A Hierarchy of Acid-Base Balances

The simplest acid-base relationships should involve the smallest number of molecular species and the smallest number of phases. Adding 

Before proceeding through a hierarchy of examples, a word about the term equilibrium is in order, particularly as it applies to the dynamically changing components of the Earth system. It is a fact that any particular chemical system itself will rarely be in true equilibrium, just as the physical systems of Earth are not ever really in a perfect steady state. The equilibrium conditions are extremely relevant because they describe the tendency of the system to which termodynamically favorable reactions tend. That is, no matter what the condition is, all systems are moving toward equilibrium. 

Not surprisingly, the acid-base balances within the Earth system almost all involve elements of high abundance, i.e., elements that have low atomic number. In many cases, the acidic molecule is an oxygen-containing oxidation product of an element. Table 16-1 lists the main acids and bases in the global environment. The sources of these acids are chemical reactions of reduced forms of the element involved. Both gas and aqueous phase reactions exist for production of acids. 

H2CO3 (carbonic acid) aqueous phase HC2H3O2 (acetic acid) aqueous, gas phases HCHOz (formic acid) aqueous, gas phases H2C2O4 (oxalic acid) aerosol particles solid phase RCCX3H (many carboxylic acids) 

HNO3 (nitric acid) gas, aqueous, solid phases HNO2 (nitrous acid) gas, aqueous phases Sulfur based  

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