[JohnP]

Consider the following reaction:
3Si₃N₄ + 27H₂O + 4H₃PO₄ --> 4(NH₄)₃PO₄ + 9H₂SiO₃  -->  ^4NH₃(g) + 4H₃PO₄ + 9H₂SiO₃ --> 4H₃PO₄ + ??

The phosphoric acid solution is 85%wt.

In your fella's opinion, what happens to the Meta Silicic Acid (H₂SiO₃)? Is the solution to acidic to form a polysilicic acid?
Will this occur: H₂SiO₃ --> H₂O + SiO₂  and form a gelatinous poly silica? Something else?
I do know that at relatively low concentrations of this silicate based product (~100PPM @ 155C) the product will come out of solution.
Appreciate your thoughts, thanks guys.

[john13579]

This reaction is going to be very slow because phosphoric acid is not very strong and Si3N4 generally only reacts with strong acid. The solution has to be below a pH of about 9 in order to form Silicic Acid (pka 9.84, 13.2). I don't think it can be too acidic to form Silicic Acid but it will make the solution more hygroscopic and possibly accelerate the dehydration.  The reaction will end in the formation of ammonium phosphate , leftover phosphoric acid and silicon dioxide. Silicic Acid  is ephemeral at any appreciable concentration like you said.

[Intanjir]

Typically any possible H2SiO3 molecule would immediately take on a water to form Si(OH)4. An Si=O exposed on the surface in aqueous environment is predominantly replaced by Si-(OH)2. However you do only have 15% water so perhaps there would be more of that than normal. In any case, Si-OH bonds will condensate to form Si-O-Si linkages. I suppose briefly you have something you might want to call a polysilicic acid but very soon it will grow to the size that you just want to call it hydrated silica. The only way the condensation will stop with something small is if the solution is thoroughly basic (ph of 12). So yes definitely the solution would be too acidic for anything so small as a polysilicic acid to be stable.

The real question then is what form will the hydrated silica take? Fully dense spheres of Si-O-Si in the center with Si-OH limited to the surface, or a convoluted gel with Si-OH dispersed throughout or something inbetween? If all of this is happening in 85% phosphoric acid then this is a rather unusual scenario so I wouldn't place too much faith in conjecturing here mostly because the influence of fleeting Si-O-P on the structure may be significant. However I would favor a rather convoluted gel since an Si-O-Si would not be broken once formed given the rarity of both H2O and OH-, leaving no opportunity to densify by minimizing surface energy by a precipitation redissolution cycle. Since the concentration of Si-(OH)4 isn't likely to be radically high the gel can't possibly fill the whole volume. You shouldn't form much of a sol though because the phosphate ions should screen the positive surface charge of silica(PH should be below the isoelectric point), allowing particles to readily approach each other and clump.