Speciation and structure of tin(II) in hyper-alkaline aqueous solution

Area of Science:

• Inorganic Chemistry
• Aqueous Solution Chemistry
• Coordination Chemistry

Background:

• Understanding tin speciation in alkaline media is crucial for various chemical processes.
• Previous studies lacked definitive identification of tin(II) hydroxide complexes in highly alkaline solutions.

Purpose of the Study:

• To identify the predominant tin(II)-hydroxide complex in hyper-alkaline aqueous solutions.
• To elucidate the structural characteristics and stability of this complex.

Main Methods:

• Potentiometric titrations with a H2/Pt electrode.
• Spectroscopic analyses including Raman, Mössbauer, and XANES.
• Extended X-ray absorption fine structure (EXAFS) for structural determination.
• Quantum chemical calculations for theoretical support.

Main Results:

• A single monomeric tin(II) complex with a 1:3 tin(II):hydroxide ratio was identified.
• Spectroscopic and computational data confirmed the predominant complex as [Sn(OH)3](-).
• EXAFS analysis revealed a short mean Sn-O bond distance (2.078 Å) for [Sn(OH)3](-).
• Complex speciation was unaffected by high chloride concentrations at pH > 13.

Conclusions:

• The trihydroxidostannate(II) complex, [Sn(OH)3](-), is the dominant species in hyper-alkaline solutions.
• The structure of [Sn(OH)3](-) is stable and independent of reactant concentrations.
• This finding clarifies tin speciation under extreme alkaline conditions.