measurementof acid neutralizing capacity- c e e cornell univ（measurementof acid neutralizing capacity - c e e cornell univ）.doc

Measurement of Acid Neutralizing Capacity Introduction Acid neutralizing capacity (ANC) is a measure of the ability of water to neutralize acid inputs. Lakes with high ANC (such as Cayuga Lake) can maintain a neutral pH even with some acid rain input whereas lakes with an ANC less than the acid input will not maintain a neutral pH. In the Adirondack region of New York State, lakes typically receive large inputs of acids during the spring thaw when the accumulated winter snow melts and runs off into the lakes. The ANC of Adirondack lakes is not always sufficient to neutralize these inputs. Theory The ANC for a typical carbonatecontaining sample is defined as: This equation can be derived from a charge balance if ANC is considered to be the cation contributed by a strong base titrant and if other ions present do not contribute significantly. Determination of ANC or Alkalinity involves determination of an equivalence point. The equivalence point is defined as the point in the titration where titrant volume that has been added equals the "equivalent" volume (Ve). The equivalent volume is defined as: where: Ns = normality (in this case Alkalinity or ANC) of sample, equivalents/L Vs = volume of sample, liters Nt = normality of titrant, equivalents/L. The titration procedure involves incrementally adding known volumes of standardized normality strong acid (or base) to a known volume of unknown normality base (or acid). When enough acid (or base) has been added to equal the amount of base (or acid) in the unknown solution we are at the "equivalence" point. (Note: the point at which we add exactly an equivalent or stoichiometric amount of titrant is the equivalence point. Experimentally, the point at which we estimate to be the equivalence point is called the titration endpoint). There are several methods for determining Ve (or the equivalence point pH) from titration data (titrant volume versus pH). The shape of the titration curve (Vt versus pH) can reveal Ve. It