Calcium hexaboride

Water solubility

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

water solubility

Type of information:

other: review of literature

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:

Molecular Orbital Theory of metal-metal bonds

GLP compliance:

no

Remarks:

(Theoretical consideration of chemical structure)

Type of method:

other: literature review

Key result

Remarks on result:

not determinable

Remarks:

Based on molecular orbital theory, the bond energy of the B-B bonds is too high to be cleaved by solvation. Only hydrolysis in strong acid would slowly cleave the B-B bonds. Therefore, CaB6 is insoluble in water.

Water solubility of calcium hexaboride is not a relevant property. Rather than being an ionic solid consisting of calcium and hexaboride ions, calcium hexaboride consists of calcium ions [Ca(II)] packed into an anionic crystalline 3-D polymeric lattice formed by hexaboride. As such, calcium may be exchanged with other cations, but the hexaboride lattice will remain intact. The crystal structure of calcium hexaboride consists of calcium cations in a close packed 3-D structure of covalently-linked octahedral hexaboride units (1). “The presence of a strong bond between boron atoms in hexaborides creates a strong frame work of boron atoms, in which each boron atom is bonded to four neighboring boron atoms in the same octahedron and to one boron atom in the next octahedron. (3 1/3 bonding electrons per boron)” (2). This results in seven intra-unit bonding orbitals forming each octahedral hexaboride and six outwardly directed orbitals forming one bond with each of six neighboring octahedral hexaboride units, for each boron in an individual octahedron (3). The result is a bond dissociation energy of ca -6.5 eV for each B-B bond within a hexaboride octahedron, a -13 eV orbital consisting of two electrons shared six ways and six hexaboride-hexaboride bonds connecting the octahedral with a dissociation energy of ca. -8 eV (3). These energies can be compared to the 4.40 eV bond energy for the covalent C-H bond and a 3.91 eV bond energy for the covalent C-C bond (4). “Dissolution” would therefore involve the breaking of strong covalent bonds within the hexaboride 3-D lattice, i.e. hydrolysis, rather than the disruption of weaker intramolecular ionic involved in dissolving ionic salts or organic molecules. However, sufficient energy to do this is not available in aqueous solution. In order to dissolve one the hexaboride, more energy is required than for water to break all C-C and C-H bonds in ethane. As ethane is not atomized in water, it is safe to assume that a hexaboride octahedron would not be cleaved from the crystal lattice in water.

Calcium hexaboride will dissolve slowly in non-oxidizing acids (1, 2). The proposed mechanism is intrusion of protons into the lattice which possibly forces the adoption of a Hexaboride Lanthanum type electronic structure. This is followed by more proton intrusion and subsequent loss of Ca(II). Lastly, water intrudes and dissolution occurs. It is unclear exactly when the boride octahedral is broken down or when oxidation of released boron occurs.

1. G. Raynor, K. Trask. Chemistry and Applications of Calcium and Potassium, Rev Ed., Academic Studio, New York, NY, 2016, p. 23.

2. G.V. Samsonov, B. Paaderno. Borides of Rare Earth Metals, Academy of the Sciences, SSR Kiev, 1961, p. 10-11.

3. K. Schmitt, C. Stückl, H. Ripplinger, B. Albert. Crystal and electronic structure of BaB₆ in comparison with CaB₆ and molecular [B₆H₆]^{2 -}. Solid State Sci. 2001, vol. 3, pp. 321-32.

4.  S. J. Blanksby, G.B. Ellison. Bond Dissociation Energies of Organic Molecules. Acc. Chem. Res. 2003, vol. 36, no. 4, pp. 255–2637.

Conclusions:

Calcium hexaboride is insoluble in water.

Executive summary:

Based on a review of literature and the crystal structure of calcium hexaboride, water solubility is not a relevant property. The structure of calcium hexaboride is a covalently linked lattice of hexaboride unit interspersed with calcium ions [Ca(II)]. The bond strength of the boron-boron connections is too great for dissolution or hydrolysis in aqueous solution. After ligand-exchange by protons, the crystalline lattice may be disrupted more easily. Acid-mediated hydrolysis can then occur in non-oxidizing mineral acid solution. This study is based on a review of the scientific literature and sound scientific concepts. It is deemed reliable with restrictions, and is suitable for Risk Assessment, Classification & Labelling, and PBT Analysis.

