D. Harvey - Modern Analytical Chemistry (794078), страница 79
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For example, apiezoelectric immunosensor has been developed that shows a high selectivity forhuman serum albumin and is capable of detecting microgram quantities.11Quantitative Calculations The result of a quantitative analysis by particulategravimetry is just the ratio, using appropriate units, of the amount of analyte to theamount of sample.1400-CH08 9/9/99 2:18 PM Page 265Chapter 8 Gravimetric Methods of AnalysisEXAMPLE 8.8A 200.0-mL sample of water was filtered through a preweighed glass fiber filter.After drying to constant weight at 105 °C, the filter was found to have increasedin mass by 48.2 mg. Determine the total suspended solids for the sample inparts per million.SOLUTIONParts per million is the same as milligrams of analyte per liter of solution; thus,the total suspended solids for the sample is48.2 mg= 241 ppm0.2000 L8D.3 Evaluating Particulate GravimetryThe scale of operation and detection limit for particulate gravimetry can be extended beyond that of other gravimetric methods by increasing the size of thesample taken for analysis.
This is usually impossible for other gravimetric methods because of the difficulty of manipulating a larger sample through the individual steps of the analysis. With particulate gravimetry, however, the part ofthe sample that is not analyte is removed when filtering or extracting. Consequently, particulate gravimetry is easily extended to the analysis of trace-levelanalytes.Except for methods relying on a quartz crystal microbalance, particulategravimetry uses the same balances as other gravimetric methods and is capableof achieving similar levels of accuracy and precision.
Since particulate gravimetry is defined in terms of the mass of the particle itself, the sensitivity of theanalysis is given by the balance’s sensitivity. Selectivity, on the other hand, is determined by either the filter’s pore size or the properties of the extracting phase.Particulate gravimetric methods based on filtration are generally less time-,labor-, and capital-intensive than other gravimetric methods since they requireonly a filtration step.8E KEY TERMSadsorbate (p.
239)coagulation (p. 242)digestion (p. 239)electrogravimetry (p. 234)gravimetry (p. 233)homogeneous precipitation (p. 241)inclusion (p. 238)occlusion (p. 239)particulate gravimetry (p. 234)peptization (p. 245)precipitant (p. 235)precipitation gravimetry (p. 234)relative supersaturation (p. 241)supernatant (p. 244)thermogram (p. 256)thermogravimetry (p. 255)volatilization gravimetry (p. 234)2651400-CH08 9/9/99 2:18 PM Page 266266Modern Analytical Chemistry8F SUMMARYIn a gravimetric analysis a measurement of mass or change inmass provides quantitative information about the amount of analyte in a sample.
The most common form of gravimetry uses a precipitation reaction to generate a product whose mass is proportional to the analyte. In many cases the precipitate includes theanalyte; however, an indirect analysis in which the analyte causesthe precipitation of another compound also is possible. Precipitation gravimetric procedures must be carefully controlled to produce precipitates that are easily filterable, free from impurities,and of known stoichiometry.In volatilization gravimetry, thermal or chemical energy is usedto decompose the sample containing the analyte.
The mass ofresidue remaining after decomposition, the mass of volatile products collected with a suitable trap, or a change in mass due to theloss of volatile material are all gravimetric measurements.When the analyte is already present in a particulate form that iseasily separated from its matrix, then a particulate gravimetricanalysis may be feasible.
Examples include the determination ofdissolved solids and the determination of fat in foods.Experiments8G Suggested EXPERIMENTSA number of gravimetric methods, such as the determination of Cl– in a soluble salt, have been part of the“standard” repertoire of experiments for introductory courses in analytical chemistry. Listed here are additionalexperiments that may be used to provide practical examples of gravimetry.Burrows, H. D.; Ellis, H.
A.; Odilora, C. A. “TheDehydrochlorination of PVC,” J. Chem. Educ. 1995, 72,448–450.Henrickson, C. H.; Robinson, P. R. “GravimetricDetermination of Calcium as CaC2O4 H2O,” J. Chem. Educ.1979, 56, 341–342.This experiment describes a simple gravimetric procedurefor determining the %w/w Cl in samples of poly(vinylchloride).A procedure is provided for the analysis of calcium insamples of CaCO3. Precipitation is done homogeneouslyusing urea and acidified ammonium oxalate. By acidifyingthe ammonium oxalate, the oxalate is introduced as oxalicacid and does not precipitate the Ca2+. Heating the solutionhydrolyzes the urea, forming NH3.
