Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods413the case, then we cannot convert the moles of titrant consumed in reachingthe end point to the moles of titrand in our sample. Second, the titrationreaction must effectively proceed to completion; that is, the stoichiometricmixing of the titrant and the titrand must result in their reaction. Third, thetitration reaction must occur rapidly. If we add the titrant faster than it canreact with the titrand, then the end point and the equivalence point willdiffer significantly. Finally, there must be a suitable method for accuratelydetermining the end point. These are significant limitations and, for thisreason, there are several common titration strategies.A simple example of a titration is an analysis for Ag + using thiocyanate,SCN – , as a titrant.+ −Ag ( aq) + SCN ( aq) Ag(SCN) () sThis reaction occurs quickly and with a known stoichiometry, satisfying twoof our requirements. To indicate the titration’s end point, we add a smallamount of Fe 3+ to the analyte’s solution before beginning the titration.When the reaction between Ag + and SCN – is complete, formation of thered-colored Fe(SCN) 2+ complex signals the end point. This is an exampleof a direct titration since the titrant reacts directly with the analyte.If the titration’s reaction is too slow, if a suitable indicator is not available,or if there is no useful direct titration reaction, then an indirect analysismay be possible. Suppose you wish to determine the concentration offormaldehyde, H 2 CO, in an aqueous solution. The oxidation of H 2 COby I 3–HCO( aq) + I − ( aq) + 3OH − ( aq) HCO − ( aq) + 3I− ( aq)+ 2H O2 3 22() lis a useful reaction, but it is too slow for a titration. If we add a known excessof I 3 – and allow its reaction with H 2 CO to go to completion, we can titratethe unreacted I 3 – with thiosulfate, S 2 O 3 2– .− 2− 2− −I ( aq) + 2S O ( aq) S O ( aq) + 3I( aq)3 2 34The difference between the initial amount of I 3 – and the amount in excessgives us the amount of I 3 – reacting with the formaldehyde. This is an exampleof a back titration.Calcium ion plays an important role in many environmental systems. Adirect analysis for Ca 2+ might take advantage of its reaction with the ligandethylenediaminetetraacetic acid (EDTA), which we represent here as Y 4– .Ca 2 +Y 4 −CaY2 −( aq) + ( aq) ( aq)Unfortunately, for most samples this titration does not have a useful indicator.Instead, we react the Ca 2+ with an excess of MgY 2–Ca 2 +MgY 2 −CaY 2 −Mg2 +( aq) + ( aq) ( aq) + ( aq)6Depending on how we are detecting theendpoint, we may stop the titration tooearly or too late. If the end point is a functionof the titrant’s concentration, thenadding the titrant too quickly leads to anearly end point. On the other hand, if theend point is a function of the titrant’s concentration,then the end point exceeds theequivalence point.This is an example of a precipitation titration.You will find more information aboutprecipitation titrations in Section 9E.This is an example of a redox titration. Youwill find more information about redoxtitrations in Section 9D.MgY 2– is the Mg 2+ –EDTA metal–ligandcomplex. You can prepare a solution ofMgY 2– by combining equimolar solutionsof Mg 2+ and EDTA.