D. Harvey - Modern Analytical Chemistry (794078), страница 78
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To determine thecompound’s empirical formula, we assume that we have 1 g of the compound,giving 0.4515 g of C, 0.0151 g of H, and 0.5335 g of Cl. Expressing each elementin moles gives 0.0376 mol C, 0.0150 mol H, and 0.0150 mol Cl. Hydrogen andchlorine are present in a 1:1 molar ratio. The molar ratio of C to moles of H orCl isMoles Cmoles C0.0376=== 2.51 ≈ 2.5Moles Hmoles Cl0.0150Thus, the simplest, or empirical, formula for the compound is C5H2Cl2.In an indirect volatilization gravimetric analysis, the change in the sample’sweight is proportional to the amount of analyte. Note that in the following example itis not necessary to apply the conservation of mass to relate the analytical signal to theanalyte.EXAMPLE 8.6A sample of slag from a blast furnace is analyzed for SiO2 by decomposing a 0.5003-gsample with HCl, leaving a residue with a mass of 0.1414 g.
After treating with HFand H2SO4 and evaporating the volatile SiF4, a residue with a mass of 0.0183 gremains. Determine the %w/w SiO2 in the sample.SOLUTIONIn this procedure the difference in the residue’s mass before and after volatilizingSiF4 gives the mass of SiO2 in the sample. Thus the sample contained0.1414 g – 0.0183 g = 0.1231 g SiO2The %w/w SiO2, therefore, is0.1231 gg SiO2× 100 =× 100 = 24.61% w/w SiO20.5003 gg sampleFinally, in some quantitative applications it is necessary to compare the result for asample with a similar result obtained using a standard.2611400-CH08 9/9/99 2:18 PM Page 262262Modern Analytical ChemistryEXAMPLE 8.7A 26.23-mg sample of MgC2O4 • H2O and inert materials is heated to constantweight at 1200 °C, leaving a residue weighing 20.98 mg. A sample of pureMgC2O4 • H2O, when treated in the same fashion, undergoes a 69.08% changein its mass. Determine the %w/w MgC2O4 • H2O in the sample.SOLUTIONThe change in mass when analyzing the mixture is 5.25 mg, thus the grams ofMgC2O4 • H2O in the sample is5.25 mg lost ×100 mg MgC 2O 4 • H 2O= 7.60 mg MgC 2O4 • H 2O69.08 mg lostThe %w/w MgC2O4 • H2O, therefore, ismg MgC 2O4 • H 2O7.60 mg× 100 =× 100 = 29.0% w/w MgC 2O4 • H 2Omg sample26.23 mg8C.3 Evaluating Volatilization GravimetryThe scale of operation, accuracy, and precision of gravimetric volatilizationmethods are similar to that described in Section 8B.4 for precipitation gravimetry.
The sensitivity for a direct analysis is fixed by the analyte’s chemical formfollowing combustion or volatilization. For an indirect analysis, however, sensitivity can be improved by carefully choosing the conditions for combustion orvolatilization so that the change in mass is as large as possible. For example, thethermogram in Figure 8.9 shows that an indirect analysis for CaC2 O 4 H 2 Obased on the weight of the residue following ignition at 1000 °C will be moresensitive than if the ignition was done 300 °C. Selectivity does not present aproblem for direct volatilization gravimetric methods in which the analyte is agaseous product retained in an absorbent trap.
A direct analysis based on theresidue’s weight following combustion or volatilization is possible when theresidue only contains the analyte of interest. As noted earlier, indirect analysesare only feasible when the residue’s change in mass results from the loss of a single volatile product containing the analyte.Volatilization gravimetric methods are time- and labor-intensive. Equipmentneeds are few except when combustion gases must be trapped or for a thermogravimetric analysis, which requires specialized equipment.⋅8D Particulate GravimetryGravimetric methods based on precipitation or volatilization reactions require thatthe analyte, or some other species in the sample, participate in a chemical reactionproducing 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 maybe readily separated from its liquid, gas, or solid matrix. When such a separation ispossible, the analyte’s mass can be directly determined with an appropriate balance.In this section the application of particulate gravimetry is briefly considered.1400-CH08 9/9/99 2:18 PM Page 263Chapter 8 Gravimetric Methods of Analysis2638D.1 Theory and PracticeTwo approaches have been used to separate the analyte from its matrix in particulate gravimetry. The most common approach is filtration, in which solid particulates are separated from their gas, liquid, or solid matrix.
A second approach uses aliquid-phase or solid-phase extraction.Filtration Liquid samples are filtered by pulling the liquid through an appropriatefiltering medium, either by gravity or by applying suction from a vacuum pump oraspirator. The choice of filtering medium is dictated primarily by the size of thesolid particles and the sample’s matrix. Filters are constructed from a variety of materials, including cellulose fibers, glass fibers, cellulose nitrate, and polytetrafluoroethylene (PTFE). Particle retention depends on the size of the filter’s pores.
Cellulose fiber filters, commonly referred to as filter paper, range in pore size from 30 µmto 2–3 µm. Glass fiber filters, constructed from chemically inert borosilicate glass,range in pore size from 2.5 µm to 0.3 µm. Membrane filters, which are made from avariety of materials, including cellulose nitrate and PTFE, are available with poresizes from 5.0 µm to 0.1 µm.Solid aerosol particulates in gas samples are filtered using either a single ormultiple stage. In a single-stage system the gas is passed through a single filter, retaining particles larger than the filter’s pore size.
