Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods515do absorb at this wavelength. The m-nitrophenolate ion has a greaterabsorbance than an equimolar solution of the p-nitrophenolate ion.Sketch the spectrophotometric titration curve for a 50.00-mL mixtureconsisting of 0.0500 M p-nitrophenol and 0.0500 M m-nitrophenolwith 0.100 M NaOH. Compare your result to the expected potentiometrictitration curves.10. The quantitative analysis for aniline (C 6 H 5 NH 2 , K b = 3.94 × 10 –10 )can be carried out by an acid–base titration using glacial acetic acidas the solvent and HClO 4 as the titrant. A known volume of samplecontaining 3–4 mmol of aniline is transferred to a 250-mL Erlenmeyerflask and diluted to approximately 75 mL with glacial acetic acid. Twodrops of a methyl violet indicator are added, and the solution is titratedwith previously standardized 0.1000 M HClO 4 (prepared in glacialacetic acid using anhydrous HClO 4 ) until the end point is reached.Results are reported as parts per million aniline.(a) Explain why this titration is conducted using glacial acetic acid asthe solvent instead of water.(b) One problem with using glacial acetic acid as solvent is its relativelyhigh coefficient of thermal expansion of 0.11%/ o C. For example,100.00 mL of glacial acetic acid at 25 o C occupies 100.22 mL at27 o C. What is the effect on the reported concentration of anilineif the standardization of HClO 4 is conducted at a temperature thatis lower than that for the analysis of the unknown?(c) The procedure calls for a sample containing 3–4 mmoles of aniline.Why is this requirement necessary?Some of the problems that follow require one ormore equilibrium constants or standard state potentials.For your convenience, here are hyperlinksto the appendices containing these constantsAppendix 10: Solubility ProductsAppendix 11: Acid Dissociation ConstantsAppendix 12: Metal-Ligand Formation ConstantsAppendix 13: Standard State Reduction Potentials11. Using a ladder diagram, explain why the presence of dissolved CO 2leads to a determinate error for the standardization of NaOH if theend point’s pH falls between 6–10, but no determinate error if the endpoint’s pH is less than 6.12. A water sample’s acidity is determined by titrating to fixed end pointpHs of 3.7 and 8.3, with the former providing a measure of the concentrationof strong acid, and the later a measure of the combinedconcentrations of strong acid and weak acid. Sketch a titration curvefor a mixture of 0.10 M HCl and 0.10 M H 2 CO 3 with 0.20 M strongbase, and use it to justify the choice of these end points.13. Ethylenediaminetetraacetic acid, H 4 Y, is a weak acid with successiveacid dissociation constants of 0.010, 2.19 × 10 –3 , 6.92 × 10 –7 , and5.75 × 10 –11 . Figure 9.46 shows a titration curve for H 4 Y with NaOH.What is the stoichiometric relationship between H 4 Y and NaOH at theequivalence point marked with the red arrow?pH141210864200 10 20 30 40Volume of NaOH (mL)Figure 9.46 Titration curve for Problem9.13.