Ammonia auxiliary complexing agent

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. 

Many cations form hydrous oxide precipitates when the pH is raised to the level required for their successful titration with EDTA. When this problem is encountered, an auxiliary complexing agent is needed to keep the cation in solution. For example, zinc(II) is ordinarily titrated in a medium that has fairly high concentrations of ammonia and ammonium chloride. These species buffer the solution to a pH that ensures complete reaction between cation and titrant in addition, ammonia forms ammine complexes with zinc(II) and prevents formation of the sparingly soluble zinc hydroxide, particularly in the early stages of the titration. A somewhat more realistic description of the reaction is then... 

The titration curve developed in Example 9.1 was hypothetical. In practice, complexometric titrations are conducted at pH values at which the titrant is protonated to some extent. Often, an auxiliary complexing agent such as ammonia is present so that there are various metal-containing species in addition to just the simple hydrated metal ions. Calculations of such titration curves can be readily accomplished within the framework of the material presented in Chapter 5, particularly that dealing with p, the conditional constant. In the treatment that follows, side reactions of the titration ligand and metal ion will be considered using the familiar Ul and... 

Figure 13-6 Guide to EDTA titrations of some common metals. Light color shows pH range in which reaction with EDTA is quantitative. Dark color shows pH range in which auxiliary complexing agent such as ammonia is required to prevent metal from precipitating. [Adapted from K. Ueno, J. Chem. Ed. 1965,...

Left 0.02 M CuSO before titration. Center. Color of Cu(ll)-ammonia complex after addition of ammonia buffer, pH 10. Right End-point color when all ammonia ligands have been displaced by EDTA. In this case, the auxiliary complexing agent accentuates the color change at the equivalence point. 

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