To find the equivalent concentration of alkalinity in terms of CaCO3 proceed as follows:
Note: We have reacted the alkalinity species (CaCO3) with the hydrogen ion, since it is the property of alkalinity to react with an acid.
From this reaction, the equivalent mass of CaCO3 is 50. Therefore, a mass equivalent of alkalinity is equal to 50 mass units expressed as CaCO3. For example, 1 gram equivalent of alkalinity is equal to 50 grams of alkalinity expressed as CaCO3.
The determination of the equivalent concentration of acidity in terms of CaC03 is a bit tricky. Calcium carbonate does not react with an 0H- which ought to determine the amount of acidity that CaC03 contains, if it does. But since no reaction can ever occur, CaC03 does not have any acidity; it has alkalinity, instead, as shown by the reaction in Equation (12.85). If CaC03 does not have any acidity, why then express acidity in terms of calcium carbonate? This is one of the biggest blunders in the environmental engineering literature, and it ought to be a big mistake; however, right or not, things can always be made arbitrary and then rationalized—this is what is done and used in practice. Arbitrarily, Equation (12.85) has been made the basis for the determination of the equivalent mass of CaC03 when expressing concentral tions of acidity in terms of calcium carbonate. The rationalization is that the two H+ in Equation (12.85) is equivalent to two 0H-. Thus, the reaction is equivalent as if CaC03 is reacting directly with 0H-. Reasonable? No, but you have to take it. This rationalization is the same as the following rationalization: Pedro and Maria are friends. Maria and Jose are also friends. Therefore, Pedro and Jose are friends. Is this correct? No, because Pedro and Jose are, in reality, irreconcilable enemies. Nonetheless, equivalent mass of CaC03 — 50, no questions asked.
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