Complex formation titrations inorganic

Inorganic complexing reagents such as Hg(N03)2, AgNOs, NiS04 and KCN can be used for complex-formation titrations. Mercury(II) ion forms neutral complexes with most of the anions that precipitate with silver nitrate such as Br, CD, SCN, CN and thiourea. 

Inorganic speciation in solution can also affect the mobility of metal ions (Doner, 1978). The formation of an ion-pair with Cl can more than double the mobility of Cd in the presence of 200molm 3NaCl. At the same chloride concentration, however, the mobilities of Cu2+ and Ni2+ are only increased slightly (5-10%), presumably because of very weak complexation with Cl. This mechanism could increase the leaching of Cd from saline soils but it may not be effective in non-saline soils because the ratio of the total concentrations of Cd Cl must be >1 106 before >50% of total Cd is complexed by Cl (estimated using the computer model TITRATOR (Cabaniss, 1987), which considered the chloro and hydroxy complexes of Cd at pH 5.0 and a total Cd concentration of 0.1 mmolm-3 equilibrium constants were taken from Lindsay (1979)). 

Simple inorganic ions and molecules like NH3, CN , Cl-, H20 form monodentate ligands, that is one ion or molecule occupies one of the spaces available around the central ion in the coordination sphere, but bidentate (like the dipyridyl ion), tridentate, and also tetradentate ligands are known. Complexes made of polydentate ligands are often called chelates, the name originating from the Greek word for the claw of the crab, which bites into an object like the polydentate ligand catches the central ion. The formation of chelate complexes is used extensively in quantitative chemical analysis (complexometric titrations). ... 

Complexation reactions have many uses in analytical chemistiy, but their classical application is in complexometric titrations. Here, a metal ion reacts with a suitable ligand to form a complex, and the equivalence point is determined by an indicator or an appropriate instrumental method. The formation of soluble inorganic complexes is not widely used for titrations, as discussed later, but the formation of precipitates, particularly with silver nitrate as the titrant, is the basis for many important determinations (see Section 13F). 

The most widely used complexometric titration employing a unidentate ligand is the titration of cyanide with silver nitrate, a method introduced by Liebig in the 1850s. This method involves the formation of the soluble Ag(CN)J, as discussed in Feature 17-2. Other common inorganic complexing agents and their applications are listed in Table 17-1. 

The formation of stable soluble complexes has an analytical advantage over precipitation titration in that it avoids the problem of coprecipitation. In view of the rather unusual behavior of iodide ions in acetone, a short study of other inorganic anions which form insoluble silver salts in aqueous solutions was made for the acetone system. The results are shown in Table I. 

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