Reference 2

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

disregarded due to major methodological deficiencies

Study period:

2017

Reliability:

3 (not reliable)

Rationale for reliability incl. deficiencies:

unsuitable test system

Qualifier:

according to guideline

Guideline:

OECD Guideline 105 (Water Solubility)

GLP compliance:

no

Type of method:

flask method

Water solubility:

0.74 mg/L

Temp.:

30 °C

Remarks on result:

other: The pH has not been measured during the test, %RSD = 4.1%

Details on results:

Single results: 0.71 / 0.77 / 0.73 mg/L
Mean value: 0.74 mg/L
Standard deviation: 0.03 mg/L
RSD: 4.1 %

The test material is not representative of the registered test substance.

Conclusions:

The test material in this study is not representative of the registered tets substance. Therefore, it is considered not reliable and a disregarded study.

Executive summary:

A sub-sample of 1 mg of calciumhexaboride, purity unknown, was weighed into a 1000 ml volumetric flask. Three sample and blank flasks, each, were prepared.  The sample and blank flasks were shaken at 30 °C ± 0.5 °C for 96 hours. After sedimentation of the remaining calciumhexaboride, the concentration of calcium in the clear supernate was measured using ICP OES, corrected by the blank value and calculated as solubility of calciumhexaboride in mg/L.

This study measured the solubility of test material with purity that is not representative of the registered test substance. In addition, the report lacks analytical details. Furthermore, OECD 105 requires that test to be performed in multiple time point to ensure an equilibrium is reached. Therefore, it is considered not reliable and a disregarded study.

Reference 3

Endpoint:

transformation / dissolution of metals and inorganic metal compounds

Type of information:

experimental study

Adequacy of study:

disregarded due to major methodological deficiencies

Study period:

March 26, 2018 to April 09, 2018

Reliability:

3 (not reliable)

Rationale for reliability incl. deficiencies:

unsuitable test system

Qualifier:

according to guideline

Guideline:

other: Analytical Method: EPA 6020 Preparation Method: EPA 3020

GLP compliance:

no

Type of method:

flask method

Type of test:

screening transformation/dissolution test - sparingly soluble metal compounds

Mean dissolved conc.:

9 997 µg/L

Element analysed:

Boron (B) concentration

Loading of aqueous phase:

100 mg/L

Incubation duration:

120 h

Type of test:

screening transformation/dissolution test - sparingly soluble metal compounds

Mean dissolved conc.:

9 608 µg/L

Element analysed:

Calcium (Ca) conctration

Loading of aqueous phase:

100 mg/L

Incubation duration:

120 h

Details on results:

The overall average measured Boron concentration for the 100 mg/L loading rate (45-mg) samples was 9997 µg/L, overall RSD% = 9.3% (n = 9). The overall average measured Calcium concentration for the 100 mg/L loading rate (45-mg) samples was 9608 µg/L, overall RSD% = 7.1% (n = 9) (Table 1).

The average measured Boron concentration for the 1000 mg/L loading rate (450-mg) samples at 120 hours was 31300 µg/L, RSD% = 10.9% (n =3). The average measured Calcium concentration for the 1000 mg/L loading rate (450-mg) samples was 54500 µg/L, RSD% = 5.35% (n =3) (Table 2).

The result between the average concentration at 45 mg/sample versus 450 mg/sample is considered to be different if %RPD ≥ 20%. The %RPD were 73% and 99% for the Boron and Calcium concentration, respectively (Table 3).

Table 1. CaB6 analytical results for 100 mg/L loading samples

Samples	B conc. (µg/L)	Ca conc. (µg/L)
Time point 1, Rep 1	9250	9110
Time point 1, Rep 2	8900	8630
Time point 1, Rep 3	9640	10200
Time point  1 Average	9263	9313
Time point  1  RSD%	4.0	8.6
Time point 2, Rep 1	11500	10700
Time point 2, Rep 2	11100	10300
Time point 2, Rep 3	9480	9470
Time point  2 Average	10693	10157
Time point  2  RSD%	10.0	6.2
Time point 3, Rep 1	10800	9960
Time point 3, Rep 2	10100	8960
Time point 3, Rep 3	9200	9140
Time point  3 Average	10033	9353
Time point 3 RSD%	8.0	5.7
Overall average	9997	9608
Overall RSD%	9.3	7.1

