Determination gravimetric methods

These include chemical methods (reduction by ferrons snlfate and titrimetric determination), gravimetric methods for 0 U ratio, moistnre analysis by conlomet-ric techniques, and determination of H, C, N, Cl, and E with a specific method for each of these elements. In addition, the isotopic composition is determined by mass spectrometric methods and several metallic and nomnetallic impnrity elements are determined by spectrochemical methods. 

A gravimetric method in which the mass of a particulate analyte is determined following its separation from its matrix. 

In the previous section we used four examples to illustrate the different ways that mass can serve as an analytical signal. These examples also illustrate the four gravimetric methods of analysis. When the signal is the mass of a precipitate, we call the method precipitation gravimetry. The indirect determination of by precipi-... 

Representative Method The best way to appreciate the importance of the theoretical and practical details discussed in the previous section is to carefully examine the procedure for a typical precipitation gravimetric method. Although each method has its own unique considerations, the determination of Mg + in water and waste-water by precipitating MgNH4P04 6H2O and isolating Mg2P20y provides an instructive example of a typical procedure. 

The determination of silicon is commonly encountered in metallurgical and mining laboratories responsible for the analysis of ores, slags, and alloys. The volatilization gravimetric method, which is appropriate for samples containing high concentrations of silicon, was described earlier in Method 8.2. 

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. 

A number of gravimetric methods, such as the determination of Ct in a soluble salt, have been part of the standard repertoire of experiments for introductory courses in analytical chemistry. Listed here are additional experiments that may be used to provide practical examples of gravimetry. 

Nickel also is deterrnined by a volumetric method employing ethylenediaminetetraacetic acid as a titrant. Inductively coupled plasma (ICP) is preferred to determine very low nickel values (see Trace AND RESIDUE ANALYSIS). The classical gravimetric method employing dimethylglyoxime to precipitate nickel as a red complex is used as a precise analytical technique (122). A colorimetric method employing dimethylglyoxime also is available. The classical method of electro deposition is a commonly employed technique to separate nickel in the presence of other metals, notably copper (qv). It is also used to estabhsh caUbration criteria for the spectrophotometric methods. X-ray diffraction often is used to identify nickel in crystalline form. 

Although gravimetric methods have been used traditionally for the determination of large amounts of tellurium, more accurate and convenient volumetric methods are favored. The oxidation of teUurium(IV) by ceric sulfate in hot sulfuric acid solution in the presence of chromic ion as catalyst affords a convenient volumetric method for the determination of tellurium (32). Selenium(IV) does not interfere if the sulfuric acid is less than 2 N in concentration. Excess ceric sulfate is added, the excess being titrated with ferrous ammonium sulfate using o-phenanthroline ferrous—sulfate as indicator. The ceric sulfate method is best appHed in tellurium-rich materials such as refined tellurium or tellurium compounds. 

Specifications, Analysis, and Toxicity. Dicyandiamide is identified quaHtatively by paper chromatography and quantitatively by ultraviolet spectrometry of the chromatogram. More commonly, total nitrogen analysis is used as a purity control or the dicyandiamide is converted by hydrolysis to guanylurea, which is determined gravimetrically as the nickel salt (50). Methods based on the precipitation of silver dicyandiamide picrate are sometimes used (51). Dicyandiamide can also be titrated with tetrabutylammonium hydroxide ia pyridine solution. Table 4 gives a typical analysis of a commercial sample. Dicyandiamide is essentially nontoxic. It may, however, cause dermatitis. 

The gravimetric method is accepted as the most accurate humidity-measuring technique. In this method a known quantity of gas is passed over a moisture-absorbing chemical such as phosphorus pent-oxide, and the increase in weight is determined. 

Ln(II) in LnFj Ln(II) were determined after samples dissolution in H PO in the presence of a titrated solution of NFI VO, which excess was titrated with the Fe(II) salt. It was found that dissolution of the materials based on CeF CeFj in H PO does not change the oxidation state of cerium, thus phosphate complexes of Ce(III, IV) can be used for quantitative spectrophotometric determination of cerium valence forms. The contents of Ln(II, III) in Ln S LnS may be counted from results of the determination of total sulfur (determined gravimetric ally in BaSO form) and sum of the reducers - S and Ln(II) (determined by iodometric method). 

ISO 9096. Stationary source emissions—Determination of concentration and mass flow rate of particulate material in gas-carrying ducts Manual gravimetric method. 1992, p. 30. 

The gravimetric method depends on the sampling of flowing, particulate-laden gas from different positions across the exhaust gas duct and the determination of the mass of the particulate material. The sample is collected over a certain time period from each point. The volumetric gas flow is measured. The result is obtained by the following steps ... 

ISO 9096. Stationary Source Emissions—Determination of Concentration and Mass Tiow Rate of Particulate Material in Cja,s-Carrying Ducts—Manual Gravimetric Method. (1992). 

It should be pointed out that the titrimetric methods described for the determination of antimony (Chapter 10) are to be preferred to the gravimetric methods as they are simpler, more rapid, and quite as accurate. 

Unique methods based on new principles have been developed within the past 10 years. Threonine (27,28,249) is oxidized by lead tetraacetate or periodic acid to acetaldehyde, which is determined by photometric analysis of its p-hydroxydiphenyl complex or iodometric titration of its combined bisulfite. Serine is oxidized similarly to formaldehyde, which is determined gravimetrically (207) as its dimedon (5,5-dimethyldihydro-resorcinol) derivative or photometric analysis (31) of the complex formed with Eegriwe s reagent (l,8-dihydroxynaphthalene-3,5-disulfonic acid). It appears that the data obtained for threonine and serine in various proteins by these oxidation procedures are reasonably accurate. [Block and Bolling (26) have given data on the threonine and serine content of various proteins. ]... 

Gravimetric and volumetric methods are practicable for the quantitative determination of the a-sulfo fatty acid esters. Using gravimetric methods the surfactant is precipitated with p-toluidine or barium chloride [105]. The volumetric determination method is two-phase titration. In this technique different titrants and indicators are used. For the analysis of a-sulfo fatty acid esters the quaternary ammonium surfactant hyamine 1622 (p,f-octylphenoxyethyldimethyl-ammonium chloride) is used as the titrant [106]. The indicator depends on the pH value of the titration solution. Titration with a phenol red indicator is carried out at a pH of 9, methylene blue is used in acid medium [106], and a mixed indicator of a cationic (dimidium bromide) and an anionic (disulfine blue VN150) dye can be used in an acid and basic medium [105]. 

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