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The uncertainty in determining the inflection point

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Unformatted text preview: the color change for an acid- base indicator marks the endpoint of an acid- base titration. This change occurs at a slight excess of NaOH; it does not mark the equivalence point. • NaOH can also be standardized by following pH as a function of the volume of the added titrant. The equivalence point in this case is not identified by an indicator color change, but by monitoring the shape of the curve that results when such data are plotted. The equivalence point is identified as the point of maximum slope in a graph of pH versus volume of titrant (Vtitrant), or the inflection point of the curve. The characteristic shape of such an acid- base titration curve also can be used to identify the pKa of the deprotonation reaction being followed. • The uncertainty in determining the inflection point of any titration curve dictates the accuracy in the equivalence volume identified. For a reaction that goes to completion (for example, the titration of a strong acid by a strong base, where Keq is large or undefined because dissociation is complete) the slope of the titration curve at the equivalence point is so steep that the error in its determination is small. However, for an acid- base reaction with a small equilibrium constant, as for a weak acid – or base, the slope will be less steep at the equivalence point, and equivalence volume determination will be less precise. Acids containing multiple acidic hydrogens (titratable protons) are called polyprotic acids. Phosphoric acid (H3PO4) is such an acid....
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