Applications of Adsorption

Adsorption underlies a wide range of practical applications. (i) Catalysis: heterogeneous solid catalysts, whose action depends on adsorbing reactants onto their surface, are used throughout industry -- iron in manufacturing ammonia, platinum in manufacturing sulfuric acid (H2SO4) by the contact process, and finely divided nickel in hydrogenating oils. (ii) Gas masks: a device packed with activated charcoal or a mixture of adsorbents, used for breathing in coal mines to avoid inhaling poisonous gases, which get adsorbed onto the charcoal instead of reaching the lungs. (iii) Control of humidity: silica gel and alumina gel are effective adsorbents of atmospheric moisture and are used to keep enclosed spaces dry. (iv) Production of high vacuum: lowering the temperature at a given pressure increases the rate at which charcoal adsorbs gas; a vessel evacuated with a vacuum pump is connected to a second vessel containing coconut charcoal cooled with liquid air, and the cooled charcoal adsorbs the remaining traces of air or moisture, driving the first vessel to a high vacuum. (v) Adsorption indicators: used to detect the end point of precipitation titrations -- for example, titrating a sodium chloride solution containing a trace of fluorescein against silver nitrate: NaCl + AgNO3 -> AgCl (white precipitate) + NaNO3; once chloride ions are used up, fluorescein is instead adsorbed onto the white AgCl precipitate itself and turns it from pale yellow to a reddish-pink colour, marking the end point. (vi) Separation of inert gases: in a mixture of noble gases, different gases adsorb to different extents on coconut charcoal, so selective adsorption lets them be separated. (vii) Froth flotation: a low-grade sulfide ore is concentrated by separating it from silica and other earthy gangue material, using pine oil as a frothing agent -- the hydrophobic pine oil preferentially wets (adsorbs onto) the sulfide ore, w …