Complexation of Silicic Acid with Tiron in Aqueous Solution Under near Natural Condition

The formation of a silicic acid–tiron (a derivative of catechol) complex was investigated in aqueous solution with various molar ratios of tiron to silicic acid (tiron/Si) or with various pH using 29 Si NMR. Only the 1:3 silicic acid–tiron complex was detected in despite of the range of tiron/Si molar ratios. This complex is stable in the pH range from 6 to 10. This indicates that the formation of a hydrogen bond (SiOH⋅⋅⋅O− in tiron), due to dissociation of H + from hydroxy groups of tiron, may be a trigger for the formation of the complex. Around pH=6, the formation of the complex occurs when the tiron/Si molar ratio is larger than 3 and most of silicic acid is converted to the complex when the ratio is larger than 25. Based on the temperature dependence of the 29 Si NMR spectra for the complex, the existence of two possible meridional and facial isomers was confirmed. The conditional formation constant for the 1:3 silicic acid–tiron complex was defined as Eq. 8 and estimated to be 2.0 mol −1 ⋅dm 3 for the facial complex and 4.1 mol −1 ⋅dm 3 for the meridional complex, respectively.