Compleximetric titrations

Buffer solutions find many applications in quantitative analysis, e.g. many precipitations are quantitative only under carefully controlled conditions of pH, as are also many compleximetric titrations numerous examples of their use will be found throughout the book. 

Other methods have involved compleximetric titration, nephelom-etry, potentiometric titration, and gravimetric methods. In 1981, a paper by Mayanna and Jayaram11 outlined the determination of caffeine in a wide variety of products including pharmaceuticals and food products using sodium N-chloro-p-toluene-sulphonamide (chloramine-T) in a titri-metric procedure. 

Argentimetric titrations Compleximetric titrations Redox 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... 

Buffers can also be composed of weak bases and their salts examples include ammonia buffer, used to control the pH of compleximetric titrations (see Chapter 6) and the common biological buffer TRIS (or tris(hydroxymethylaminomethane), C4HnN03), used to control the pH of protein solutions. 

Disodium edetate has the structure shown in Figure 6.5 and ionises with the release of two H+ ions. For this reason, compleximetric titrations involving disodium edetate require an alkaline pH and a buffer to ensure that the released protons do not lower the pH. The usual buffer is ammonia solution, which buffers to around pH 10. Careful choice of buffer conditions can allow the assay of several different metal ions in the same sample for example, in the assay of Intraperitoneal Dialysis Solution BPC, both Ca2+ and Mg2+ are assayed by titration with 0.02 m disodium edetate. 

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. 

Matsui and coworkers reported the use of cobalt ion MIPs for chromatography based recognition studies on imprinted compounds. The authors chose to utilize an imprinting system described previously for the catalysis of aldol condensations (vide supra). This system was shown to be amenable to the study of MIP-metal ion mediated recognition. Preliminary studies were conducted to provide evidence for the complex formation between cobalt, polymerizable ligands, and dibenzoyl-methane, 28. Compleximetric titration of 28 in a model prepolymerization reaction mixture containing cobalt (II) acetate and pyridine in chloroform/methanol (5 1) showed formation of a complex with 1 1 stoichiometry between 28 and Co(II) (Fig. 19). 

Compleximetric titrations are used mainly to determine metal ions by use of complex-forming reactions. Although in theory many complexing agents (cyanide, thiocyanate, fluoride, 1,2-diaminoethane, etc.) could be used for this purpose, in practice the titrants are almost always compounds having the iminodiacetic acid functional group ... 

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. 

Rgure 1 (A) Symmetric titration curve compleximetric titration of copper ion, using a copper-selective electrode. (B) Asymmetric titration curve compleximetric titration of nickel using a copper-selective electrode. 

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. 

The chloride content was determined by the Volhard method, the H2P0 content was determined alkalimetrically after ion exchange calcium content was determined by a compleximetric titration. Where the amount of phosphate ion was small, it was determined gravimetrlcally as ammonium phosphomolybdate. 

Almost all compleximetric titrations of metal ions are carried out with EDTA or related compounds as titrant. The equivalence point, when all the metal ion... 

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