Reactions of acid with limestone and

Arrhenius s definition of an acid recognized the importance of the hydrogen ion, H. Arrhenius said that a chemical species is an acid if it can dissociate in aqueous solution and yield H+ ions to the solution. H+ ions were responsible for the sour taste of acids, for the effect of acids on litmus paper, and for the reaction of acids with metals and sedimentary materials like limestone. Examples of Arrhenius acids included hydrochloric acid (HCl), nitric acid (HNOj), sulfuric acid (H SO ), acetic acid (HC HjOj), phosphoric acid (H PO ), and oxalic acid (H CjOj. 

Another simple test that distinguishes acids from bases is the reaction of acids with ionic compounds that contain the carbonate ion,, to form carbon dioxide gas, water, and another compound, as shown in Figure 14.3. A similar reaction, also shown in Figure 14.3, is the source of the destructive action of acidic pollution on marble and limestone sculptures. Bases do not react with carbonates. 

The investigations (26—29) illustrated the difference between reactive and nonreactive foams. During the tests, core permeabilities ranged from 0.5 to 5.0 md. Fluid loss of the nonreactive foam was approximately half of the reactive foam, although the stability of each did not show a significant difference. This result suggests two possible scenarios. The first is that the foamed acid is destabilized in its reaction with limestone, and this destablization causes greater fluid loss of the gas phase. The second is that the permeability is increased as the add dissolves the limestone. 

The reaction occurs naturally when acid rain-water percolates through limestone strata to form stalactites in underground caves the acid rain dissolves the rock to form soluble bicarbonate, which then decomposes back to carbonate when it deposits on the stalactite. The same process takes place in the sea, both with limestone and with other alkaline sediments washed out by rivers. This serves ultimately to hold the pH of the ocean almost constant, but the equilibrium process between the river sediment and the ocean bulk is very slow. Reaction (3.3) does, however, provide a potential route for neutralizing the acidity of the injected carbon dioxide, via the deliberate addition of appropriate quantities of alkaline mineral. Such a procedure would be most suited to point sources of carbon dioxide (e.g., power stations) that are located close to both the ocean and large deposits of the mineral. 

Formation con osition. The chemical and physical compositions of the formation are very important in defining the acid spending time, and, subsequently, the acid penetration distance. Acid spends very rapidly in highly reactive (>95%) carbonates. Acid spending time can be much slower in formations with lower HCl reactivity (65%-85%). As mentioned previously, the reaction rate of acid in limestone is about twice that in dolomites (at lower temperatures). Therefore, live acid penetration can be deep in low-solubility, lower-temperature dolomites. 

Calcium carbonate, which is present in limestone and marble, is another water-insoluble solid that is soluble in strong and weak acids. Here the solid produces a low concentration of C03 ions, which combine with H to form the weak acid H2CO3. This causes more of the solid to dissolve, and so on. Carbonic acid, H2CO3, is a very unstable substance that decomposes into H2O and C02(g). The net ionic equation for the reaction of CaC03 with an acid is given below. 

Neutralization Acidic or basic wastewaters must be neutrahzed prior to discharge. If an industry produces both acidic and basic wastes, these wastes may be mixed together at the proper rates to obtain neutral pH levels. Equahzation basins can be used as neutralization basins. When separate chemical neutralization is required, sodium hydroxide is the easiest base material to handle in a hquid form and can be used at various concentrations for in-line neutralization with a minimum of equipment. Yet, lime remains the most widely used base for acid neutr zation. Limestone is used when reaction rates are slow and considerable time is available for reaction. Siilfuric acid is the primary acid used to neutralize high-pH wastewaters unless calcium smfate might be precipitated as a resmt of the neutralization reaction. Hydrochloric acid can be used for neutrahzation of basic wastes if sulfuric acid is not acceptable. For very weak basic waste-waters carbon dioxide can be adequate for neutralization. 

Sulfur dioxide emissions may affect building stone and ferrous and nonferrous metals. Sulfurous acid, formed from the reaction of sulfur dioxide with moisture, accelerates the corrosion of iron, steel, and zinc. Sulfur oxides react with copper to produce the green patina of copper sulfate on the surface of the copper. Acids in the form of gases, aerosols, or precipitation may chemically erode building materials such as marble, limestone, and dolomite. Of particular concern is the chemical erosion of historical monuments and works of art. Sulfurous and sulfuric acids formed from sulfur dioxide and sulfur trioxide when they react with moisture may also damage paper and leather. 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials Corrosive to most metals with the evolution of flammable and explosive hydrogen gas Stability During Transport Stable Neutralizing AgerUs for Acids and Caustics Flush with water and apply powdered limestone, slaked lime, soda ash, or sodium bicarbonate Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

In an attempt to find out how fast the waste was reacting with limestone, a 3-h backflushing experiment, in which waste was allowed to flow back out of the injection well, yielded some unexpected results. The increase in pH of the neutralized waste could not be fully accounted for by the solution of limestone as determined from the calcium content of the backflushed liquid the additional neutralization apparently resulted from reactions between nitric acid and alcohols and ketones in the original waste induced by increased pressure in the injection zone compared to surface conditions.41... 

Retarded acids are primarily applicable to sandstone acidizing. Fluoroboric acid slowly hydrolyzes to form the more reactive hydrofluoric acid (109,110). The time required for this hydrolysis process may enable deeper penetration of the HF into the formation although one report contradicts these findings (111). Na TiF and similar salts also slowly generate HF in acid media (112). Phosphorous acid addition to hydrochloric acid has been used to reduce the HC1 reaction rate with limestone (113). 

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