Reaction mass of Ammonium bisethanedioato bisaquo niobate(V) (anhydrous form) and Ammonium hydrogen ethanedioate ethanedioic acid (anhydrous form)

Hydrolysis

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

hydrolysis

Type of information:

other: inorganic constituent

Remarks:

The substance is water-soluble, dissociating readily into ammonium ions (NH+), oxalate-niobate (V) and oxalate anions, respectively. The presented data refers to the inorganic component Nb (V) representing a dissociation product.

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Well-documented publication

Qualifier:

no guideline followed

Principles of method if other than guideline:

Solubilities of the crystalline B-Nb2O5 were determined in five NaClO4 solutions (0.1 ≤ Im/mol·kg−1 ≤ 1.0) over a pH range of 1 to 13 at 25.0±0.1 °C and at 0.1 MPa.

GLP compliance:

no

Radiolabelling:

no

Analytical monitoring:

yes

Details on sampling:

- Sampling intervals for the parent/transformation products:after 5, 10, 15 and 30 days
- Sampling intervals/times for pH measurements: every sampling

Duration:

30 d

pH:

4

Initial conc. measured:

20 g/L

Duration:

30 d

pH:

7

Initial conc. measured:

20 g/L

Duration:

30 d

pH:

9

Initial conc. measured:

20 g/L

Positive controls:

not specified

Negative controls:

not specified

Transformation products:

not measured

Remarks on result:

not measured/tested

Remarks on result:

not measured/tested

The solubility study indicated the possibleexistence of five mononuclear Nb(V) species in solution,viz., Nb(OH)₃²⁺, Nb(OH)₄⁺, Nb(OH)₅⁰, Nb(OH)₆^-, Nb(OH)₇^2-in equilibrium with B-Nb₂O₅, according to the generalized reaction:

0.5Nb₂O₅(cr) + 0.5(2n-5) H₂O(l)  → Nb(OH)_n^5-n + (n-5) H⁺ (n = 4 – 7)

 

However, the trihydroxo species Nb(OH)₃²⁺could only be detected in the analysis conducted at 70 °C (I_m= 1.0 mol·kg⁻¹). The sudden appearance of this species at the highest temperature study is counterintuitive, because hydrolysis of metal ions is known to increase with increasing temperature such that Nb(OH)₃²⁺should be destabilized with respect to Nb(OH)₄⁺and Nb(OH)₅⁰with increasing temperature.

 

All of the solubility data at pH_m> 9 show a discontinuity, probably favoring the transformation of Nb₂O₅to a more insoluble solid phase, especially when one considers that the solution speciation must follow continuously from that at pH < 9 such that the Nb(OH)₇^2-would immediately predominate. 

 

XRD analyses of solids recovered after equilibration with acidic (10⁻²mol·kg⁻¹, HNO₃) and basic (10⁻²mol·kg⁻¹, NaOH) solutions at room temperature showed sharp peaks which are consistent only with crystalline B-Nb₂O_5.The result indicated that any mixed oxide/hydroxide solid phase(s) which may be formed in the latter medium is/are either present as a thin crystalline film on the B-Nb₂O₅particles or as a thin amorphous layer.

Reference 2

Endpoint:

hydrolysis

Type of information:

other: inorganic constituent

Remarks:

The substance is water-soluble, dissociating readily into ammonium ions (NH+) and oxalate-niobate (V) and oxalate anions respectively. The presented data refers to the inorganic component Nb (V).

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Well-documented publication

Radiolabelling:

no

Analytical monitoring:

yes

Description of key information

Solubility study of crystalline Nb2O5 indicated the presence of four species, Nb(OH)5−n/n (where n = 4–7).

Additional information

Solubility measurements of well-characterized B-Nb2O5 as a function of temperature (10 – 70 °C), ionic strength (0.1 – 0.6 mol kg−1), and pH (1 – 9) have provided a unique thermodynamic platform for determination of the speciation of Nb(OH)_n^5-n, where n = 4 - 7.

For acidic (pH ≤ 4) solutions, equilibrium (i.e., constant niobium concentration) was attained after 15 days. In the solutions with pH range of 4 to 9, a minimum of 30 days was necessary to reach equilibrium. The ionic strength, I_m, in the range of 0.1 – 6.0 mol kg−1 seemed to have no effect on the kinetics of dissolution of niobium oxide.