow estimate of 1.76, identical to that of TPA. The octanol/water partition coefficient of isophthalic acid has been determined according to the shake-flask procedure, in a system buffered at pH 7. The mean log Kow obtained for IPA under these conditions was -2.34. This value implies a much higher relative solubility of IPA in the aqueous phase than the log Kow indicated by KOWWIN QSAR. This is likely to have been caused by the presence of the buffer used to maintain the test system at pH 7: IPA would have been converted under these conditions to salts with higher aqueous solubility than that of the free acid. Similar behaviour, with a log Kow similarly lower than that of the free acid, may be expected for TPA under comparable conditions, following conversion of the parent monomer to its more soluble salts.

Wherever other models of the US EPA's EPISUITE QSAR programs have been used to generate estimates of physico-chemical or environmental fate and behaviour characteristics that require input of log Kow, these secondary models selected the database (measured) log Kow of 2.00 as the default value for terephthalic acid.

Water solubility

The limit of solubility of terephthalic acid (TPA) in water at 25 degrees C is ca. 17 to 19 mg/L. TPA is therefore considered to be slightly soluble in water. (Park & Sheehan, 1996; Roehrscheid, 2005).

Solubility in organic solvents

Limits of the solubility of terephthalic acid at 25 degrees C are published for four organic solvents: glacial acetic acid (0.013 %, w/w), methanol (0.1 % w/w), dimethylformamide (6.7 % w/w) and DMSO (19 % w/w) (Park & Sheehan, 1996).

Surface tension

In accordance with Column 2 adaptation statement of REACH Annex VII, information requirement 7.6, this study does not need to be conducted if, based on structure, surface activity is not expected and no surface-active properties would be predicted for this compound. Surface activity is not a desired property of the material.

Flash point

In accordance with Section 2 of REACH Annex XI, information requirement section 7.9, this study does not need to be conducted based on the physical state of the molecule. According to ECHA guidance, flash point is only relevant to liquids and low melting point solids.


Terephthalic acid is a solid that sublimes at 402-404 degrees C at atmospheric pressure, without evidence of self-heating. In the absence of self-heating at 400 degrees C an experimental determination of auto-flammability (the lowest temperature at which terephthalic acid will spontaneously ignite in the absence of an external ignition source) is not required.


A preliminary flammability screening test was performed by igniting a loosely-packed linear pile of TPA. Combustion self-extinguished after 218 seconds and without propagating 200 mm along the pile. According to the UN Recommendations on the Transport of Dangerous Goods, terephthalic acid is therefore not classified as a readily combustible solid of Division 4.1 and further flammability testing is not required. (Atwal & Tremain, 2010).


According to theoretical considerations based on chemical structure, terephthalic acid does not to possess explosive or oxidising properties. Terephthalic acid is unlikely to undergo rapid decomposition accompanied by the evolution of gases or release of heat and therefore does not present a risk of explosion (Curl & Wright, 2010a).

Oxidising properties

According to theoretical considerations based on chemical structure, terephthalic acid does not possess oxidising properties. Terephthalic acid is unlikely to cause or contribute to the combustion of other material during transport, storage or use (Curl & Wright, 2010b).

Stability in organic solvents and identity of relevant degradation products

In accordance with REACH Annex XI Section 2, with reference to the guidance mentioned in REACH Art 13(3) the test guidance “ECHA guidance on information requirements and chemical safety assessment Chapter R.7a Endpoint Specific Guidance”; this study does not need to be conducted since the stability in organic solvents is not considered critical. This would be assessed in individual studies where organic solvents are used.


The pH of a saturated aqueous solution of terephthalic acid at 25 degrees C is 3.88 (Sheehan, 1986).

Dissociation constant

The pK1 and pK2 values for terephthalic acid in aqueous solution at 25 degrees C are 3.54 and 4.46, respectively (Park & Sheehan, 1996).


In accordance with Section 2 of REACH Annex XI, information requirement section 7.17, this study cannot be conducted on solid materials or gases. According to ECHA Chapter 7 guidance, viscosity measurement is only relevant to liquids.