Other Complexones

Other complexones exist, but their use is more limited than that of EDTA  

The given formula is probably not the only appropriate one to represent its true structure. There certainly exist other microforms, the existence of which is in connection with the occurrence of several protonation sites. Of course, all the microforms are in equilibria. This complexone often gives complexes more stable than those formed with EDTA, but it reacts more slowly than the latter ethyleneglycolbis(2-aminoethyl ether)N, N, N, N -tetraa-cetic acid (ECTA) and triethylenetetramine-N, N, N, N , N , N -hexa-acetic acid (TTHA)  

The liberated Zn + gives itself a complex with the indicator, making the solution blue  

At the final point, the EGTA in excess displaces the indicator from its complex with Zn +. The liberated zincon gives the orange-red color of its free form  

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The ability of polyaminocarboxylic acids to form stable, water-soluble chelates over a wide pH range accounts for their diversity of uses. Ethylenediaminetetraacetic acid, H4EDTA, has played a central role in this development and stimulated interest in other complexones with a view to finding ligands with increased affinity and selectivity for metal ions. 

In 1937 I.G.-Farbenindustrie patented a method for the preparation of polyaminoacetonitriles, corresponding acids and their derivatives . The process, known as the hydrogen cyanide process, utilizes sodium cyanide in acid solution. The cyanomethylation takes place by treatment of the amine with formaldehyde and hydrogen cyanide. In the case of ethylenediamine reaction (2) takes place ethylenediaminetetraacetonitrile (1) is isolated to ensure that the resulting tetrasodium salt, after hydrolysis, is not contaminated by by-products. This synthesis can be performed as a continuous process and is also used to obtain other complexones. The use of hydrogen cyanide and of an acid medium gives rise to corrosion and safety problems. 

We begin by discussing the most important reason why complexometric titrations were weakly developed before the advent of EDTA and various other complexones. 

Other types of complexons for polyanions (e.g. polycarboxylates and phosphates) are linear ligands with polyguanidinium cations or polyammonium cations as functional groups 33). 

The vast majority of complexation titrations are carried out using multidentate ligands such as EDTA or similar substances as the complexone. However, there are other more simple processes which also involve complexation using monodentate or bidentate ligands and which also serve to exemplify the nature of this type of titration. This is demonstrated in the determination outlined in Section 10.44. 

Providing no other acidic species are present and water has been used as the absorption medium in either the 02 flask or pyrohydrolytic tube decomposition, the acidimetric titration of hydrogen fluoride is the simplest. Unfortunately this is often not the case, so one of the other finishes such as colorimetry and titrimetry has to be used. One of the best colorimetric procedures employs Ce-Alizarin complexone reagent5 which changes from wine red to dark blue in the presence of fluoride ion. The color takes a while to develop fully, but it is very reliable. 

Yellow Alizarin Complexone (AC) reacts with La or Ce(III) ions to form a red chelate, which, in turn, reacts with fluoride ions to give a blue ternary complex (Alizarin Fluorine Blue). The fluoride anion displaces the water molecule bound with the metal ion (formula 20.1) [19,20]. Other suggestions regarding the mechanism of this reaction have also been proposed [21,22]. 

Other organic reagents recommended for determination of Mo include Alizarin Complexone [103], l-nitroso-2-naphthol [104], morin [105], rutin [106], quercetin [107], 1,5-diphenylcarbazone [108], quinalizarin in the presence of surfactant [109], tiron [12,31,39], and 2,2 -biquinoxalyl [110]. The FIA technique has been applied in the determination of Mo with the use of carminic acid [111]. 

Complexones such as EDTA (complexone III) [1-3] and DCTA (complexone IV) [4,5] are suitable eluents, but other complexing agents, such as citrate [3,6] and sulphate [7] are also applied. Barium has been separated from strontium and other metals by cation-exchange chromatography using mixed HCl-organic solvent eluents [8]. Strontium has been enriched and determined in sea water [5] and in milk [2]. 

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