Sodium sulfide (Na2(Sx))

Melting point / freezing point

Description of key information

Liquid: The freezing point of sodium polysulfide solution was -6.2 °C.

Solid: The melting point of the test item disodium tetrasulfide (EC no. 215-686-9) is 230°C – 278°C

Key value for chemical safety assessment

Melting / freezing point at 101 325 Pa:

230 °C

Additional information

Liquid: The freezing point of the test item sodium polysulfide solution was determined according to OECD guideline no. 102 [adopted on 27 July 1995] and EU test method A.1 [Regulation 92/69/EEC, Official Journal L 383 A 1992] with the with the freezing temperature method. Approx. 10 ml sample are filled into the vessel with a stirrer and thermometer. The vessel is closed and for a preliminary test the cryostat is set to maximum cooling. When the sample crystallizes this is usually associated with excessive supercooling. The sample is melted by raising the temperature again. The sample is then cooled again with the cryostat set to a final temperature 5 ± 2 °C below the crystallization temperature of the preliminary test. If the temperature rise exceeds 2 °C seed crystals should be added to the sample at the preliminary crystallization temperature. The test is performed at least twice.

The freezing point of sodium polysulfide solution was -6.2 °C.

Solid: The melting temperature of the test item disodium tetrasulfide (EC no. 215-686-9) was determined by differential scanning calorimetry according to OECD guideline no. 102 (adopted on July 27, 1995) and EU test method A.1 [Council Regulation (EC) No 440/2008 of 30 May 2008]. In a measurement with differential scanning calorimetry a sample of the test item and a reference material (identical crucible without the test item) are subjected to the same controlled temperature program. The difference in the temperatures of the test item and the reference material is recorded and applying a calibration function converted to a heat flow signal. When the sample undergoes a transition involving a change in enthalpy (endothermic on melting or boiling), that change is indicated by a departure from the base line of the heat flow record. A thermogravimetric measurement was carried out as a preliminary test before the DSC measurements.

The heat flow curves then showed two endothermic effects, a smaller peak at 245°C (extrapolated onset temperature at 230°), and a larger peak at 287°C (extrapolated onset temperature at 278°). This indicates probably the melting of two phases. The melting point of the test item disodium tetrasulfide (EC no. 215-686-9) is 230°C – 278°C.