Acid Rain

Acid precipitation affects stone in two ways: dissolution and alteration.

The calcite dissolves when sulfurous, sulfuric, and nitric acids in polluted air react with the calcite in marble and limestone.

Exposed areas of building and statues develop rough surfaces. Some of the material that makes them up are removed or corroded, thus, details and carvings are lost.

Even sheltered areas of stone buildings and monuments are affected by acid precipitation. However, sheltered areas on limestone and marble buildings and monuments show blackened crusts that have peeled off in some places, revealing crumbling stone beneath. This black crust is primarily composed of gypsum, a mineral that forms from the reaction between calcite, water, and sulfuric acid. Gypsum is soluble in water; although it can form anywhere on carbonate stone surfaces that are exposed to sulfur dioxide gas (SO2), it is usually washed away. It remains only on protected surfaces that are not directly washed by the rain. Gypsum is white, but the crystals form networks that trap particles of dirt and pollutants, so the crust looks black. Eventually the black crusts blister and spall off, revealing crumbling stone.When marble is exposed to acid rain, sharp edges and carving details become rounded, gradually. Black crust forms on sheltered buildings and monuments.

A classic buffer is a combination of a weak acid and its conjugate salt; for instance, carbonic acid (H2CO3) and sodium bicarbonate (NaHCO3), or even sodium bicarbonate and calcium carbonate. In any buffer system, the boost in [H+] increases the reaction rate H+ + salt => weak acid and takes some H+ out of circulation.

As it does so, it increases weak acid concentration, so the reverse reaction rate starts to increase

until you get a new equilibrium. Similarly, titration with a strong base decreases the H+ + salt => weak acid rate, and so (since the weak acid dissociation is still happening), the weak acid => H+ + salt adds some H+ to the solution. Thus the pH changes less than it would if you titrated pure water - it's buffered.

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