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Melting point / freezing point

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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.
Endpoint:
melting point/freezing point
Type of information:
other: peer reviewed data base
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 - 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
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).

Key value for chemical safety assessment

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.