For complexometric titration 34,

Fig. 12. Titration curves for complexometric titrations. Titration of 60.0 ml of a solution that is 0.02 mol dm-3 in M curve A represents a 0.02 mol dm-3 solution of the tetradentate ligand D to give MD as product curve B represents a 0.04 mol dm-3 solution of the bidentate ligand B to give MB2 and curve C represents a 0.08 mol dm-3 solution of the un-identate ligand A to give MA4. The overall formation constant for each product is 1.0 x 1020. Adapted from ref. 115.

EDTA (ethylenediaminetetraacetic acid) (H02CCH2)2NCH2CH2N-(CH2C02H)2, the most widely used reagent for complexometric titrations. It forms 1 1 complexes with virtually all cations with a charge of 2 or more, effective formation constant Equilibrium constant for formation of a complex under a particular stated set of conditions, such as pH, ionic strength, and concentration of auxiliary complexing species. Also called conditional formation constant. 

Analytical Reagents. Various chelators give specific color changes on combining with different metal salts, such as vanadium, iron, cobalt, nickel, copper, and palladium salts and thus identify the corresponding metal ions [74], Other chelators, such as C.I. Mordant Black 11 (Eriochrome Black T) (2 see Section 3.11.1), are employed as indicators for complexometric titrations. 

Describe three general methods for performing EDTA titrations. What are the advantages of each 17-3. Why are multidentate ligands preferable to unidentate ligands for complexometric titrations 17-4. Write chemical equations and equilibrium-constant expressions for the stepwise formation of (a) Ni(CN)5-. 

This test is based upon formation of barium sulfate in the presence of excess barium, followed by chelation of the remaining barium with methylthymol blue, a metallochrome indicator recently developed for complexometric titrations (13, 14, 15, 16, 17). The reagent is a solution containing equimolar amounts of barium chloride and methylthymol blue at a concentration equivalent to the largest amount of sulfate ion anticipated in the test solution 18). 

In this chapter, equations will be developed for complexometric titration curves along similar lines used in the previous chapter for Bronsted acid-... 

EDTA (ethylenediaminetetraacetic acid) (H02CCH2)2NCH2CH2N-(CH2C02H)2, the most widely used reagent for complexometric titrations. It forms 1 1 complexes with virtually all cations with a charge of 2 or more. 

Some selected indicators for complexometric titrations are given in Table 2. 

Used as a soln. in EtOH for titrimetric detn. of Tl as 0.2% soln. in EtOH in photometric detn. of Cu ( max 545 nm, s 49000), Th indicator used for complexometric titration of Pb. Orange-red cryst. Mod. sol. alkalis, EtOH, Mc2CO. 

Now that we know something about EDTA s chemical properties, we are ready to evaluate its utility as a titrant for the analysis of metal ions. To do so we need to know the shape of a complexometric EDTA titration curve. In Section 9B we saw that an acid-base titration curve shows the change in pH following the addition of titrant. The analogous result for a titration with EDTA shows the change in pM, where M is the metal ion, as a function of the volume of EDTA. In this section we learn how to calculate the titration curve. We then show how to quickly sketch the titration curve using a minimum number of calculations. 

How to sketch an EDTA complexometric titration curve see text for explanation. 

Sketching the Titration Curve As we have done for acid-base, complexometric titrations, and redox titrations, we now show how to quickly sketch a precipitation titration curve using a minimum number of calculations. 

Alkaline-earth metals are often deterruined volumetricaHy by complexometric titration at pH 10, using Eriochrome Black T as indicator. The most suitable complexing titrant for barium ion is a solution of diethylenetriaminepentaacetic acid (DTPA). Other alkaline earths, if present, are simultaneously titrated, and in the favored analytical procedure calcium and strontium are deterruined separately by atomic absorption spectrophotometry, and their values subtracted from the total to obtain the barium value. 

Such reactions are not available for Bi but this can readily be determined by complexometric titration using ethylenediaminetetraacetic acid or similar complexones ... 

Solochrome dark blue or calcon ( C.1.15705). This is sometimes referred to as eriochrome blue black RC it is in fact sodium l-(2-hydroxy-l-naphthylazo)-2-naphthol-4-sulphonate. The dyestuff has two ionisable phenolic hydrogen atoms the protons ionise stepwise with pK values of 7.4 and 13.5 respectively. An important application of the indicator is in the complexometric titration of calcium in the presence of magnesium this must be carried out at a pH of about 12.3 (obtained, for example, with a diethylamine buffer 5 mL for every 100 mL of solution) in order to avoid the interference of magnesium. Under these conditions magnesium is precipitated quantitatively as the hydroxide. The colour change is from pink to pure blue. 

E. Detection of the colour change. With all of the metal ion indicators used in complexometric titrations, detection of the end point of the titration is dependent upon the recognition of a specified change in colour for many observers this can be a difficult task, and for those affected by colour blindness it may be... 

Full details are given for the determination of aluminium by this method. Many other metals may be determined by this same procedure, but in many cases complexometric titration offers a simpler method of determination. In cases where the oxine method offers advantages, the experimental procedure may be readily adapted from the details given for aluminium. 

Pyridylazo-ligands (Scheme 2) have widely been used in colorimetric analyses of various metal ions. For example, l-(2-pyridylazo)-2-naphthol (Hpan) is one of the most well-known reagents for the colorimetric determination and complexometric titration of... 

Braun and Richter [923] have described an application of CE in additive analysis, namely quantitative analysis of heat stabilisers in PVC, such as Irgastab 17A and 18 MOK-N, which are metal-based (in the past usually Cd, Ba and Pb, now nontoxic Ca, Zn and Sn). Quantitative metal analysis is of interest for PVC recycling purposes. Various alternative approaches are possible for such quantitative analysis, such as XRF [924], polarog-raphy [925] and AAS [923], The performance of AAS, CE and complexometric titrations in the analysis of the heavy metal content in PVC was compared [923]. For all methods investigated the metals must be separated from the polymer and transferred into an aqueous phase. 

Applications Quantitative dry ashing (typically at 800 °C to 1200°C for at least 8h), followed by acid dissolution and subsequent measurement of metals in an aqueous solution, is often a difficult task, as such treatment frequently results in loss of analyte (e.g. in the cases of Cd, Zn and P because of their volatility). Nagourney and Madan [20] have compared the ashing/acid dissolution and direct organic solubilisation procedures for stabiliser analysis for the determination of phosphorous in tri-(2,4-di-t-butylphenyl)phosphite. Dry ashing is of limited value for polymer analysis. Crompton [21] has reported the analysis of Li, Na, V and Cu in polyolefins. Similarly, for the determination of A1 and V catalyst residues in polyalkenes and polyalkene copolymers, the sample was ignited and the ash dissolved in acids V5+ was determined photo-absorptiometrically and Al3+ by complexometric titration [22]. 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

For analysis in solutions, the most frequently used CL reaction is alkaline oxidation of luminol and lucigenin in the presence of hydrogen peroxide as oxidant, although sodium hypochlorite, sodium perborate, or potassium ferricyanide may also be used. CL reactions involving alkaline oxidation have been used to indicate acid-base, precipitation, redox, or complexometric titration endpoints either by the appearance or the quenching of CL when an excess of titrant is present [114, 134], An example of these mechanisms is shown in Figure 14. 

The end points of precipitation titrations can be variously detected. An indicator exhibiting a pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving the formation of red silver chromate with the appearance of an excess of silver ions, is an important example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an indication of the presence of excess thiocyanate ions, is another. A series of indicators known as adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned under complexometric titrations. 

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