Sodium dithionite

Melting point / freezing point

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

melting point/freezing point

Type of information:

other: Handbook data

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Qualifier:

no guideline followed

Principles of method if other than guideline:

Not applicable - Handbook data

GLP compliance:

not specified

Type of method:

other: no data

Key result

Decomposition:

yes

Decomp. temp.:

52 °C

Sublimation:

no

Conclusions:

According to the reference, sodium dithionite decomposes at 52 °C.

Reference 2

Endpoint:

melting point/freezing point

Type of information:

other: Handbook data

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Remarks:

peer reviewed data base

Qualifier:

no guideline followed

Principles of method if other than guideline:

Not applicable - peer reviewed data base

GLP compliance:

not specified

Type of method:

other: no data

Decomposition:

yes

Decomp. temp.:

> 90 °C

Sublimation:

no

The anhydrous salt decomposes exothermically in air on prolonged heating  above 90 °C (decomposition/oxidation products: sodium sulfate and sulfur  dioxide). Above ca. 150 °C, (exclusion of air) vigorous decomposition,  giving mainly sodium sulfite, sodium thiosulfate, sulfur dioxide, and a  small amount of sulfur. In the absence of air, moisture only causes a  small degree of decomposition. Sodium dithionite in powder form can  decompose in air on contact with a small amount of water with such  intense heat formation that it burns with a flame.

Aqueous dithionite solutions decompose slowly in the cold and rapidly in  the warm

Main decomposition products are thiosulfate and hydrogensulfite. To a  small amount (2 -4 %) sulfide and consecutively sulfur occurs.

2 Na2S2O4 -(H2O)-> Na2S2O3 + Na2S2O5 (NaHSO3 respectively)

NaS2O4 + Na2S2O3 -(H2O)-> Na2S + 3 NaHSO3

Conclusions:

The anhydrous salt decomposes exothermically in air on prolonged heating above 90 °C (decomposition/oxidation products: sodium sulfate and sulfur dioxide).
Above ca. 150 °C, (exclusion of air) vigorous decomposition, giving mainly sodium sulfite, sodium thiosulfate, sulfur dioxide, and a small amount of
sulfur. In the absence of air, moisture only causes a small degree of decomposition.
Sodium dithionite in powder form can decompose in air on contact with a small amount of water with such intense heat formation that it burns with
a flame. Aqueous dithionite solutions decompose slowly in the cold and rapidly in the warm Main decomposition products are thiosulfate and hydrogensulfite. To a small amount (2 -4%) sulfide and consecutively sulfur occurs.

2 Na2S2O4 -(H2O)-> Na2S2O3 + Na2S2O5 (NaHSO3 respectively) NaS2O4 + Na2S2O3 -(H2O)-> Na2S + 3 NaHSO3

Reference 3

Endpoint:

melting point/freezing point

Type of information:

other: Handbook data

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

data from handbook or collection of data

Qualifier:

no guideline followed

Principles of method if other than guideline:

Not applicable - Handbook data

GLP compliance:

no

Type of method:

other: no data

Decomposition:

yes

Sublimation:

no

Anhydrous sodium dithionite is combustible and can decompose  exothermically if subjected to moisture. Sulfur dioxide is given off  violently if the dry salt is heated above 190 °C. At room temperature, in  the absence of oxygen, alkaline (pH 9-12) aqueous solutions of dithionite decompose slowly over a matter of days. Increased temperature dramatically increases the decomposition rate. A representation of the  decomposition chemistry is as follows:

2 S2O4(2 -) + H2O ---> 2 HSO3(-) + S2O3(2 -)

The decomposition of dithionite in aqueous solution is accelerated by  thiosulfate, polysulfide, and acids. The addition of mineral acid to a dithionite solution produces first a red color which turns yellow on standing; subsequently, sulfur precipitates and evolution of sulfur  dioxide takes place.

Conclusions:

Anhydrous sodium dithionite is combustible and can decompose exothermically if subjected to moisture.
Sulfur dioxide is given off violently if the dry salt is heated above 190 °C.
At room temperature, in the absence of oxygen, alkaline (pH 9-12) aqueous solutions of dithionite decompose slowly over a matter of days.
Increased temperature dramatically increases the decomposition rate. A representation of the decomposition chemistry is as follows: 2 S2O4(2 -) + H2O ---> 2 HSO3(-) + S2O3(2 -) The decomposition of dithionite in aqueous solution is accelerated by thiosulfate, polysulfide, and acids.
The addition of mineral acid to a dithionite solution produces first a red color which turns yellow on standing;
subsequently, sulfur precipitates and evolution of sulfur dioxide takes place.

Description of key information

Sodium dithionite decomposes at 52 °C (handbook).

Additional information

For the evaluation of this endpoint data from one reliable handbook (CRC handbook) were used as key information, supported by two further references.

Handbook data considered to be from a trusted source acc. to "ECHA, Guidance on information requirements and chemical safety assessment – Chapter R.7a: Endpoint specific guidance" and therefore, the description of sodium dithionite could be regarded as valid.

Further to the information that sodium dithionite composes before melting following information are available: The anhydrous salt decomposes exothermically in air on prolonged heating above 90 °C (decomposition/oxidation products: sodium sulfate and sulfur dioxide). Above ca. 150 °C, (exclusion of air) vigorous decomposition,  giving mainly sodium sulfite, sodium thiosulfate, sulfur dioxide, and a  small amount of sulfur. In the absence of air, moisture only causes a small degree of decomposition. Sodium dithionite in powder form can decompose in air on contact with a small amount of water with such intense heat formation that it burns with a flame.

Main decomposition products are thiosulfate and hydrogensulfite. To a  small amount (2 -4 %) sulfide and consecutively sulfur occurs.

At room temperature, in  the absence of oxygen, alkaline (pH 9 -12) aqueous solutions of dithionite decompose slowly over a matter of days.Increased temperature dramatically increases the decomposition rate.