D. Harvey - Modern Analytical Chemistry (794078), страница 75
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In this section we review the generalapplication of precipitation gravimetry to the analysis of inorganic and organiccompounds.Inorganic Analysis The most important precipitants for inorganic cations arechromate, the halides, hydroxide, oxalate, sulfate, sulfide, and phosphate. A summary of selected methods, grouped by precipitant, is shown in Table 8.1. Many inorganic anions can be determined using the same reactions by reversing the analyte2471400-CH08 9/9/99 2:18 PM Page 248248Modern Analytical ChemistryTable 8.1Selected Gravimetric Method for Inorganic CationsBased on PrecipitationAnalytePrecipitantPrecipitate FormedPrecipitate WeighedBa2+Pb2+Ag+Hg22+Al3+Be2+Fe3+Ca2+Sb3+As3+Hg2+Ba2+Pb2+Sr2+Be2+Mg2+Sr2+Zn2+(NH4)2CrO4K2CrO4HClHClNH3NH3NH3(NH4)2C2O4H2SH2SH2SH2SO4H2SO4H2SO4(NH4)2HPO4(NH4)2HPO4KH2PO4(NH4)2HPO4BaCrO4PbCrO4AgClHg2Cl2Al(OH)3Be(OH)2Fe(OH)3CaC2O4Sb2S3As2S3HgSBaSO4PbSO4SrSO4NH4BePO4NH4MgPO4SrHPO4NH4ZnPO4BaCrO4PbCrO4AgClHg2Cl2Al2O3BeOFe2O3CaCO3 or CaOSb2S3As2S3HgSBaSO4PbSO4SrSO4Be2P2O7Mg2P2O7Sr2P2O7Zn2P2O7Table 8.2Selected Gravimetric Methods for Inorganic AnionsBased on PrecipitationAnalytePrecipitantPrecipitate FormedPrecipitate WeighedCN–I–Br –Cl–ClO3–SCN–SO42–AgNO3AgNO3AgNO3AgNO3FeSO4/AgNO3SO2/CuSO4BaCl2AgCNAgIAgBrAgClAgClCuSCNBaSO4AgCNAgIAgBrAgClAgClCuSCNBaSO4and precipitant.
For example, chromate can be determined by adding BaCl2 andprecipitating BaCrO4. Methods for other selected inorganic anions are summarizedin Table 8.2. Methods for the homogeneous generation of precipitants are shown inTable 8.3.The majority of inorganic precipitants show poor selectivity.
Most organic precipitants, however, are selective for one or two inorganic ions. Several common organic precipitants are listed in Table 8.4.Precipitation gravimetry continues to be listed as a standard method for theanalysis of Mg2+ and SO42– in water and wastewater analysis. A description of theprocedure for Mg2+ was discussed earlier in Method 8.1. Sulfate is analyzed by precipitating BaSO4, using BaCl2 as the precipitant. Precipitation is carried out in an1400-CH08 9/9/99 2:18 PM Page 249Chapter 8 Gravimetric Methods of AnalysisTable 8.3Reactions for the HomogeneousPreparation of SelectedInorganic PrecipitantsPrecipitantReactionOH–SO42–S2–IO3–PO42–C2O42–CO32–(NH2)2CO + 3H2O 2NH4+ + CO2 + 2OH–NH2HSO3 + 2H2O NH4+ + H3O+ + SO42–CH3CSNH2 + H2O CH3CONH2 + H2SHOCH2CH2OH + IO4– 2HCHO + H2O + IO3–(CH3O)3PO + 3H2O 3CH3OH + H3PO4(C2H5)2C2O4 + 2H2O 2C2H5OH + H2C2O4Cl3CCOOH + 2OH– CHCl3 + CO32– + H2OTable 8.4tttttttSelected Gravimetric Methods for Inorganic Cations Based on Precipitation withOrganic PrecipitantsAnalytePrecipitantNi2+dimethylgloximeFe3+StructureNNOHOHNOcupferronPrecipitate FormedPrecipitate WeighedNi(C4H7O2N2)2Ni(C4H7O2N2)2Fe(C6H5N2O2)3Fe2O3CuC14H11O2NCuC14H11O2NCo(C10H6O2N)3Co or CoSO4K[B(C6H5)4]K[B(C6H5)4]C20H16N4HNO3C20H16N4HNO3NO – NH4+Cu2+cupronH5C6C6H5NOHOHCo2+249NO1-nitroso-2-naphtholOHK+sodium tetraphenylborateNO3–nitronNa[B(C6H5)4]NC6H5–NNC6H5N+C6H51400-CH08 9/9/99 2:18 PM Page 250250Modern Analytical ChemistryTable 8.5Selected Gravimetric Methods for the Analysis of Organic Functional Groupsand Heteroatoms Based on PrecipitationAnalyteTreatmentPrecipitantPrecipitateOrganic halidesR-XX = Cl, Br, IOrganic HalidesR-XX = Cl, Br, IOrganic sulfurOrganic sulfurOxidation with HNO3 in presence of Ag+AgNO3AgXCombustion in O2 (with Pt catalyst) in presence of Ag+AgNO3AgXOxidation with HNO3 in presence of Ba2+Combustion in O2 (with Pt catalyst) to produce SO2 and SO3,which are collected in dilute H2O2Reaction with HI to produce RIBaCl2BaSO4BaCl2AgNO3BaSO4AgIReaction with HI to produce RIAgNO3AgIAlkoxy groupsR’-ORR = CH3 or C2H5orOR'CORAlkimide groupNRR = CH3, C2H5N may be 1°, 2°, or 3°acidic solution (acidified to pH 4.5–5.0 with HCl) to prevent the possible precipitation of BaCO3 or Ba3(PO4)2 and performed near the solution’s boiling point.
