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Dissociation constant

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Endpoint:
dissociation constant
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
Well documented, scientifically sound study that used appropriate instrumentation for quantifying effective dissociation constants.
Justification for type of information:
1. HYPOTHESIS FOR THE CATEGORY APPROACH: The hypothesis is that properties are likely to be similar or follow a similar pattern because of the presence of a common metal ion, in this case tungstate.
2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES):
Source: Sodium metatungstate
Target: Ammonium metatungstate
3. CATEGORY APPROACH JUSTIFICATION: See Annex 3 in CSR
4. DATA MATRIX: See Annex 3 in CSR
Reason / purpose for cross-reference:
read-across: supporting information
Principles of method if other than guideline:
The speciation and equilibria of sodium metatungstate was determined by a combination of 183W NMR spectroscopy and gravimetry over the pH range 4.5-10.0.
GLP compliance:
not specified
Dissociating properties:
yes
No.:
#1
pKa:
-118
Temp.:
20 °C
Remarks on result:
other: See remarks on results for equation
No.:
#2
pKa:
-20
Temp.:
20 °C
Remarks on result:
other: See remarks on results for equation
No.:
#3
pKa:
-195
Temp.:
20 °C
Remarks on result:
other: See remarks on results for equation

Reactions represented by pKas above:

  No. Reaction  log K   pKa
 1  12WO42- + 14H+=

[H2W12O42]10–+ 6H2O

 118  -118
 2

7[H2W12O42]10–+ 2OH- = 12W7O246-+ 8H2O

 20  -20
 3   

12W7O246-+ 30H+ =

7[H2W12O40]6 - + 8H2O

195  -195

The kinetics of equilibrium for the system were found to be slow, requiring up to 8 months to stabilize.

Sodium paratungstate B, Na10[H2W12O42].xH2O, which constituted a major component of this system, was found to be insoluble and precipitated as large, well-defined crystals. The solubility product for sodium paratungstate B ([H2W12O42]10–) was determined to be 6.89 (± 0.80) × 10–6

Metatungstate (H2W12O407-) is stable up to p[H] 7.5. Paratungstate B ([H2W12O42]10–

(as solid) is stable up to p[H] 9.6. The orthotungstate ion (WO42-)is stable above p[H] 6.4 and paratungstate A (W7O246-)has a stability range from p[H] 5.5 to 7.5.

Conclusions:
The major stable isopolytungstate species present, when sodium metatungstate solutions (pH 4.5-10) were measured via NMR, were WO4-–,(W7O24)6–, (H2W12O42)10- (as solid sodium salt) and (H2W12O40)6-.
Executive summary:

Due to the similar chemical constituency and structure between the source substance (sodium metatungstate) and the target substance (ammonium metatungstate), the physico-chemical properties would be expected to be the same or sufficiently similar such that application of read-across is appropriate for this endpoint. For more details, refer to the read-across category approach description in the Category section of this IUCLID submission or Annex 3 of the CSR.

Endpoint:
dissociation constant
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
Data from peer-reviewed handbook or collection of data.
Justification for type of information:
1. HYPOTHESIS FOR THE CATEGORY APPROACH: The hypothesis is that properties are likely to be similar or follow a similar pattern because of the presence of a common metal ion, in this case tungstate.
2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES):
Source: Ammonium
Target: Ammonium metatungstate
3. CATEGORY APPROACH JUSTIFICATION: See Annex 3 in CSR
4. DATA MATRIX: See Annex 3 in CSR
Principles of method if other than guideline:
Data in this peer-reviewed publication were taken from many sources, including both compilations and primary literature. Where conflicts were found, the value deemed most reliable was chosen for inclusion in the handbook. Reference sources were not provided for this parameter, nor was the experimental method.
GLP compliance:
not specified
Dissociating properties:
yes
pKa:
9.24
Temp.:
25 °C

NH3 + H2O = NH4 + + OH-

pKb = 4.76

Executive summary:

Ammonia is appropriate to use in assessing the dissociation of ammonium metatungstate as ammonia is one of the constituents of ammonium metatungstate and will affect the pKa for the substance such that application of read-across is appropriate for this endpoint. For more details, refer to the read-across category approach description in the Category section of this IUCLID submission or Annex 3 of the CSR.

Description of key information

The speciation of tungsten compounds in water is complex and several species exist coincidingly, depending on the pH of the system ammonium metatungstate is soluble and dissolution will occur at environmentally relevant pHs based on the Tranformation/Dissolution (T/D) testing (endpoint 4.8) conducted on ammonium metatungstate. Dean JA (ed.)(1985) indicates that the pKa for the ammonium constituent is 9.24. The major stable isopolytungstate species present, when sodium metatungstate solutions (pH 4.5-10) were measured via NMR, were WO4(2-), orthotungstate, W7O24(6-), paratungstate A, H2W12O42(10-), paratungstate B (as solid) and H2W12O40(6-), metatungstate (Smith and Patrick, 2000). Similar ionic species are expected to be present in solutions of ammonium metatungstate between these pHs. Other resources also support that dissociation would occur at environmentally relevant pHs.

Key value for chemical safety assessment

Additional information

No dissociation constant data are available for ammonium metatungstate (target substance). However, transformation/dissolution (T/D) data are available for ammonium metatungstate and sodium metatungstate, and pKa data are available for ammonium, which are used in a weight-of-evidence (WoE) approach for read-across. For more details, refer to the read-across category approach description in the Category section of this IUCLID submission or Annex 3 of the CSR.