Titrations with Indirect Metal Indicators

The indicator becomes free. Before, the indicator was in the form of its complex with Mg +, the latter coming from the displacement reaction  

For such a titration to be satisfactory, a judicious choice of the experimental conditions must be made. The variable to adjust is, for this example, the concenttation [Mgy2 ]. 

In the general case, suppose that we want to titrate the metallic ion Mi with the complexing reagent L, which gives the complex MjL, and that the colored indicator is sensitive to the metallic ion Mn with formation of the complex Muln. Mu also forms the complex MnL. (L is a common ligand to both metallic ions.) The problem is to calculate pMi at the point at which the indicator color changes, that is, when pMn = log A Mnin- Expressing the concentration [L] at equilibrium from the expressions A mil and A miil leads to the equality  

To illustrate the use of this relation, let s investigate the titration of a 10 mol/L Ca + solution with EDTA at pH = 10 in the presence of eriochrome black T. What must the value of the concentration of added [MgY] be in order to obtain a satisfactory color change (It is considered that at the color change, pMg = 5.4. A satisfactory titration of Ca + in these conditions is carried out if pCa = 6.6 at the equivalence point. The conditional stability constants are Ar aY = 10 , AT gv = 10. ) 

Identifying Mi with Ca and Mu with Mg + and taking the numerical values already given into account lead to 

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The basis for titrations with indirect indicators of metallic ions is different from one that supports replacement titrations (see below). 

An interesting indirect end-point detection method is based on the work of Siggia et al [143], and Reilley and Schmid [119], who originally developed the technique for chelometric titrations of electro-inactive metals with a mercury indicator electrode. Assume that metal ion M is titrated by titrant Y (ionic charges are omitted for the sake of simplicity),... 

Back titrations based on the addition of excess EDTA followed by back titration of the excess reagent are useful when reactions are slow or a suitable direct indicator is not available. The excess generally is determined by titration with standard solutions of magnesium or zinc ion. These titrants are chosen because they form EDTA complexes of relatively low stability, thereby preventing the possible titration of EDTA bound by the sample metal ion. Examples of the indirect method are the following ... 

One of the most significant advances in chemical analysis as applied to pharmaceuticals during the last decade is undoubtedly the introduction and development of the complexometric titration. The ability of certain amino-polycarboxylic acids to react stoichiometrically and instantaneously with certain metal ions was first recognised and described by Schwarzenbach in 1945. Later, the same author, together with co-workers, described the first metal indicator, murexide, and then, perhaps the most important of all, Eriochrome Black T (usually referred to in this country as solo-chrome black). This was followed quite shortly by the first description of the now classic use of the complexometric titration for the determination of temporary and permanent hardness in water. It was some time, however, before metal indicators capable of functioning at an acid pH were developed with the availability of such indicators a rapid increase in the application of complexometric titrations took place and there are now few metal ions that are not capable of determination by this means. In the present book reference will be found to the use of complexometric methods for determination of aluminium, bismuth, calcium, copper, iron, lead, magnesium, manganese, mercury and zinc. In addition, indirect methods are described for the determination of certain anions such as fluoride, phosphate and sulphate. 

For end-point detection, we commonly use metal ion indicators, a glass electrode, an ion-selective electrode, or a mercury electrode. When a direct titration is not suitable, because the analyte is unstable, reacts slowly with EDTA, or has no suitable indicator, a back titration of excess EDTA or a displacement titration of Mg(EDTA)2- may be feasible. Masking prevents interference by unwanted species. Indirect EDTA titrations are available for many anions and other species that do not react directly with the reagent. 

Reilley and Schmid made the important observation that the mercury-mercury (II)-EDTA electrode can be used indirectly as an indicator electrode for various metal ions, and this method was applied to the determination of 29 different metal ions by either direct- or back-titration procedures. Under some conditions,such as when a high concentration of buffer is present, formation of a mercury(I) precipitate or a complex with mercury(II) may result in incorrect end points. 

Cahnagite indicator gives a somewhat improved end point over Eriochrome Black T for the titration of calcium and magnesium with EDTA. It also has a longer shelf life. Xylenol orange is useful for titration of metal ions that form very strong EDTA complexes and are titrated at pH 1.5 to 3.0. Examples are the direct titration of thorium(IV) and bismuth(III), and the indirect determination of zirconium(IV)... 

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