Theprecipitate is digested at 80–90 °C for at least 2 h. Ashless filter paper pulp is addedto the precipitate to aid in filtration. After filtering, the precipitate is ignited to constant weight at 800 °C. Alternatively, the precipitate can be filtered through a fineporosity fritted glass crucible (without adding filter paper pulp) and dried to constant weight at 105 °C. This procedure is subject to a variety of errors, includingocclusions of Ba(NO3)2, BaCl2, and alkali sulfates.Organic Analysis Several organic functional groups or heteroatoms can be determined using gravimetric precipitation methods; examples are outlined in Table 8.5.Note that the procedures for the alkoxy and alkimide functional groups are examples of indirect analyses.Quantitative Calculations In precipitation gravimetry the relationship betweenthe analyte and the precipitate is determined by the stoichiometry of the relevantreactions.
As discussed in Section 2C, gravimetric calculations can be simplifiedby applying the principle of conservation of mass. The following exampledemonstrates the application of this approach to the direct analysis of a singleanalyte.1400-CH08 9/9/99 2:18 PM Page 251Chapter 8 Gravimetric Methods of AnalysisEXAMPLE 8.1An ore containing magnetite, Fe3O4, was analyzed by dissolving a 1.5419-gsample in concentrated HCl, giving a mixture of Fe2+ and Fe3+. After addingHNO3 to oxidize any Fe2+ to Fe3+, the resulting solution was diluted with waterand the Fe 3+ precipitated as Fe(OH) 3 by adding NH 3 .
After filtering andrinsing, the residue was ignited, giving 0.8525 g of pure Fe2O3. Calculate the%w/w Fe3O4 in the sample.SOLUTIONThis is an example of a direct analysis since the iron in the analyte, Fe3O4, ispart of the isolated precipitate, Fe2O3. Applying a conservation of mass to Fe,we write3 × moles Fe3O4 = 2 × moles Fe2O3Using formula weights, FW, to convert from moles to grams in the precedingequation leaves us with3 × g Fe3O42 × g Fe2 O3=FW Fe3O4FW Fe2 O3which can be solved for grams of Fe3O4 and %w/w Fe3O4 in the sample.2 × g Fe2 O3 × FW Fe3O42 × 0.8525 g × 231.54 g/mol== 0.82405 g Fe3O43 × FW Fe2 O33 × 159.69 g/molg Fe3O40.82405 g× 100 =× 100 = 53.44% w/w Feg sample1.5419 gAs discussed earlier, the simultaneous analysis of samples containing two analytesrequires the isolation of two precipitates.
As shown in Example 8.2, conservation ofmass can be used to write separate stoichiometric equations for each precipitate.These equations can then be solved simultaneously for both analytes.EXAMPLE 8.2A 0.6113-g sample of Dow metal, containing aluminum, magnesium, and othermetals, was dissolved and treated to prevent interferences by the other metals.
Thealuminum and magnesium were precipitated with 8-hydroxyquinoline. Afterfiltering and drying, the mixture of Al(C9H6NO)3 and Mg(C9H6NO)2 was foundto weigh 7.8154 g. The mixture of dried precipitates was then ignited, convertingthe precipitate to a mixture of Al2O3 and MgO. The weight of this mixed solid wasfound to be 1.0022 g.
