Lab Report > University of Houston - Chem 3369 Chapter06-1-2ac. Good Titrations: Volumetric Analysis-1

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Good Titrations: Volumetric Analysis-1 Ch6-2 CHEM 3369 1. Volumetric Analysis A titration is a procedure in which increments of the known reagent solution – the titrant – are added to analy... te until the reaction is complete. Titrant is usually delivered from a buret. Each increment of titrant should be completely and quickly consumed by reaction with analyte until the analyte is used up. Common titrations are based on: • acid-base reactions • oxidation-reduction reactions • precipitation reactions • complex formation reactions Ch6-3 CHEM 3369 How to Determine the Reaction Has Gone to Completion (1) detecting a sudden change in voltage or current between a pair of electrodes (Chapters 10 & 14) Methods of determining when analyte has been consumed include: (2) monitoring the absorbance of light by species in the reaction (Section 19-3) (3) observing an indicator color change (Sections 6-6, 9-6, 10-4, 13-3, and 16-2) Ch6-4 CHEM 3369 When the Titration Reaction Stops Equivalence point: the point at which stoichiometric amounts of reactants have been added (theoretical): moles of analyte originally in flask moles of titrant added from buret = End point: what we actually measure – marked by a sudden physical or chemical change in solution. ?end point - ?equivalence point = ?titration error ® want to make titration error as small as possible ® choose appropriate end point detection ® do a blank titration If one mole of titrant reacts with one mole of analyte Ch6-5 CHEM 3369 Example 1: An Acid-Base Titration After an initial buret reading, base (OH– ions) is slowly added to the acid (H+ ions). H+(aq) + Cl–(aq) + Na+(aq) + OH–(aq) ® H2O(l) + Cl–(aq) + Na+(aq) Near the end of the titration, the indicator momentarily changes to its base color but reverts to its acid color with swirling. When the end point is reached, a tiny excess of OH– is present, shown by the permanent change of color of the indicator. Base Acid plus phenolphthalein (colorless in acid & pink in base) Ch6-6 CHEM 3369 change occurs over ~2 pH units An acid/base indicator is a weak organic acid or base, whose undissociated form differs in color from its conjugate base or its conjugate acid form low basic blu mixture of yellow and yel e lueColor change of the indicator bromothymol blue acidic Ch6-7 CHEM 3369 Na+(aq) + Cl–(aq) + Ag+(aq) + NO3–(aq) ® AgCl(s) + NO3–(aq) + Na+(aq) AgNO3(aq) NaCl(aq) Prior to the titration, the solution is yellow, due to the dichlorofluorescein indicator. Milky-white AgCl suspension appears during the titration. The pink suspension appears at the end point, when the anionic indicator becomes adsorbed to the cationic particles of precipitate. Example 2: A Precipitation Titration COLOR PLATE 1 The Fajans titration of Cl– with Ag+. Ch6-8 CHEM 3369 A crystal growing in the presence of excess lattice anions (anions that belong in the crystal) has a negative charge because… See Figure 6-6 Ions from solution are adsorbed on the surface of a growing crystallite. … anions are predominantly adsorbed. A crystal growing in the presence of excess lattice cations has a positive charge and… …can therefore adsorb a negative indicator ion. Ch6-9 CHEM 3369 Example 3: A Redox Titration 5C 2O42– + 2MnO4– + 16H+ ® 10CO2 + 2Mn2+ + 8H2O Up to the equivalence point, all the added permanganate is consumed by oxalate, and the titration solution remains colorless. After the equivalence point, unreacted permanganate builds up until there is enough to see. Titrant is also an indicator Ch6-10 CHEM 3369 Two Types of Volumetric Titration In a direct titration, titrant is added until the end point is observed. analyte Unknown titrant Known + ® product Titrant (All colorless) Permanganate (purple) Analyte Oxalic acid (colorless) 5HO–C–C–OH + 2MnO4– + 6H+ ® 10CO2 + 2Mn2+ + 8H2O O || O || Example: addition of permanganate titrant to oxalic acid analyte [Show More]

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