T

i behavior. Water, then, is the very limit of acidity. Notice the arrows pointing upward from the weakest to the strongest acids.

The most fundamental reaction of a base, B, is its reaction with H2O,

As seen from this reaction, the base, B, accepts the proton donated by H20, producing HB. HB is the conjugate acid of B. The proton donor, H20 transforms into the hydroxide ion, OH-. It is this transformation into the hydroxide ion that determines the strength of a base. Strong bases ionize completely (or nearly so) producing 100% equivalent of the hydroxide ions. Weak bases only ionize partially, which, correspondingly produce a relatively small amount of hydroxide ions. As in the case of acids, they normally ionize to about 10% or less of the original base. From the table, under the column on bases, the ethoxide ion, amide ion, hydride ion, and methide ion are the strongest bases. As indicated, they ionize to 100% OH- and, since they ionize practically completely, the activity of the base molecule is also practically zero.

The equilibrium constant for a base, Kb, is

For strong bases, since they are completely ionized, Kb would be equal to infinity. Kb is also called ionization constant for the base.

From the table, the bases between water and the hydroxide ion are the weak bases. They are the ones that ionize to 10%. Note that in the case of bases, the other boundary limit is demarcated by the hydroxide ion rather than the hydronium ion. This is so, because the hydronium ion is not a base. thus, it cannot form as a boundary for the bases. One the other hand, H20 is both an acid and a base. Thus, it consistently forms as a boundary limit in both the acids and the bases. Compounds that act both as an acid and a base are called amphoteric substances. H20 is an amphoteric substance. Above water in the table, the compounds do not exhibit any observable basic behavior. Water, then, is the very limit of basicity. Notice the arrows pointing downward from the weakest to the strongest bases.

Alkalinity and acidity. Related to acids and bases are the concepts of alkalinity and acidity. Alkalinity is defined as the capacity of a substance to neutralize an acid, and acidity is defined as the capacity of a substance to neutralize a base. Consider one mole per liter of HCl. Its neutralization reaction with OH- is

Note that the symbol is used instead of " ^ ." This is so, because HCl is a strong acid and it ionizes completely. From the reaction, one mole per liter of HCl neutralizes one mole per liter of OH- and its acidity is therefore one mole per liter.

Now, consider one mole per liter of a weak acid HCO-, Its reaction with OH- is

Now, note that the symbol is used instead of If we look at Table 1, CO3 is a stronger base than H2O. Thus, it can grab H+ from H2O forming back the left-hand side of the reaction. Forming back the left-hand side of the reaction means that Reaction (53) is a reversible reaction, so, the symbol is used. Reversible reactions are a characteristic of weak acids. Because of the reversibility of the reactions of weak acids, a mole of the acid will not neutralize a mole of a base but a quantity less than one mole. This relationship also holds true for weak bases: A mole of a weak base will not neutralize a mole of an acid but a quantity less than one mole. Again, the reason is the reversibility of the reactions of weak bases, as in the case of weak acids. In the case of strong bases, however, one mole of a strong base will neutralize one mole of an acid.

An important application of the concept of alkalinity is the determination of the amount of base needed to raise the pH of a solution. Let us apply this concept to a system composed entirely of the carbonate system. The alkalinity of the system at any instant of time will be given by the concentration of the constituent species.

Call the current pH as pHcur and the pH to be adjusted to or the destination pH as pH(0. pHcur is less than pH(0 and a base is needed. A natural water will always have acidity. Until it is all consumed, this acidity will resist the change in pH when a base is added to the water. Let the current total acidity be [Accur]geq in gram equivalents per liter. Also, let the total alkalinity to be added as [Aadd]geq in gram equivalents per liter. Assuming no acidity present, the total base to be added is 10 F cur -10 F gram moles per liter of the equivalent hydroxide ions. But since there is always acidity present, the total alkalinity to be added [Afldd]geq must include the amount for neutralizing the natural acidity, which would be [Accur]geq. Thus, the total alkalinity to be added is

where "b is the fractional dissociation of the hydroxide ion from the base supplied. For strong bases, "b is unity. Weak bases do not produce 100% hydroxide ions. Thus, the quotient "b is incorporated to yield the correct amount of base that will produce the amount of base needed to raise the pH.

Example 13 A raw water containing 140 mg/L of dissolved solids is subjected to a coagulation treatment using Fe2(SO4)3. For optimum coagulation, the pH should

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