The hydrogen ion concentration corresponding to pHcur is 10 P cur gram moles per liter and that corresponding to pH(0 is 10~pH'° gram moles per liter.

Note: The gram moles per liter of hydrogen ion is equal to its number of equivalents per liter.

Assuming no alkalinity present, the total acid to be added is 10 p - 10 p cur gram moles per liter. Alkalinity is always present, however, so more acid must be added to counteract the natural alkalinity, [Acur]geq. Thus, the total acidity to be added, [Acadd]geq, is

where Ya is the fractional dissociation of the hydrogen ion from the acid supplied. For strong acids, Ya is unity; for weak acids, it may be calculated from equilibrium constants (Table 11.5).

To determine the equivalent mass of carbon dioxide needed, write the following chemical reactions, noting that carbon dioxide must react with existing alkalinity:

From these reactions, the equivalent mass of CO2 is CO2/2.

Let us calculate some fractional dissociations of H2CO3. Its dissociation reaction is as follows:

First, assume a 0.1 M concentration of carbonic acid and let x be the concentration of H+ and HCO- at equilibrium. At equilibrium, the concentration of carbonic acid will be 0.1 - x. Thus, substituting into the equilibrium expression produces

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