Precipitation analytical methods based

Techniques responding to the absolute amount of analyte are called total analysis techniques. Historically, most early analytical methods used total analysis techniques, hence they are often referred to as classical techniques. Mass, volume, and charge are the most common signals for total analysis techniques, and the corresponding techniques are gravimetry (Chapter 8), titrimetry (Chapter 9), and coulometry (Chapter 11). With a few exceptions, the signal in a total analysis technique results from one or more chemical reactions involving the analyte. These reactions may involve any combination of precipitation, acid-base, complexation, or redox chemistry. The stoichiometry of each reaction, however, must be known to solve equation 3.1 for the moles of analyte. 

Gravimetric methods based on precipitation or volatilization reactions require that the analyte, or some other species in the sample, participate in a chemical reaction producing a change in physical state. For example, in direct precipitation gravimetry, a soluble analyte is converted to an insoluble form that precipitates from solution. In some situations, however, the analyte is already present in a form that may be readily separated from its liquid, gas, or solid matrix. When such a separation is possible, the analyte s mass can be directly determined with an appropriate balance. In this section the application of particulate gravimetry is briefly considered. 

Thus far we have examined titrimetric methods based on acid-base, complexation, and redox reactions. A reaction in which the analyte and titrant form an insoluble precipitate also can form the basis for a titration. We call this type of titration a precipitation titration. 

In modern terms, asphaltene is conceptually defined as the normal-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. The generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale (8,9). With this extension there has been much effort to define asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous source. It was demonstrated that the elemental compositions of asphaltene fractions precipitated by different solvents from various sources of petroleum vary considerably (see Table I). Figure 1 presents hypothetical structures for asphaltenes derived from oils produced in different regions of the world. Other investigators (10,11) based on a number of analytical methods, such as NMR, GPC, etc., have suggested the hypothetical structure shown in Figure 2. 

Analytical determinations with the fluoride ion-selective electrode These are based either on direct potentiometry of fluorides [37, 84, 85, 88, 430] or on titration determinations of either fluorides or of other ions and also on titrations with fluoride ions as indicator. The advantages of potentiometry with an ISE over other analytical methods for determining fluorides were pointed out by Crosby etai [67], Further comparison studies [42, 56, 191, 433] came to the same conclusions, confirmed also by a study of 16 methods [365]. Fluoride ions are titrated either with La (for concentrations greater than 1 mM) or Th (in the concentration range 0.2-1 mM F ) [13, 102, 103, 113,233, 234]. Titration with fluoride ions can be used for the determination of Al with formation of the AIF4 complex up to nanomolar concentrations, especially in ethanol-water mixtures [25] (see also [267,384]). Precipitation titrations can also be used to determine La, Th and UOJ [241, 384] as well as Li in... 

In one analytical method for air filters, the air filters are ashed, silica content is volatilized with hydrogen fluoride, uranium is extracted with triisooctylamine, purified by anion exchange chromatography and coprecipitated with lanthanum as fluoride. The precipitated uranium is collected by filtration, dried, and a-spectroscopy is performed (EPA 1984b). The activities of and are determined based on... 

Two further analytical methods are based on the formation of metal complexes. Reaction of dopamine with KzCrjOy gives a Cr complex which can be assayed by atomic absorption spectroscopy, carbon rod absorption spectroscopy or, if Kf CrjO has been used, by liquid scintillation spectroscopy. Since the complex still gives a reaction with fluorescamine, the side-chain is thought not to be involved in the formation of the complex. Methamphetamine hydrochloride can be precipitated as a Bi complex. Determination of the amount of Bi remaining in solution by atomic absorption spectroscopy provides an indirect method of assay for the amphetamine. ... 

The term radiometric analysis is often used in a broad sense to include all methods of determination of concentrations using radioactive tracers. In a more restricted sense it refers to a specific analytical method which is based on a two-phase titration in the presence of a radioactive isotope. The endpoint of the titration is indicated by the disappearance of the radioisotope from one of the phases. Figure 9.4 illustrates two cases, (a) the determination of Ag in a solution by titration with Nal solution containing ( y t 1.57x10 y),and (b) the determination of Fe in an aqueous solution, to which trace amounts of radioactive Fe (EC 2.73 y) has been added. In case (a) the Agl precipitate is radioactive but the solution has little radioactivity until all the Ag has been precipitated. The activity of the solution is measured by a liquid flow GM-detector (Ch. 8). In the latter case (b) a two-phase liquid-liquid analytical technique is used ( 9.2.6) the titrant contains a substance (oxine) which extracts Fe(II) from the aqueous to the chloroform phase. The radioactivity of the organic phase is followed by liquid scintillation (sampling) to determine the end point of the titration. 

Small amounts (about 1 g.) of very pure Nb Ps and Ta Os can be obtained from the corresponding crude oxides via a simple fractional precipitation of oxalate complex solutions with tannin. The procedure is based on the analytical method of Schoeller (1937). 

Analytical method. The most serious errors are those in the method itself. Examples of method errors include (1) incomplete reaction for chemical methods, (2) unexpected interferences from the sample itself or reagents used, (3) having the analyte in the wrong oxidation state for the measurement, (4) loss of analyte during sample preparation by volatilization or precipitation, and (5) an error in calculation based on incorrect assumptions in the procedure (errors can evolve from assignment of an incorrect formula or molecular weight to the sample). Most analytical chemists developing a method check all the compounds likely to be present in the sample to see if they interfere with the determination of the analyte unlikely interferences may not have been checked. Once a vahd method is developed, an SOP for the method should be written so that it is performed the same way every time it is run. 

For many years, analytical chemistry relied on chemical reactions to identify and determine the components present in a sample. These types of classical methods, often called wet chanical methods, usually required that a part of the sample be taken and dissolved in a suitable solvent if necessary and the desired reaction carried out. The most important analytical fields based on this approach were volumetric and gravimetric analyses. Acid-base titrations, oxidation-reduction titrations, and gravimetric determinations, such as the determination of silver by precipitation as silver chloride, are all examples of wet chemical analyses. These types of analyses require a high degree of skill and attention to detail on the part of the analyst if accurate and precise results are to be obtained. They are also time consuming, and the demands of today s high-throughput pharmaceutical development labs, forensic labs, commercial environmental labs, and industrial quality control... 

Procedures especially suited to silicate rocks, minerals, and refractory silicates and aluminosilicates have been described by Bennett and Reed (290). A history of analytical methods was published by Andersson (291), and three new spectro-photometric procedures were developed. Available chemical methods listed by Meites (292), in addition to the conventional gravimetric and colorimetric methods, also include precipitation of the silicate ion as the cobalt salt, which is then determined by chelometric titration, and as a nitrogen base salt which is titrated with perchloric acid. 

An alternative analytical method, also based on the oxidation with potassirrm permanganate was introduced in 1895 by Stahr [154] and later developed as qtran-titative proeedrrre by Kimz [155-157]. In Starh s method, citric acid is oxidized to acetone diearboxyUc acid, which reacts with bromine to give precipitate of penta-bromoacetone, Br3CCOCHBr2. 

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