Dimerization of monosilicic acid

Figure 11. Second-order rate constant for dimerization of monosilicic acid under conditions typified by solution giving spectra shown in Fig. 10 as function of hydrochloric acid concentration. Result ( Z ) due to Engelhardt et al. (11). Solutions are effectively buffered below [HCl] = 10 mol/dm. ...

At the third stage the dimer catechol ether of monosilicic acid bonded with silica matrix is formed. 

At the eleventh stage the dimer tricatechol ether of monosilicic acid is separated from the dissolving matrix. 

We decided that it would be more effective to use vicinal dihydroxy-organic reagents. The ammonium salt of tricatechol ether of monosilicic acid (formed from biogenic silica from diatomite) was obtained. This salt contains the cation of ammonium, catechol anion and a hexacoordinated atom of silicon as 2 3 1. This salt is a dimer and is formed from the reaction (Eq. 1) ... 

Silica may be transferred to the site of silcrete development by wind and/or water. Quartz dust, for example, may be transported considerable distances from desert areas by the wind (Goudie and Middleton, 2001). Similarly, plant phytoliths, sponge spicules and diatoms can be subject to aeolian transport (Clarke, 2003). All other transfers rely upon silica transport in solution as undissociated monosilicic acid, either as the monomer H4Si04 or the dimer H6Si207 (Dove and Rimstidt, 1994). Organic or inorganic complexes may also be formed. 

The most doubtful point is the reported pK<, of 10.7 for disilicic acid, which would mean it is a weaker acid than monosilicic acid, of which the pK is 9.8. Analogy with other inorganic acids would suggest that disilicic acid should be a stronger acid than monomer. Unfortunately the dimer is difficult if not impossible to prepare and keep in sufficiently pure state for strength measurements, although a solution in which probably at least 50% of the silica was dimeric was prepared by Couduricr, Baudru, and Donnet (36). 

Figure 3.44. Earliest stages in the polymerization of 0.5m monosilicic acid at -2 C and pH 2.0 /I, monomer B. probably cyclic trimer. may also be dimer and linear trimer C. cyclic tetramer D. higher polysilicic acids. (Hocbbcl and Wicker (84).J...

---