On the Stability of the AlOSi ( OH ) 32+ Complex in Aqueous Solution

• The complexation of aluminium(III) and silicon(IV) was studied in a simplified seawater medium (0.6 M Na(Cl)) at 25 °C. The measurements were performed as potentiometric titrations using a hydrogen electrode with OH − ions being generated coulometrically. The total concentrations of Si(IV) and Al(III) respectively [Si tot ] and [Al t ot], and −log[H +] were varied within the limits 0.3 < [Si tot ] < 2.5 mM, 0.5 < [Al tot ] < 2.6 mM, and 2 ≤ -log[H +] ≤ 4.2. Within these ranges of concentration, evidence is given for the formation of an AlSiO(OH) 3 2+ complex with a formation constant log β1,1-1 = −2.75 ± 0.1 defined by the reaction Al 3++Si (OH)4 ↔ AlOSi(OH) 3 2+ +H + An extrapolation of this value to I=0 gives log β1,1-1 = −2.30. The calculated value of log K (Al 3++SiO(OH) 3 − ↔ AlOSi(OH) 3 2+ ) = 6.72 (I=0.6 M) can be compared with corresponding constants for the formation of AlF 2+ and AlOH 2+ , which are equal to 6.16 and 8.20. Obviously, the stability of these Al(III) complexes decreases within the series OH −>SiO(OH) 3 −  > F−

Nyckelord

aluminium

equilibrium analysis

silica

solution chemistry

speciation

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