Reaction of Polysilicic Acid with Metal Cations

Reaction of Polysilicic Acid With Metal Cations 

The interaction of monosilicic acid with some metal ions has been discussed earlier in this chapter. Polysilicic acid behaves in quite a different way, mainly because it is more polyfunctional and can enter into chelate-like bonding with a metal atom, M  

The reaction of polysilicic acid with metal salts must be distinguished from the reaction of a soluble silicate, such as sodium silicate with a metal salt, as described in Chapter 2. In the latter case, metal silicates in which the silica is only slightly polymerized may be obtained. On the other hand, polysilicic acid, being already polymerized, combines with far less metal than corr ponds to a metal silicate. It is the nature of this combination that is discussed here. 

When a solution of a metal salt is added to a solution of polysilicic acid, combination occurs if the pH is above a critical value, which depends largely on the metal. For example. Hazel, Schock, and Gordon (204) found with silicic acid prepared by base exchange that, in the pH range 2-3, ferric ion reacted strongly, aluminum and chromium less, in that order barium, lanthanum, and copper did not react at this pH. 

The combination of the metal ion with the silicic acid may be regarded as an adsoprtion of the metal hydroxide on the silica. Britton (205) has tabulated the pH at which the silicates and the hydroxides precipitate from solutions of many metal salts. Combination with silicic acid does not occur much below the pH at which the metal hydroxide would be formed (either as a sol or as a precipitate) on prolonged standing. This is perhaps not surprising, when it is recalled that silicic acid is an extremely weak acid. 

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