272_Exp4BC

In a potentiometric titration the equivalence point

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Unformatted text preview: ed is called the equivalence point. In an acid- base titration, no visible change occurs at this point. However, if an acid- base indicator is added to the sample being analyzed, the indicator will undergo its characteristic color change just beyond equivalence. The color change marks the endpoint of the titration. In a potentiometric titration, the equivalence point is determined not by a color change, but as the point where the maximum change in the concentration of the species being titrated as a function of the volume of added titrant is observed. In the titration of an acid by a base, the concentration of the species being titrated, [H+], is measured as pH and the titrant is NaOH. The equivalence point is therefore identified as the point of maximum slope in a graph of pH vs. the volume of titrant (Vtitrant) added; this is also known as the inflection point of the curve. The uncertainty in determining the inflection point, and thus the accuracy in the equivalence point, is a function of the strength of the acid or base being titrated. • The equivalence point of a titration of an acid is reached when the number of moles of hydroxide ion (OH- ) added equal the number of moles of proton (H+) present. • When quantitative analysis titration is used to standardize an NaOH titrant, the concentration of the titrant is determined by dividing the number of acid equivalents present in a known number of moles of a primary standard, like KHP, by the volume of NaOH needed to deliver an equivalent number of moles of OH)- . • The volume of titrant required to reach...
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