Metal-EDTA Titration Curves

Only some metal chelates are stable enough to allow titrations in acid solution others require alkaline solution. 

Minimum pH for effective titration of various metal ions with EDTA. (Reprinted with permission from C. N. Reilley and R. W. Schmid, Anal. Chem., 30 (1958) 

---

The calculation that we just did was oversimplified because we neglected any other chemistry of such as formation of MOH, M(OH)2(a ), M(0H )2(5), and M(0H)3. These species decrease the concentration of available and decrease the sharpness of the titration curve. Mg " is normally titrated in ammonia buffer at pH 10 in which Mg(NH3) also is present. The accurate calculation of metal-EDTA titration curves requires full knowledge of the chemistry of the metal with water and any other ligands present in the solution. 

Now that we know something about EDTA s chemical properties, we are ready to evaluate its utility as a titrant for the analysis of metal ions. To do so we need to know the shape of a complexometric EDTA titration curve. In Section 9B we saw that an acid-base titration curve shows the change in pH following the addition of titrant. The analogous result for a titration with EDTA shows the change in pM, where M is the metal ion, as a function of the volume of EDTA. In this section we learn how to calculate the titration curve. We then show how to quickly sketch the titration curve using a minimum number of calculations. 

The greater the effective formation constant, the sharper is the EDTA titration curve. Addition of auxiliary complexing agents, which compete with EDTA for the metal ion and thereby limit the sharpness of the titration curve, is often necessary to keep the metal in solution. Calculations for a solution containing EDTA and an auxiliary complexing agent utilize the conditional formation constant K" = aM aY4- Kt, where aM is the fraction of free metal ion not complexed by the auxiliary ligand. 

The equivalence point of a complexation titration occurs when stoichiometri-cally equivalent amounts of analyte and titrant have reacted. For titrations involving metal ions and EDTA, the equivalence point occurs when Cm and Cedxa are equal and may be located visually by looking for the titration curve s inflection point. 

Plot the titration curve (potential in millivolts vs S.C.E. against volume of standard EDTA solution) and evaluate the end point. In general, results accurate to better than 0.1 per cent are obtained. Brief notes on determinations with various metal ion solutions follow. 

Figure 12-11 Theoretical titration curves for the reaction of 50.0 mL of 0.040 0 M metal ion with 0.080 0 M EDTA at pH 10.00.

As can be seen in Figure 1, the shape of the titration curve of a solution of the acid H4L differs from that obtained in presence of an equal quantity of a metal ion, indicating that some reactions take place between edta and the cation. Let us consider the case of three different metal ions Li+, Mg2+ and Cu2+, for which the stability constant A, = [ML]/([M][L]) is equal to 1028, 108-7 and 10l8 > respectively (/ = 0.1 M (KC1) and 20 °C). 

Figure 1 Titration curves for H,edta. Curve 1 H4edta curve 2 H4edta + lithium curve 3 H4edta + magnesium curve 4 H4edta + copper. The quantity a is in moles of strong base per mole H4edta. Total ligand and metal concentration ...

The ligand interchange reaction between metal S-diketonates and edta was utihzed to establish the amount of metal -diketonate by conductometry in DMF or DMSO. The -diketonates studied were of Co(ll), Cu(ll), Mn(ll), Fe(III) and Cr(III). The combination ratios of the metal /3-diketonate with edta were 4 1, 2 1 and 1 1. Presence of less than 1% H2O, inorganic acids or organic solvents did not affect the inflection points in the conductometric titration curves . 

The principles illustrated in Examples 17-2 and 17-3 can be u.sed in the derivation of the titration curve for a metal ion with EDTA in a solution of fixed pH. Example 17-4 demonstrates how the titration curve is constructed with a spreadsheet. 

Consider an idealized titration (no metal complexation or pH effects on ligand or metal) of 50 mL of 0.003 M Zn + with 0.01 M Na2H2Y(EDTA) and draw the titration curve. Develop the spreadsheet by block filling A with log[Y ] values from -18 to -3 in 0.3 increments. In successive columns, place values of [Y" ], with the help of Equation... 

Explain the analogies between the titration of a metal with EDTA and the titration of a strong acid (H" ) with a weak base (A ). Make comparisons in all three regions of the titration curve. 

The high frequency titration (h.f.t.) (1,2) as a method for complexing study in solution was used by R.HARA and P.WEST (3) for the first time. This method is based on the determination of the molar ratio of the metal to the addend from an inflection point in the titration curve due to a change in conductivity in the moment of completion of complex formation, especially with complexing agent, which will liberate hydrogen ions, such as EDTA. 

Direct Titration Curve of a Metallic Ion with EDTA... 

In the now familiar pattern discussed above, the titration involves the buret addition of EDTA solution to the metal ion solution, which generates a titration curve with an abrupt change in — log[M " ] (pM). This is governed by the equilibrium constant for the formation of the metal-EDTA complex ... 

For a complex-forming metal ion detectable by its own metal electrode, e.g. in the titration of Cu2+ with EDTA by means of a Cu electeode and a double junction calomel electrode, p/ curves are obtained of a nature comparable to those in Fig. 2.20 and with a Cu range of about 20 (cf., stability constant... 

Majer65 in 1936 proposed measuring, instead of the entire polarographic curve, only the limiting current at a potential sufficiently high for that purpose if under these conditions one titrates metal ions such as Zn2+, Cd2+, Pb2+, Ni2+, Fe3+ and Bi3+ with EDTA66, one obtains a titration as depicted in Fig. 3.55 i, decreases to a very low value, in agreement with the stability constant of the EDTA-metal complex and the titration end-point is established by the intersection of the ij curves before and after that point correction of the i values for alteration of the solution volume by the titrant increments as in conductometric titration is recommended. 

Figure 9.4 shows the minimum pH at which different metals can be titrated with EDTA. The points on the curve represent the pH at which the conditional formation constant K for each metal is 10 (log K = 6), which was arbitrarily chosen as the minimum needed for a sharp end point. Note that the smaller Kf, the more alkaline the solution must be to obtain a K of 10 (i.e., the larger 0 4 must be). Thus, Cd with Kf only about 10 requires a pH of about 8 or above. The... 

Examples. What proportion of Cu(II) remains uncomplexed by EDTA at equivalence when the titration is performed in ammonia solution at pH 10 with [NH3] 0.2 M Let the total copper(II) be 0.010 M at equivalence. The EDTA forms 1 1 complexes with metal ions and theKi with Cu " is 10 (Table 9-2). The ligand in question is the ion, whose ao curve is shown in Figure 9-5. At pH 10, aoL... 

Fig. 28.4 Shapes of the curves obtained during the titration of 10 ml of a 10 mol/L metal cation with 10 2 mol/L EDTA...

---