Complexometric titration methods

Complexometric titration methods of analysis of rare earth elements. 

The amount of calcium in different samples of milk powder (in mg of calcium per g of milk powder) were analyzed by two methods, one employing extraction followed by analysis using atomic absorption spectroscopy, the other using a complexometric titration method. The results of nine analyses are shown in table 3.4. 

The indirect determination of certain organic substances can be made by complexometric titration methods. Such methods depend on the formation of an insoluble product between the organic material and a metal then, either the excess metal in solution is determined by a suitable titration with EDTA, or the metal-containing precipitate is decomposed and the liberated metal ions titrated. Thus, for example, narcotine, papaverine, codeine, strychnine and brucine have been determined by formation of iodobis-muthate complexes, chlorpromazine and quinine" as cadmium iodide complexes, purines and nicotinic acid derivatives by precipitation with mercury and barbiturates by precipitation with zinc. ... 

Similar investigations were done in the temperature range of 700-800°C by Volkovich. The saturated solutions of oxides were prepared by placing pressed oxide tablets in melt and holding them for 2-3 h. The analysis of the saturated solution samples was performed by complexometric titration method. 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving redox titrations are similar to those described earlier in the chapter for acid-base and complexometric titrimetric methods. As with acid-base titrations, redox titrations can be extended to the analysis of mixtures if there is a significant difference in the ease with which the analytes can be oxidized or reduced. Figure 9.40 shows an example of the titration curve for a mixture of Fe + and Sn +, using Ce + as the titrant. The titration of a mixture of analytes whose standard-state potentials or formal potentials differ by at least 200 mV will result in a separate equivalence point for each analyte. 

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. 

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. 

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. 

The method is particularly suited to complexometric titrations of mixtures or where there is no suitable visual indicator. 

How would your carry out complexometric btrations by the direct titration method Discuss the assay of the following pharmaceutical drugs explicitely ... 

How does residual titration method help in the complexometric btrations Elaborate the assay of the following drugs by this technique ... 

Titrations are veiy powerful techniques that contain two very different kinds of information and thus serve two different purposes (a) titrations are used for quantitative analytical applications, e.g. the determination of the concentration of an acid by an acid-base titration or the determination of a metal ion by a complexometric titration (b) titrations serve also as a method for the determination of equilibrium constants, e.g. the determination of the strength of the interaction between a metal ion and a ligand. Naturally, both objectives can be combined and the analysis of one titration can deliver both types of information. 

A precipitation titration method has been reported where chlorpromazine hydrochloride is precipitated from solutions using excess standard Cdlj in the presence of K1 or HI, and the excess Cdl2 in the filtrate is then determined complexometrically using xylenol orange as the indicator [58]. 

Complexometric titration with EDTA is the usual winery procedure for determining calcium (4, 6, 22, 113), but atomic absorption spectrophotometry (51,53,112) and flame photometry and a rapid micro method based on oxalate precipitation (114) have been used successfully. 

A complexometric titration is a rapid, accurate, and inexpensive method to analyze metal ions. However, its application in metal analysis in environmental samples is very much limited, because the more common atomic absorption/emis-sion spectrometry method gives a lower detection limit. 

A simple and rapid method for the iodometric determination of microgram amounts of chromium(ni) in organic chelates is based on the oxidation of chromium(III) with periodate at pH 3.2, removal of the umeacted periodate by masking with molybdate and subsequent iodometric determination of the liberated iodate . Iodometric titration was also used for determination of the effective isoascorbate (see 2) concentration in fermentation processes . The content of calcium ascorbate can be determined with high sensitivity by complexometric titration with edta, which is superior to iodometry. The purity of /3 -diketonate complexes of Al, Ga, In and Ni was determined by complexometric titration with edta at pH 5.5-3, with RSD < 0.01 for determining 5-30% metal ion. Good analytical results were obtained by a similar procedure for the metal content of 15 lanthanide organic complexes. ... 

Complexometric titrations are mainly used to determine the concentration of cations in solution. The method is based on the competition between a metal ion (for example) and two ligands, one of which acts as an indicator and the other is a component of a standard solution. 

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. 

Go to http //chemistry.brookscole.com/skoogfac/. From the Chapter Resources menu, choose Web Works, and locate the Chapter 17 section, where you will find links to several good educational sites that give additional help on complexation equilibria and complexometric titrations. Several Web sites describe experiments that can be done in the laboratory based on complexation methods. Find the abstract of the paper from the Journal of Chemical Education that deals with the determination of zinc by EDTA titration. Find the indicator and the buffer pH used in the titration. There is also a link to additional information on chemistry applied to aquatic systems. Compare some of the complexation equilibria described in these documents to those discussed in this chapter. 

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-. 

The solubility of CaSe04-2H20(cr) in water was determined between 270.6 and 375.4 K. The calcium concentration was found by the oxalate-permanganate method and by a complexometric titration. As the solubility data are not readily available they are reproduced in Table A-16. 

Complexometric titrations in the clinical laboratory are limited to those substances that occur in fairly high concentrations since volumetric methods are generally not too sensitive. The most important complexometric titration is the determination of calcium in blood (see Ref. 8). Chelating agents such as. EDTA are used in the treatment of heavy-metal poisoning, for example, when children ingest chipped paint that contains lead. The calcium chelate (as Na2CaY) is administered to prevent complexation and removal of calcium in the bones. Heavy metals such as lead form more stable EDTA chelates than calcium does and will displace the calcium from the EDTA. The chelated lead is then excreted via the kidneys. 

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