The term borate species refers to the various chemical forms (species) in which boron can exist in a solution. The form depends strongly on the pH value .
Important borate species
1. Boric acid (H₃BO₃) – undissociated, neutral
- Predominant at pH < 7
- Acts as a weak Lewis acid
- Exists mainly as uncharged molecules
Reaction in water:
2. Tetrahydroxoborate ion ([B(OH)₄]⁻) – anionic
- Predominant at pH > 9
- Formed by the reaction of boric acid with hydroxide ions (OH⁻)
- Important for the titration of boron with NaOH
3. Polycondensed borates
- At higher concentrations and certain pH ranges, boron can form borate oligomers or polyborates
- An example is the tetraborate ion [B₄O₇]²⁻
4. Boron-mannitol complex
- By adding mannitol, boron forms a stable complex
- This complex acts like a strong acid
- Can be titrated by NaOH
Summary
| pH range | Predominant borate species |
|---|---|
| pH < 7 | Boric acid (H₃BO₃) |
| pH 7 – 9 | Equilibrium between H₃BO₃ and [B(OH)₄]⁻ |
| pH > 9 | Tetrahydroxoborate ion ([B(OH)₄]⁻) |
| With mannitol | Boron-mannitol complex (titratable with NaOH) |
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