Criteria For Water Stability At Normal Conditions

In the preceding discussions, a criterion for stability was established using the equilibrium constant called Ksp. At normal conditions, as especially used in the water works industry, specialized forms of water stability criteria have been developed. These are saturation pH, Langelier index, and the precipitation potential of a given water.

11.2.1 Saturation pH and the Langelier Index

Because pH is easily determined by simply dipping a probe into a sample, determination of the saturation pH is a convenient method of determining the stability of water. The concentrations of any species at equilibrium conditions are in equilibrium with respect to each other. Also, for solids, if the condition is at equilibrium no precipitate or scale will form. One of the concentration parameters of equilibrium is the hydrogen ion concentration, which can be ascertained by the value of the pH. Thus, if the pH of a sample is determined, this can be compared with the equilibrium pH to see if the water is stable or not. Therefore, we now proceed to derive the equilibrium pH. Equilibrium pH is also called saturation pH.

In natural systems, the value of the pH is strongly influenced by the carbonate

equilibria reactions. The CO3 species of these reactions will pair with a cation, thus "guiding" the equilibrium reactions into a dead end by forming a precipitate. For example, the complete carbonate equilibria reactions are as follows:

Cation2 (CO3 )c (s)i ^ 2Cationc+ + c CO2_ Ksp (11.17)

c is the charge of the cation that pairs with CO3 forming the precipitate Cation2(CO3)c(s). We call the formation of this precipitate as the dead end of the carbonate equilibria, since the carbonate species in solution are diminished by the precipitation.

Let us digress for a moment from our discussion of the saturation pH in order to find the dead end cation for the carbonate system equilibria. Several of these cations can possibly pair with the carbonate. The pairing will be governed by the value of the solubility product constant, Ksp. A small value of the Ksp means that only small values of the concentration of the constituent species are needed to form a product equal to the Ksp. This, in turn, means that solids with smaller Ksp's will easily form the solids. Thus, of all the possible cations that can pair with the carbonate, the one with the smallest Kp value is the one that can form a dead end sp _5

for the carbonate equilibria reactions. Mg forms MgCO3 with a Ksp of 10 . Ca forms CaCO3 with a Ksp of 4.8(10_9). Table 11.3 shows other carbonate solids with the respective solubility product constants.

From the previous table, the smallest of the Ksp's is that for Hg2CO3. Thus, considering all of the possible candidates that we have written, Hg2CO3 is the one that will form a dead end for the carbonate equilibria; however, of all the possible cations, Ca2+ is the one that is found in great abundance in nature compared to the rest. Thus, although all the other cations have much more smaller K's than calcium,

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