Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods469available for locating the end point, including indicators and sensors thatrespond to a change in the solution conditions.Fi n d i n g t h e En d p o i n t w it h a n In d i c a t o rMost indicators for complexation titrations are organic dyes—known asmetallochromic indicators—that form stable complexes with metalions. The indicator, In m– , is added to the titrand’s solution where it forms astable complex with the metal ion, MIn n– . As we add EDTA it reacts firstwith free metal ions, and then displaces the indicator from MIn n– .n 4 2MIn + Y → MY + In− − − m−If MIn n– and In m– have different colors, then the change in color signalsthe end point.The accuracy of an indicator’s end point depends on the strength of themetal–indicator complex relative to that of the metal–EDTA complex. Ifthe metal–indicator complex is too strong, the change in color occurs afterthe equivalence point. If the metal–indicator complex is too weak, however,the end point occurs before we reach the equivalence point.Most metallochromic indicators also are weak acids. One consequenceof this is that the conditional formation constant for the metal–indicatorcomplex depends on the titrand’s pH. This provides some control over anindicator’s titration error because we can adjust the strength of a metal–indicator complex by adjusted the pH at which we carry out the titration.Unfortunately, because the indicator is a weak acid, the color of the uncomplexedindicator also changes with pH. Figure 9.30, for example, showsthe color of the indicator calmagite as a function of pH and pMg, whereH 2 In – , HIn 2– , and In 3– are different forms of the uncomplexed indicator,and MgIn – is the Mg 2+ –calmagite complex. Because the color of calmagite’smetal–indicator complex is red, it use as a metallochromic indicatorhas a practical pH range of approximately 8.5–11 where the uncomplexedindicator, HIn 2– , has a blue color.Table 9.14 provides examples of metallochromic indicators and themetal ions and pH conditions for which they are useful. Even if a suitableindicator does not exist, it is often possible to complete an EDTA titrationTable 9.14 Selected Metallochromic IndicatorsIndicator pH Range Metal Ions acalmagite 8.5–11 Ba, Ca, Mg, Zneriochrome Black T 7.5–10.5 Ba, Ca, Mg, Zneriochrome Blue Black R 8–12 Ca, Mg, Zn, Cumurexide 6–13 Ca, Ni, CuPAN 2–11 Cd, Cu, Znsalicylic acid 2–3 Fea metal ions in italic font have poor end pointsFigure 9.30 is essentially a two-variableladder diagram. The solid lines are equivalentto a step on a conventional ladder diagram,indicating conditions where two (orthree) species are equal in concentration.