EDTA and Water Hardness

Thus a one-to-one reaction is involved, and in fact, all reactions of EDTA with metal ions (most metals do react with EDTA) are one-to-one. Therefore, we will not be concerned with a formal scheme for determining equivalent weights, as we were with acid-base reactions. 

Ethylenediaminetetraacetic acid is frequently symbolized H4Y, and the disodium salt, Na2H2Y. The hydrogens in these formulas are the acidic hydrogens associated with the carboxyl groups, as in any weak organic carboxylic acid they dissociate from the EDTA ion in a series of equilibrium steps  

FIGURE 5.20 The disodium dihydrogen salt of ethylenediaminetetraacetic acid. 

When Na2H2Y is dissolved in water and mixed with a solution of a metal ion, such as calcium, at a basic pH, the metal ion will react with the predominant EDTA species such that the following equilibrium is established  

EDTA titrations are routinely used to determine water hardness in a laboratory. Raw well water samples can have a significant quantity of dissolved minerals that contribute to a variety of problems associated with the use of such water. These minerals consist chiefly of calcium and magnesium carbonates, sulfates, etc. The problems that arise are mostly a result of heating or boiling the water over a period of time such that the water is evaporated, and the calcium and magnesium salts become concentrated and precipitate in the form of a scale on the walls of the container, hence the term hardness. This kind of problem is evident in boilers, domestic and commercial water heaters, humidifiers, tea kettles, and the like. 

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