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Triborate Formation Constants and Polyborate Speciation under Hydrothermal Conditions by Raman Spectroscopy using a
Swaroop Sasidharanpillai1, Hugues Arcis1, Liliana Trevani1
1Department of Chemistry , University of Guelph , 50 Stone Road W. , Guelph , Ontario , Canada N1G 2W1.
The Journal of Physical Chemistry. B
|June 12, 2019
Summary
This study quantifies polyborate speciation in high-temperature boric acid solutions. The triborate ion is the dominant species, with formation constants agreeing with literature up to 200°C.
Area of Science:
- Geochemistry
- Aqueous Geochemistry
- Solution Chemistry
Background:
- Understanding boron speciation in aqueous solutions is crucial for various geochemical processes.
- High-temperature and high-pressure conditions significantly influence the equilibrium of boric acid and borate species.
- Previous studies have limited quantitative speciation data for polyborates at temperatures above 200 °C.
Purpose of the Study:
- To quantitatively determine the speciation of polyborate ions in aqueous boric acid + sodium borate solutions.
- To investigate the speciation across a temperature range of 25 to 300 °C at 20 MPa.
- To provide thermodynamic formation constants for polyborate species at elevated temperatures.
Main Methods:
- Solvent-corrected reduced isotropic Raman spectroscopy was employed.
- Measurements were conducted using a custom-made titanium flow cell with sapphire windows.
- Perchlorate anion (ClO4-) was used as an internal standard for quantitative analysis.
Main Results:
- The study reports the first quantitative speciation of polyborate ions in high-temperature aqueous solutions (>200 °C).
- At low sodium/boron ratios, boric acid (B(OH)3) and the triborate ion ([B3O3(OH)4]-) were identified as major species.
- The triborate ion ([B3O3(OH)4]-) was the predominant polyborate species across the entire temperature range (25–300 °C).
Conclusions:
- The triborate ion is the dominant polyborate species in boric acid-rich solutions from 25 to 300 °C.
- Calculated thermodynamic formation constants for the triborate species align with literature values up to 200 °C.
- At 300 °C, the formation constant for triborate is approximately three times larger than extrapolated literature values, indicating a significant temperature effect.
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