[CaCO3] = -3 = •"'•3 - = 7.97( 10-6) gmol/L

/ CaHCO3 cK 2K CaHCO3 c


[CaHCO+] = »WW )](10-67) = 1.74(10-14) gmol/L 0.94( 10 26 )

YCa = 0.77 KCaOHc = 10-149 Yso4 = 0.77 [ so4-] = =312( 10-6) gmol/L


„ 2+] = 7.0( 10-4) - 7.97( 10-6) - 1.74(10-14) [Ca ] = -—--—

11.2.3 Total Alkalinity as Calcium Carbonate

The unit of concentration that we have used for alkalinity is equivalents per unit volume. This unit follows directly from the chemical reaction, so this method of expression is fairly easy to understand. In practice, however, alkalinity is also expressed in terms of CaCO3. Expressing alkalinities in these terms is a sort of equivalence, although this practice can become very confusing. Depending upon the chemical reaction it is involved with, calcium carbonate can have more than one value for its equivalent mass, and this is the source of the confusion. Thus, to understand the underpinnings of this method of expression, one must look at the reference chemical reaction.

As shown previously, the equivalence of all forms of alkalinity was unified by using a common end point—the methyl orange end point that corresponds to a pH

of about 4.5. For example, the OH- alkalinity was assumed to go to completion at the methyl orange end point, although the reaction is complete long beforehand at around a pH of 10.8. The same argument held for the carbonate and the bicarbonate alkalinities, which, of course, would be accurate, since their end points are legitimately at the methyl orange end point. This was done in order to have a common equivalence point. Thus, to express alkalinities in terms of CaCO3, the reaction of calcium carbonate must also be assumed to complete at the same end point. Even without assuming, this, of course, happens to be true. The reaction is

and CaCO3 has an equivalent mass of CaCO3/2, because the number of reference species is 2.

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