As the NH3 neutralizes theacid in solution, oxalate acid is converted to oxalate andCaC2O4 H2O precipitates. The acid–base indicator methylred is used to signal the completion of the precipitation.Carmosini, N.; Ghoreshy, S.; Koether, M. C. “TheGravimetric Analysis of Nickel Using a Microwave Oven,”J. Chem. Educ. 1997, 74, 986–987.A procedure for using a microwave oven to digest samplesof Ni ore and to dry precipitates of nickel–dimethylgloxime isdescribed in this experiment.Harris, T.
M. “Revitalizing the Gravimetric Determination inQuantitative Analysis Laboratory,” J. Chem. Educ. 1995, 72,355–356.This experiment investigates the accuracy of a gravimetricanalysis when modifying a standard procedure. Thegravimetric procedure is the determination of Ba2+ as BaSO4.Modifications that are investigated include the addition of apotential interferent (Ca2+), changing the pH at whichprecipitation occurs, changing the rate at which theprecipitant is added, changing the conditions for digestingthe precipitate, and changing the procedure for filtering anddrying the precipitate. Errors introduced by modifying thestandard procedure can be explained by considering theprocess by which precipitation occurs.⋅⋅Snow, N.
H.; Dunn, M.; Patel, S. “Determination of CrudeFat in Food Products by Supercritical Fluid Extraction andGravimetric Analysis,” J. Chem. Educ. 1997, 74, 1108–1111.The %w/w fat in candy bars is determined by an indirectparticulate gravimetric analysis. Supercritical CO2 is used toextract the fat from the sample, and the change in thesample’s weight is used to determine the fat content.Thompson, R.
Q.; Ghadiali, M. “Microwave Drying ofPrecipitates for Gravimetric Analysis,” J. Chem. Educ. 1993,70, 170–171.This article describes conditions for using a householdmicrowave oven to dry precipitates for the determination ofCl– as AgCl, the determination of SO42– as BaSO4, and thedetermination of Ca2+ as CaC2O4 H2O.⋅1400-CH08 9/9/99 2:18 PM Page 267Chapter 8 Gravimetric Methods of Analysis2678H PROBLEMS1. Starting with the equilibrium constant expressions forreactions 8.1, and 8.3–8.5, verify that equation 8.7 is correct.2.
Equation 8.7 shows how the solubility of AgCl varies as afunction of the equilibrium concentration of Cl–. Derive asimilar equation to describe the solubility of AgCl as afunction of the equilibrium concentration of Ag+. Graph theresulting solubility function and compare it with that shownin Figure 8.1.3. Derive a solubility diagram (solubility versus pH) forZn(OH)2 that takes into account the following soluble zinchydroxide complexes: Zn(OH)+, Zn(OH)3–, Zn(OH)42–.What is the optimum pH for the quantitative precipitation ofZn(OH)2?4.
For what pH range will the following precipitates have theirlowest solubility?a. CaC2O4b. PbCrO4c. BaSO4d. SrCO3e. ZnS5. When solutions of 1.5 M KNO3 and 1.5 M HClO4 are mixed,a white precipitate of KClO4 is formed. If traces of MnO4– arepresent, an inclusion of KMnO4 is possible.
Impureprecipitates of KClO4 are colored purple by the includedKMnO4. Following are the descriptions and results for twoexperiments in which KClO4 is precipitated in the presence ofMnO4–. Explain why the two experiments lead to differentresults (see Color Plate 6).Experiment 1. Place 1 mL of 1.5 M KNO3 in a test tube, add3 drops of 0.1 M KMnO4, and swirl to mix. Add 1 mL of 1.5M HClO4 dropwise, agitating the solution between drops.Destroy the excess KMnO4 by adding 0.1 M NaHSO3dropwise. The resulting precipitate of KClO4 has an intensepurple color.Experiment 2. Place 1 mL of 1.5 M HClO4 in a test tube, add3 drops of 0.1 M KMnO4, and swirl to mix.
Add 1 mL of 1.5M KNO3 dropwise, agitating the solution between drops.Destroy the excess KMnO4 by adding 0.1 M NaHSO3dropwise. The resulting precipitate of KClO4 is pale purple orwhite in color.6. When solutions of Ba(SCN)2 and MgSO4 are mixed, aprecipitate of BaSO4 forms. Following are the descriptionsand results for several experiments in which only theconcentrations of Ba(SCN)2 and MgSO4 are different.
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