Arsenious acid Adsorption

Sodium hydroxide is adsorbed appreciably by ferric hydroxide and the presence of the alkali in solution diminishes the amount of arsenic adsorbed. On the other hand, salts such as potassium or ammonium chloride have no effect on the adsorption. The ferric hydroxide, especially when fresh, is liable to become peptised by arsenious acid in certain dilutions. Apparently peptisation results when a definite quantity of arsenious acid has been adsorbed by each particle.3 Excess of arsenious acid, however, causes flocculation. 

The Adsorption of Arsenious Acid. — Biltz has studied the adsorption combination of arsenious acid with colloidal iron oxide. The complex was previously given a definite chemical formula and was regarded by Bunsen, who discovered that the hydrogel of iron was an... 

Colloidal saccharated iron is sometimes used in place of ferric hydroxide as an antidote in arsenical poisoning, but its adsorptive capacity depends on the alkalinity of the medium.4 Thus a commercial preparation containing 0-75 per cent, of sodium hydroxide was found to adsorb 12-57 per cent, of arsenious oxide (reckoned on the amount of iron present) addition of alkali increased the adsorption until, with 1-28 per cent, of sodium hydroxide present, there was a maximum adsorption of 27 per cent. The addition of acid correspondingly diminished the adsorption. A gel of ferric magnesium hydroxide, if prepared without boiling, also adsorbs arsenic from sodium arsenite solutions.5... 

In the case of arsenious sulfide sulfoarsenious acid may be formed and the anion adsorbed by the particles. At present it is difficult to determine which of these points of view is the correct one. In any event the negative charge is occasioned by adsorption of ions, or by ionization on the surface of the particles. The colloidal chemistry of the future will undoubtedly have to deal with the adsorption of ions by ultramicrons. 

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