Ammonium wolframate

Dissociation constant

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

dissociation constant

Type of 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. Read-across using data from ammonia is appropriate to use in assessing dissociation of ammonium wolframate as ammonia is one of the constituents of ammonium wolframate and will affect the pKa for the substance.

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: Ammonia
Target : Ammonium paratungstate
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:

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^-

pK_b = 4.76

Executive summary:

Read-across using data from ammonia is appropriate to use in assessing dissociation of ammonium paratungstate as ammonia is one of the constituents of ammonium paratungstate and will affect the pKa for the substance. For additional information see Annex 3 in the CSR.

Reference 2

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:

comparable to guideline study with acceptable restrictions

Remarks:

Well documented, scientifically sound study that used appropriate instrumentation for quantifying effective dissociation constants. Read-across from sodium metatungstate to ammonium wolframate for this endpoint is appropriate as they are both highly soluble and are expected to have similar tungsten species in solution after dissociation occurs.

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 paratungstate
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 n.m.r. 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 pK_as above:

No.	Reaction	log K	pKa
1	12WO4^2- + 14H^+= [H2W12O42]^10–+ 6H2O	118	-118
2	7[H2W12O42]^10–+ 2OH^- = 12W7O24^6-+ 8H2O	20	-20
3	12W7O24^6-+ 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 (H₂W₁₂O₄₀^7-) 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 (WO₄^2-)is stable above p[H] 6.4 and paratungstate A (W₇O₂₄^6-)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 n.m.r., 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 paratungstate), 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.

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 paratungstate is soluble and dissolution will occur at environmentally relevant pHs according to the Tranformation/Dissolution testing (endpoint 4.8).  Dean JA (ed.)(1985) indicates that the pKa for the ammonium constituent has a pKa of 9.24. The major stable isopolytungstate species present, when sodium metatungstate solutions (pH 4.5-10) were measure via n.m.r., were WO4--(orthotungstate),(W7O24)6- (paratungstate A), (H2W12O42)10- (paratungstate B) (as solid) and (H2W12O40)6- (metatungstate)(Smith and Patrick, 2000). The pKas for these species can be found in the various endpoint records. Similar ionic species are expected to be present in solutions of ammonium paratungstate between these pHs. Other resources also support that dissociation would occur at environmentally relevant pHs.