 

 Table 2. CaB6 analytical results for 1000 mg/L loading samples at 120 hours

Samples	B conc. (µg/L)	Ca conc. (µg/L)
Sample 1	32000	57000
Sample 2	27600	51300
Sample 3	34300	55200
Average	31300	54500
RSD%	10.9	5.35

 

Table 3. Comparison of measured concentration at 100 mg/L and 1000 mg/L loading rate

Samples	B conc. (µg/L)	Ca conc. (µg/L)
100 mg/L loading rate *	9997	9608.0
1000 mg/L loading rate**	31300	54500
%RPD	73.0	99.0

 *Overall average of measured concentration for all time points

**Average measured concentration for triplicate samples

Conclusions:

This study is considered not reliable and a disregarded study, the test system was unsuitable for the test material.

Executive summary:

Triplicate solid CaB6 material at loading rate of 100 mg/L were incubated at a setting of 30 °C in a controlled temperature incubator/shaker (100 rpm orbital shaking) for three time point of nominal 72 hours, 96 hours and 120 hours. After each time point, samples were removed and allowed to equilibrate at ambient temperature (nominal 22 ± 2 °C) for ≥ 24 hours. The samples were then filtered through Amicon regenerated cellulose member ultracentrifuge filters with a 3000 MW cut off. A few microliters of sample were removed for pH measurement using pH strip and pH were recorded. The samples were then preserved with 2 mL of ultra-trace nitric acid to a pH < 2 and the bottle capped and stored at ambient temperature out of direct light until being analyzed for calcium (Ca) and boron (B) using ICP/MS at the analytical facility.

Additionally, triplicate samples at loading rate of 1000 mg/L and incubation time of 120 hours were also prepared for analysis in parallel with the 45-mg loaded sample set. These higher loading samples were used to demonstrate that solubility determined from the 100 mg/L sample does not increase at 1000 mg/L sample, as it should not increase the level of Ca and B if it is the CaB6 that dissolves. If it is an impurity that dissolves, then the 1000 mg/L sample can be expected to have a higher concentration results than the 100 mg/L samples. A results between the average concentration at 100 mg/L sample versus 1000 mg/L sample will be considered different if RPD ≥ 20%.

The overall average measured Boron concentration for the 100 mg/L loading rate samples was 9997 µg/L, overall RSD% = 9.3% (n = 9). The overall average measured Calcium concentration for the 100 mg/L loading rate samples was 9608 µg/L, overall RSD% = 7.1% (n = 9). However, the average measured Boron concentration for the 1000 mg/L loading rate samples at 120 hours was 31300 µg/L, RSD% = 10.9% (n =3). The average measured Calcium concentration for the 1000 mg/L loading rate samples was 54500 µg/L, RSD% = 5.35% (n =3). The result between the average concentrations at 100 mg/L sample versus 1000 mg/L sample is considered to be different with %RPD of 73% and 99% for the Boron and Calcium concentration, respectively.

These results demonstrated that impurity from CaB6 also dissolve. In addition, adding nitric acid to adjust sample pH < 2 is not appropriate for water solubility measurement. Therefore, the test system was unsuitable for the test material, and this study is considered not reliable and a disregarded study.

Description of key information

Calcium hexaboride is insoluble in water.

Additional information

Based on consideration of the three-dimensional crystal structure of calcium hexaboride and the strength of the covalent bonding between adjacent hexaboride subunits, calcium hexaboride is insoluble in water. The material consists of a network of covalently bonded hexaboride subunits, so that any dissolution would be due to hydrolysis of boron-boron bonds. The energy involved this cleavage is greater than is available in aqueous solution. Two studies of water solubility used a test material with purity that is not representative of the registered test substance. Both studies used ICP methods, and one examined only calcium. The ICP method is not capable of distinguishing the form of boron (hexaboride v. borate). The second study, which examined both calcium and boron, demonstrated that impurities in the tested material were soluble in water. However, in both cases the test methods was considered unreliable, and the results are not considered further.