When sampling a gas line, the filteris placed directly in line. Atmospheric gases are sampled with a high-volume sampler that uses a vacuum pump to pull air through the filter at a rate of approximately 75 m3/h. In either case, the filtering medium used for liquid samples also canbe used for gas samples. In a multiple-stage system, a series of filtering units is usedto separate the particles by size.Solid samples are separated by particle size using one or more sieves.
By selecting several sieves of different mesh size, particulates with a narrow size range can beisolated from the solid matrix. Sieves are available in a variety of mesh sizes, rangingfrom approximately 25 mm to 40 µm.Extraction Filtering limits particulate gravimetry to solid particulate analytes thatare easily separated from their matrix. Particulate gravimetry can be extended to theanalysis of gas-phase analytes, solutes, and poorly filterable solids if the analyte canbe extracted from its matrix with a suitable solvent.
After extraction, the solvent canbe evaporated and the mass of the extracted analyte determined. Alternatively, theanalyte can be determined indirectly by measuring the change in a sample’s massafter extracting the analyte. Solid-phase extractions, such as those described inChapter 7, also may be used.More recently, methods for particulate gravimetry have been developed inwhich the analyte is separated by adsorption onto a metal surface, by absorptioninto a thin polymer or chemical film coated on a solid support, or by chemicallybinding to a suitable receptor covalently bound to a solid support (Figure 8.10). Adsorption, absorption, and binding occur at the interface between the metal surface,the thin film, or the receptor, and the solution containing the analyte.
Consequently, the amount of analyte extracted is minuscule, and the resulting change inmass is too small to detect with a conventional balance. This problem is overcomeby using a quartz crystal microbalance as a support.The measurement of mass using a quartz crystal microbalance is based on thepiezoelectric effect.10 When a piezoelectric material, such as a quartz crystal, experiences a mechanical stress, it generates an electrical potential whose magnitude isproportional to the applied stress. Conversely, when an alternating electrical field isAAAAAAALALALAAA(a)(b)(c)(d)Figure 8.10Four possible mechanisms for solid-stateextraction: (a) adsorption onto a solidsubstrate; (b) absorption into a thin polymeror chemical film coated on a solid substrate;(c) metal–ligand complexation in which theligand is covalently bound to the solidsubstrate; and (d) antibody–antigen bindingin which the receptor is covalently bound tothe solid substrate.1400-CH08 9/9/99 2:18 PM Page 264264Modern Analytical Chemistryapplied across a quartz crystal, an oscillatory vibrational motion is induced in thecrystal.
Every quartz crystal vibrates at a characteristic resonant frequency that is afunction of the crystal’s properties, including the mass per unit area of any materialcoated on the crystal’s surface. The change in mass following adsorption, absorption,or binding of the analyte, therefore, can be determined by monitoring the change inthe quartz crystal’s characteristic resonant frequency.
The exact relationship betweenthe change in frequency and mass is determined by a calibration curve.8D.2 Quantitative ApplicationsParticulate gravimetry is commonly encountered in the environmental analysisof water, air, and soil samples. The analysis for suspended solids in water samples, forexample, is accomplished by filtering an appropriate volume of a well-mixed samplethrough a glass fiber filter and drying the filter to constant weight at 103–105 °C.Microbiological testing of water also is accomplished by particulate gravimetry.For example, in the analysis for coliform bacteria an appropriate volume of sampleis passed through a sterilized 0.45-µm membrane filter.
The filter is then placed ona sterilized absorbent pad saturated with a culturing medium and incubated for22–24 h at 35 ±0.5 °C. Coliform bacteria are identified by the presence of individualbacterial colonies that form during the incubation period. As with qualitative applications of precipitation gravimetry, the signal in this case is a visual observationrather than a measurement of mass.Total airborne particulates are determined using a high-volume air samplerequipped with either cellulose fiber or glass fiber filters. Samples taken from urbanenvironments require approximately 1 h of sampling time, but samples from ruralenvironments require substantially longer times.Grain size distributions for sediments and soils are used to determine theamount of sand, silt, and clay present in a sample.
For example, a grain size of 2 mmserves as a boundary between gravel and sand. Grain size boundaries for sand–siltand silt–clay are given as 1/16 mm and 1/256 mm, respectively.Several standard methods for the quantitative analysis of food samples arebased on measuring the sample’s mass following a selective solvent extraction. Forexample, the crude fat content in chocolate can be determined by extracting withether for 16 h in a Soxhlet extractor.
After the extraction is complete, the ether is allowed to evaporate, and the residue is weighed after drying at 100 °C. This analysishas also been accomplished indirectly by weighing a sample before and after extracting with supercritical CO2.Quartz crystal microbalances equipped with thin-film polymer or chemicalcoatings have found numerous quantitative applications in environmental analysis.Methods have been reported for the analysis of a variety of gaseous pollutants, including ammonia, hydrogen sulfide, ozone, sulfur dioxide, and mercury.10 Biochemical particulate gravimetric sensors also have been developed.
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