Analytical techniques gravimetry

Physicochemical methods, i.e. adsorption of probe molecules followed by varied analytical techniques (gravimetry, chromatography, calorimetry, spectroscopic techniques, etc.) are currently used for estimating more precisely the concentration of the potential active sites.[34 36] However, very few methods are well adapted for this purpose most of the methods employed for the characterization of the acidity of solid catalysts lead to values of the total concentrations of the acid sites (Brpnsted + Lewis) and to relative data on their strength, whereas few of them discriminate between Lewis and Brpnsted acid sites. It is however the case for base adsorption (often pyridine) followed by IR spectroscopy, from which the concentrations of Brpnsted and Lewis sites can be estimated from the absorbance of IR bands specific for adsorbed molecules on Brpnsted or Lewis sites. 

Figure 1.3.A shows the scheme for another analytical information hierarchy that is complementary to the previous ones. Thus, gravimetries, titrimetries, classical qualitative analyses and sensors provide onedimensional information of the form F = where x is the signal concerned. On the other hand, instrumental techniques provide two-dimensional information that can be of two types depending on whether the signal (x) is combined with an instrumental parameter (y), time (f) or space (s). Some modem analytical techniques (several of which use hybrid instruments) furnish three-dimensional information by combining signals with one or two instrumental parameters (y, z), time and space. The great...

Gravimetry is the analytical technique of obtaining a stable solid compound, of known stoichiometric composition so that the amount of an analyte in the sample may be found by weighing. 

It must be emphasised that infrared and Raman spectroscopy should not be used to the exclusion of other techniques such as H and C nuclear magnetic resonance, which are particularly useful characterisation techniques. Other useful techniques are mass spectroscopy, ultraviolet-visible spectroscopy, chromatography, thermo-analytical techniques (such as differential scanning calorimetry (DSC), thermal gravimetry (TG) etc.), or combined techniques such as GC-MS (gas chromatography combined with mass spectrometry)... 

Gravimetry, i, 13) j. quantitative analysis based on the weight of a reaction product, is the oldest of the analytical techniques and one of the most useful for major and minor constituents. It is capable of high accuracy, especially when corrections for solubility are made. Much of the early atomic weight work, with precisions as high as 0.001 %, was based on this technique. 

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. 

Particulate interferents can be separated from dissolved analytes by filtration, using a filter whose pore size retains the interferent. This separation technique is important in the analysis of many natural waters, for which the presence of suspended solids may interfere in the analysis. Filtration also can be used to isolate analytes present as solid particulates from dissolved ions in the sample matrix. For example, this is a necessary step in gravimetry, in which the analyte is isolated as a precipitate. A more detailed description of the types of available filters is found in the discussion of precipitation gravimetry and particulate gravimetry in Chapter 8. 

Analytical Procedures. Mn was determined by atomic absorption spectrophotometry (AAS) or the formaldioxime method (27J. Ca, Mg and Fe were determined by AAS. Silicate, phosphate, sulphate and chloride were determined using techniques described in Standard Methods (28). The molybdosi1icate method was used for silicate. Phosphate was determined using the vanadomolybdophosphoric acid method. Sulphate was determined by BaSO gravimetry. Chloride was determined by the mercuric chloride method. Salicylate and phthalate were determined by UV spectrophotometry. 

To reliably perform qualitative and quantitative analyses on body fluids and tissue, the clinical laboratorian must understand the basic principles and procedures that affect the analytical process and operation of the clinical laboratory. These include the knowledge of (1) the concept of solute and solvent, (2) units of measurement, (3) chemicals and reference materials, (4) basic techniques, such as volumetric sampling and dispensing, centrifugation, measurement of radioactivity, gravimetry, thermometry, buffer solution, and processing of solutions, and (5) safety. ... 

In the past century, there has been a tremendous growth in pharmaceutical analyses and the role the analytical group plays in the development of new products. The baton was passed from techniques such as gravimetry, titrimetry, spectroscopy after extraction, and thin-layer paper chromatography to HPLC, gas chromatography, and various autoanalyzers. Emphasis was on tests such as assay, content and blend uniformity, and determination of impurities and residual solvents. 

The first quantitative analytical fields to be developed were for quantitative elemental analysis, which revealed how much of each element was present in a sample. These early techniques were not instrumental methods, for the most part, but relied on chemical reactions, physical separations, and weighing of products (gravimetry), titrations... 

The first quantitative analytical fields to be developed were for quantitative elemental analysis, which revealed how much of each element was present in a sample. These early techniques were not instrumental methods, for the most part, but relied on chemical reactions, physical separations, and weighing of products (gravimetry), titrations (titrimetry or volumetric analysis), or production of colored products with visual estimation of the amount of color produced (colorimetry). Using these methods, it was found, for example, that dry sodium chloride, NaCl, always contained 39.33% Na and 60.67% Cl. The atomic theory was founded on early quantitative results such as this, as were the concept of valence and the determination of atomic weights. Today, quantitative inorganic elemental analysis is performed by atomic absorption spectrometry (AAS), AES of many sorts, inorganic MS (snch as ICP-MS), XRF, ion chromatography (1C), and other techniques discussed in detail in later chapters. 

Electrochemistry, in general, is a technique that lends itself to be combined with other techniques. One such technique is gravimetry. Electrogravimetry, discussed earlier in the chapter, is a technique that relies upon the application of a current or potential to deposit an electroactive species onto an electrode. After the material is deposited, its mass is determined analytically. A quartz crystal microbalance (QCM) is an instrument that monitors mass changes in real time. 

The major pitfall of chemical separations is the varying and unavoidable loss of material. Nonetheless, there are several ways of controlling the losses by calculating the chemical recoveries for the whole procedure. The amount of carrier can be determined in the measurement source at the end of separation and compared with the initial value. Recovery should be corrected for the presence of the element in the original sample. The amount of carrier can be determined by classical analytical methods, such as gravimetry or by nuclear techniques. In the case of short-lived nuclides, reactivation is an appropriate technique. 

Classical or wet chemistry analysis techniques such as titrimetry and gravimetry remain in use in many laboratories and are still widely taught in Analytical Chemistry courses. They provide excellent introductions to the manipulative and other skills required in analytical work, they are ideal for high-precision analyses, especially when small numbers of samples are involved, and they are sometimes necessary for the analysis of standard materials. However, there is no doubt that most analyses are now performed by instrumental methods. Techniques using absorption and emission spectrometry at various wavelengths, many different electrochemical methods, mass spectrometry, gas and liquid chromatography, and thermal and radiochemical methods, probably account for at least 90% of all current analytical work. There are several reasons for this. 

If we look back some forty or fifty years, chemical analysis concentrated on perhaps three main areas qualitative testing, quantitative determinations, particularly by classical techniques such as titrimetry and gravimetry, and structural analysis by procedures requiring laborious and time-consuming calculations. The analytical chemist of today has an armoury of instrumental techniques, automated systems and computers which enable analytical measurements to be made more easily, more quickly and more accurately. 

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