Acid, stannic electrolysis

In some cases, particularly with iaactive metals, electrolytic cells are the primary method of manufacture of the fluoroborate solution. The manufacture of Sn, Pb, Cu, and Ni fluoroborates by electrolytic dissolution (87,88) is patented. A typical cell for continous production consists of a polyethylene-lined tank with tin anodes at the bottom and a mercury pool (ia a porous basket) cathode near the top (88). Pluoroboric acid is added to the cell and electrolysis is begun. As tin fluoroborate is generated, differences ia specific gravity cause the product to layer at the bottom of the cell. When the desired concentration is reached ia this layer, the heavy solution is drawn from the bottom and fresh HBP is added to the top of the cell continuously. The direct reaction of tin with HBP is slow but can be accelerated by passiag air or oxygen through the solution (89). The stannic fluoroborate is reduced by reaction with mossy tin under an iaert atmosphere. In earlier procedures, HBP reacted with hydrated stannous oxide. 

Electrical charges on the particles operate in such a manner that many hydrosols behave during electrolysis precisely as electrolytes with large complex ions. Colloidal stannic acid, the purple of Cassius and many others may be cited as examples. The reactions of hydrosols are often determined by the electrical charge and are generally characteristic of the sign. They are dependent upon the nature of the disperse phase just as in the case of crystalloidal solutions. The particles of reversible and also the irreversible hydrosols behave similarly to molecules and ions in that they are adsorbed by various substances. 

The electrolysis of stannic acid bydrosol, or the purple of Cassius takes place in precisely the same manner as that of the crystalloids, methyl-orange, or the sodium salts of dyestuffs. In both cases under the microscope the separation of the alkali soluble constituent may be seen at the cathode. Where the particles are small enough no movement can be perceived even under the ultramicroscope. Determinations of the migration numbers have shown that in one case seven and in others ten gram mols of stannic acid were transported for every equivalent of silver deposited in the voltmeter. A portion of the stannic acid was deposited on the anode. 

Electric Charge. — The amicrons of the hydrosol are charged negatively and migrate toward the anode in neutral, alkaline, or weakly acid solution. During, the electrolysis of neutral solutions silicic acid does not separate out at the anode as stannic acid and many other colloids do, but falls slowly to the bottom in striations much after the nature of sulfuric acid. 

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