Experimental Determination of Solubilities of Sodium Polyborates In MgCl<sub>2</sub>Solutions: Solubility Constant of Di-Sodium Hexaborate Tetrahydrate, and Implications For the Diagenetic Formation of Ameghinite
Bibliographic record
Abstract
In this paper, solubility measurements were conducted for sodium polyborates in MgCl<sub>2</sub> solutions at 22.5 ± 0.5 °C. According to solution chemistry and XRD patterns, di-sodium tetraborate decahydrate (borax) dissolves congruently, and is the sole solubility-controlling phase, in a 0.01 mol/kg MgCl<sub>2</sub> solution: Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub>•10H<sub>2</sub>O(cr) ⇌ 2Na<sup>+</sup> + 4B(OH)<sub>4</sub> + 2H<sup>+</sup> + H<sub>2</sub>O(l). However, in a 0.1 mol/kg MgCl<sub>2</sub> solution borax dissolves incongruently and is in equilibrium with di-sodium hexaborate tetrahydrate: 2Na<sub>2</sub>B<sub>6</sub>O<sub>10</sub>•4H<sub>2</sub>O(cr) + 2Na<sup>+</sup> + 23H<sub>2</sub>O(l) ⇌ 3Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub>•10H<sub>2</sub>O(cr) + 2H<sup>+</sup>. In this study, the equilibrium constant (log K<sup>0</sup>) for Reaction 2 at 25 °C and infinite dilution was determined to be –16.44 ± 0.13 (2σ) based on the experimental data and the Pitzer model for calculations of activity coefficients of aqueous species. In accordance with the log K<sup>0</sup> for Reaction 1 from a previous publication from this research group, and log K<sup>0</sup> for Reaction 2 from this study, the equilibrium constant for dissolution of di-sodium hexaborate tetrahydrate at 25 °C and at infinite dilution, Na<sub>2</sub>B<sub>6</sub>O<sub>10</sub>•4H<sub>2</sub>O(cr) + 10H<sub>2</sub>O(l) ⇌ 2Na<sup>+</sup> + 6B(OH)<sub>4</sub><sup>-</sup> + 4H<sup>+</sup> was derived to be –45.42 ± 0.16 (2σ). The equilibrium constants determined in this study can find applications in many fields. For example, in the field of nuclear waste management, the formation of di-sodium hexaborate tetrahydrate in brines containing magnesium will decrease borate concentrations, making less borate available for interactions with Am(III). In the field of experimental investigations, based on the equilibrium constant for Reaction 2, the experimental systems can be controlled in terms of acidity around neutral pH by using the equilibrium assemblage of borax and di-sodium hexaborate tetrahydrate at 25 °C. As salt lakes and natural brines contain both borate and magnesium as well as sodium, the formation of sodium hexaborate tetrahydrate may influence the chemical evolution of salt lakes and natural brines. Di-sodium hexaborate tetrahydrate is a polymorph of the mineral ameghinite [chemical formula Na<sub>2</sub>B<sub>6</sub>O<sub>10</sub>•4H<sub>2</sub>O; structural formula NaB<sub>3</sub>O<sub>3</sub>(OH)<sub>4</sub> or Na<sub>2</sub>B<sub>6</sub>O<sub>6</sub>(OH)<sub>8</sub>]. Finally, di-sodium hexaborate tetrahydrate could be a precursor of ameghinite and could be transformed when borate deposits are subject to diagenesis.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.002 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".