Auxiliary Complexing Agents

Direct Titrations. The most convenient and simplest manner is the measured addition of a standard chelon solution to the sample solution (brought to the proper conditions of pH, buffer, etc.) until the metal ion is stoichiometrically chelated. Auxiliary complexing agents such as citrate, tartrate, or triethanolamine are added, if necessary, to prevent the precipitation of metal hydroxides or basic salts at the optimum pH for titration. Eor example, tartrate is added in the direct titration of lead. If a pH range of 9 to 10 is suitable, a buffer of ammonia and ammonium chloride is often added in relatively concentrated form, both to adjust the pH and to supply ammonia as an auxiliary complexing agent for those metal ions which form ammine complexes. A few metals, notably iron(III), bismuth, and thorium, are titrated in acid solution. 

We can account for the effect of an auxiliary complexing agent, such as NH3, in the same way we accounted for the effect of pH. Before adding EDTA, a mass balance on Cd + requires that the total concentration of Cd +, Ccd, be... 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

EDTA titration solutions frequently need to be buffered to a high pH to ensure stoichiometric formation of the complex. Unfortunately many metals will precipitate as hydroxides or hydrated oxides under these conditions. Use can be made of secondary or auxiliary complexing agents to retain the metal ion in solution. Typical reagents for this purpose are... 

EDTA Titration with an Auxiliary Complexing Agent... 

K j is the effective formation constant at a fixed pH and fixed concentration of auxiliary complexing agent. Box 12-2 describes the influence of metal ion hydrolysis on the effective formation constant. 

Figure 12-14 Guide to EDTA titrations of common metals. Light color shows pH region in which reaction with EDTA is quantitative. Dark color shows pH region in which auxiliary complexing agent is required to prevent metal from precipitating. Calmagite is more stable than Eriochrome black T (EB) and can be substituted for EB. [Adapted from K. Ueno, Guide tor Selecting Conditions of EDTA Titrations." J. Chem. Ed. 1965,42,432.]...

Auxiliary complexing agents such as NH3, tartrate, citrate, or triethanolamine may be employed to prevent metal ion from precipitating in the absence of EDTA. For example, Pb2+ is titrated in NH, buffer at pH 10 in the presence of tartrate, which complexes Pb2+ and does not allow Pb(OH)2 to precipitate. The lead-tartrate complex must be less stable than the lead-EDTA complex, or the titration would not be feasible. 

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. 

SB Auxiliary complexing agent. Use the equation derived in Problem 12-18 for this exercise. 

Color Plate 6 Titration of Cu(II) with EDTA, Using Auxiliary Complexing Agent (Section 12-5)... 

Similarly in the presence of an auxiliary complexing agent A (e.g. NH3, en, phen, halogen-, Ac-, CN-, etc.) the effective amount of cation M"+ will be less than the total amount CM in solution. Specifically ... 

Figure 11 shows such sigmoid curves for a hypothetical system where curve (a) is drawn for complex ML2 with [HL]org = 10-3 M and Kex = 102. Curves (b), (c) and (d) are drawn for [HL]org = 10-5 M, Ktx = 102 for complexes ML, ML2 and ML3 respectively. Curve (e) represents a complex ML2 for which [HL]org = 10-5 M but Kex = 10"2. The control that can be exercised by change of [HL]org and pH will be obvious and separations will always be possible if values of for two metals are different, for besides the variables mentioned above auxiliary complexing agents may be included in the aqueous phase to mask one or more of the unwanted cations (Section 10.3). Specific examples will be described for only one particular extractant (dithizone) to illustrate the general principle (Section 10.4.4.5). 

As with all other titrations by a non-selective reagent the analysis of one metal present in admixture with others can only be encompassed with the assistance of auxiliary complexing agents. Table 9 gives an indication of what can be done in this direction but standard texts are needed to give precise operational details.4 86 87... 

In analogy to a pH titration curve, pM (— log [M]) may be plotted against the fraction titrated. Under the usual titration conditions, in which the concentration of metal ions is small compared with the concentrations of the buffer and the auxiliary complexing agents, the fractions ay and are essentially constant during the titration. The titration curve then can be calculated directly from the conditional formation constant since it also remains constant. 

Often, auxiliary complexing agents must be used in EDTA titrations to prevent precipitation of the analyte as a hydrous oxide. Such reagents cause the end points to be less sharp. 

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