Indicator compleximetric titrations

With hydrazine hydrochloride, thioindoxyl-1,1-dioxide has been reported to afford the azine (110) by prior formation of a bishydrazone.132 Murexide (111), a purple indicator used in compleximetric titrations, is formed from ammonium acetate, glacial acetic acid, and alloxantin (152 R = H). The murexide test is well known for detecting uric acid and other purines.133... 

The indicators used in compleximetric titrations are usually themselves complexing agents, which form weak complexes with the metal ion when added initially As the edetate solution is titrated, the weak complex is displaced by the stronger edetate complex to reveal the free colour of the indicator. The most commonly used indicator is known by the sinister name of mordant black. This indicator forms wine-red complexes with metal ions, but changes to a dark blue colour at the end point when the edetate has displaced all of the metal ions from the indicator complex. 

A wide range of visual indicators is available for compleximetric titrations. These generally function by forming a colored complex with the metal ion being titrated, which causes a color change when the metal ion is removed from the complex by reaction with EDTA and releases the free ligand. These indicators are described in detail in another article. 

Many metal ions (e.g., calcium, cadmium, aluminum, lead) can be titrated with standard ethylenediamine-tetraacetic acid (EDTA) or other compleximetric titrants, using an appropriate indicator electrode. When no direct appropriate indicator electrode exists, the addition of indicator metal ions can permit a determination. For example, barium may be titrated with EDTA in the presence of silver-EDTA complex as an indicator reagent using a silver electrode. Examples of compleximetric titrations are given in Table 3. 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

Additional experiments with compleximetric, redox, or precipitation-based titrations have been designed using the same concepts as described above, employing colorimetric indication of the end point. Pertinent details can be found in Ref. 183, in Chapter 2.4.4 (Fig. 2.23), Chapter 4.9... 

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