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Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.

The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

Diss Factsheets

Physical & Chemical properties

Dissociation constant

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Administrative data

Link to relevant study record(s)

Description of key information

The HYDROTOPES Category comprises the following 6 substances:
STS - Sodium toluene 4-sulphonate (CAS 657-84-1, EC 211-522-5)
SXS - Sodium (xylenes and 4-ethylbenzene) sulphonate (EC 701-037-1)
NH4XS - Ammonium (xylenes and 4-ethylbenzene) sulphonate (EC 943-024-5)
SCS - Sodium p-cumenesulphonate (CAS 15763-76-5, EC 239-854-6)
KCS - Potassium p-cumenesulphonate (CAS 164524-02-1, EC 629-764-9)
NH4CS - Ammonium p-cumenesulphonate (CAS 680972-33-2, EC 811-484-5)


All the category members except STS were tested and the pKa values are:
SXS: pKa = 7.10
NH4XS pKa = 7.78
SCS: pKa < 2 (The dissociation constant was estimated on the free non-neutralised cumenesulfonic acid. The degree of dissociation at relevant pH values (5-9) is equal or larger than 99.9 %.)
KCS: pka < 2 (The dissociation constant was estimated on the free non-neutralised cumenesulfonic acid. The degree of dissociation at relevant pH values (5-9) is equal or larger than 99.9 %.)
NH4CS: pKa = 6.59


All the category members are sodium, potassium and ammoniun salt. When dissolved in water, hydrotropes are completely ionised.

Key value for chemical safety assessment

pKa at 20°C:
6.59

Additional information

The lowest value of the dissociation constant of Hydrotropes was selected for the risk assessment.