Silica deposit, colloidal, relative

A colloidal silica deposit having a density of 0.129g/square meter can reduce the relative light transmittance by as much as... 

V. Electrolyte.—The nature of the anion often has a very important influence on the physical form of the deposited metal for example, lead from lead nitrate solution is rough, but smooth deposits are obtained from silicofluoride and borofluoride solutions. The valence state of the metal may affect the nature of the deposit thus, from plumbic solutions lead is deposited in a spongy form whereas relatively large crystals are formed in plumbous solutions. In an analogous manner, smooth deposits of tin are obtained from stannate baths, but from stannite solutions the deposits are of poor quality. The difference in the behavior of different electrolytes is sometimes due to the possibility of the formation of colloidal matter which serves to give a fine-grained deposit this may be the case in the deposition of lead from silicofluoride and borofluoride solutions where a certain amount of colloidal hydrous silica or boron trioxide may be formed by hydrolysis. 

The rate of addition of active silica to the reaction solution from which silica is being deposited is preferably, but not necessarily, sufficiently slow for all of the silica to be deposited on the substrate and not form colloid. This can be judged by determining the yield of silica on the substrate. However, in coating coarse powders or other substrates of relatively low surface area, the silica yield is not as important as long as the colloid formed can be separated from the substrate. 

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