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Dissociation constant

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dissociation constant
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Description of key information

According to “ANNEX IX- STANDARD INFORMATION REQUIREMENTS FOR SUBSTANCES MANUFACTURED OR IMPORTED IN QUANTITIES OF 100 TONNES OR MORE, study for Dissociation constant  does not need to be conducted if:
- The substance is readily oxidisable in water.
Expert Judgement
The high water solubility and ionic character identify Reaction mass of SEX as a dissociable compound.
Reaction mass of SEX reacts with water and there are three decomposition pathways of xanthates in aqueous solution:
A. Xanthates dissociate forming alkali metal cations and xanthate anions. The solution undergoes further hydrolysis to xanthic acid which decomposes into carbon disulphide and alcohol.
ROCS2Na + H2O ——→ ROCS2H + NaOH
ROCS2H ——→CS2 + ROH
B. Xanthate is oxidised to dixanthogen. The extent of this reaction is very small and dependent on the pH. Equilibrium is reached after about 5–10% of the xanthate is oxidised, and the reaction increases with a fall in the pH.
2ROCS–2 + H2O + _O2 ——→ (ROCS2)2 + 2OH–
C. In neutral and alkaline media, xanthates decompose by hydrolytic decomposition.
6ROCS–2 + 3H2O ——→ 6ROH + CO3 2 – + 3CS2 + 2CS3 2 –
Further hydrolysis of sodium trithiocarbonate to sodium carbonate and hydrogen sulphide and carbon disulphide to carbon dioxide and hydrogen sulphide may occur. The reaction is catalysed by the alcohol formed from the xanthic acid and is self accelerating.
Therefore testing for Dissociation constant does not need to be performed.
  
Ref. 1) European Chemicals Agency (ECHA), Guidance for the implementation of REACH, Guidance on information requirements and chemical safety assessment, Chapter R.7a: Endpoint specific guidance, May 2008, pages 168-171.

Key value for chemical safety assessment

Additional information

According to “ANNEX IX- STANDARD INFORMATION REQUIREMENTS FOR SUBSTANCES MANUFACTURED OR IMPORTED IN QUANTITIES OF 100 TONNES OR MORE, study for Dissociation constant does not need to be conducted if:

- The substance is readily oxidisable in water.

Expert Judgement

The high water solubility and ionic character identify sodium ethyl xanthate as a dissociable compound.

Reaction mass of SEX reacts with water and there are three decomposition pathways of xanthates in aqueous solution:

A. Xanthates dissociate forming alkali metal cations and xanthate anions. The solution undergoes further hydrolysis to xanthic acid which decomposes into carbon disulphide and alcohol.

ROCS2Na + H2O ——→ ROCS2H + NaOH

ROCS2H ——→CS2 + ROH

B. Xanthate is oxidised to dixanthogen. The extent of this reaction is very small and dependent on the pH. Equilibrium is reached after about 5–10% of the xanthate is oxidised, and the reaction increases with a fall in the pH.

2ROCS–2 + H2O + _O2 ——→ (ROCS2)2 + 2OH–

C. In neutral and alkaline media, xanthates decompose by hydrolytic decomposition.

6ROCS–2 + 3H2O ——→ 6ROH + CO3 2 – + 3CS2 + 2CS3 2 –

Further hydrolysis of sodium trithiocarbonate to sodium carbonate and hydrogen sulphide and carbon disulphide to carbon dioxide and hydrogen sulphide may occur. The reaction is catalysed by the alcohol formed from the xanthic acid and is self accelerating.

Reaction C is the main reaction in alkaline solution while A and B occur in acidic solutions. During use in mining processes, reaction C is the principal decomposition pathway and carbon disulphide the principal decomposition product. Part of the carbon disulphide formed may decompose further to carbonate and thiocarbonate salts, some of it may evaporate and some may build up in the xanthate solution. Once the solubility of carbon disulphide is exceeded it forms a separate layer below the Reaction mass of SEX solution.

Therefore testing for Dissociation constant does not need to be performed.

Ref. 1) European Chemicals Agency (ECHA), Guidance for the implementation of REACH, Guidance on information requirements and chemical safety assessment, Chapter R.7a: Endpoint specific guidance, May 2008, pages 168-171.