Calculate the %w/w Al and %w/w Mg in the alloy.SOLUTIONThis is an example of a direct analysis in which the two analytes are determinedwithout a prior separation. The weight of the original precipitate and theignited precipitate are given by the following two equationsg Al(C9H6NO)3 + g Mg(C9H6NO)2 = 7.8154g Al2O3 + g MgO = 1.00222511400-CH08 9/9/99 2:18 PM Page 252252Modern Analytical Chemistrycontaining four unknown terms. To solve this pair of equations, we must findtwo additional equations relating the four unknowns to one another. Theseadditional equations describe the stoichiometric relationships between the twocompounds containing aluminum and the two compounds containingmagnesium and are based on the conservation of Al and Mg.
Thus, for Al wehave2 × moles Al2O3 = moles Al(C9H6NO)3Converting from moles to grams and solving yields an equation relating thegrams of Al2O3 to the grams of Al(C9H6NO)32 × g Al 2 O3g Al(C 9 H 6 NO)3=FW Al 2 O3FW Al(C 9 H 6 NO)3g Al 2 O3 =g Al(C 9 H 6 NO)3 × 101.96 g/molg Al(C 9 H 6 NO)3 × FW Al 2 O3=2 × FW Al(C 9 H 6 NO)32 × 459.45 g/mol= 0.11096 × g Al(C 9 H 6 NO)3For Mg we haveMoles MgO = moles Mg(C9H6NO)2g MgOg Mg(C 9 H 6 NO)2=FW MgOFW Mg(C 9 H 6 NO)2g MgO =g Mg(C 9 H 6 NO)2 × FW MgO g Mg(C 9 H 6 NO)2 × 40.304 g /mol=FW Mg(C 9 H 6 NO)2312.61 g /mol= 0.12893 × g Mg(C 9 H 6 NO)2Substituting the equations for g MgO and g Al2O3 into the equation for thecombined weights of MgO and Al2O3 leaves us with two equations and twounknowns.g Al(C9H6NO)3 + g Mg(C9H6NO)2 = 7.81540.11096 × g Al(C9H6NO)3 + 0.12893 × g Mg(C9H6NO)2 = 1.0022Multiplying the first equation by 0.11096 and subtracting the second equationgives–0.01797 × g Mg(C9H6NO)2 = –0.1350which can be solved for the mass of Mg(C9H6NO)2.g Mg(C9H6NO)2 = 7.5125 gThe mass of Al(C9H6NO)3 can then be calculated using the known combinedmass of the two original precipitates.7.8154 g – g Mg(C9H6NO)2 = 7.8154 g – 7.5125 g = 0.3029 g Al(C9H6NO)3Using the conservation of Mg and Al, the %w/w Mg and %w/w Al in thesample can now be determined as in Example 8.1, where AW is an atomicweight.1400-CH08 9/9/99 2:18 PM Page 253Chapter 8 Gravimetric Methods of AnalysisMoles Mg = moles Mg(C9H6NO)2Moles Al = moles Al(C9H6NO)3g Mgg Mg(C9 H 6 NO)2=AW MgFW Mg(C9 H 6 NO)2g Alg Al(C 9 H 6 NO)3=AW AlFW Al(C 9 H 6 NO)3g Mg ==g Mg(C9 H 6 NO)2 × AW MgFW Mg(C9 H 6 NO)2g Al =7.5125 g × 24.305 g/mol312.61 g/mol== 0.5841 g%Mg =0.3029 g × 26.982 g /mol459.45 g /mol= 0.0178 gg Mg× 100g sample0.5841 g× 100 = 95.55% w/w Mg0.6113 gg Al(C 9 H 6 NO)3 × AW AlFW Al(C 9 H 6 NO)3%Al =g Al× 100g sample0.0178 g× 100 = 2.91% w/w Al0.6113 gIn an indirect analysis the precipitate does not contain the analyte, but is theproduct of a reaction involving the analyte.
Despite the additional complexity, astoichiometric relationship between the analyte and the precipitate can be writtenby applying the conservation principles discussed in Section 2C.EXAMPLE 8.3An impure sample of Na3PO3 weighing 0.1392 g was dissolved in 25 mL ofwater. A solution containing 50 mL of 3% w/v mercury(II) chloride, 20 mL of10% w/v sodium acetate and 5 mL of glacial acetic acid was then prepared. Thesolution containing the phosphite was added dropwise to the second solution,oxidizing PO33– to PO43– and precipitating Hg2Cl2.
After digesting, filtering,and rinsing, the precipitated Hg2Cl2 was found to weigh 0.4320 g. Report thepurity of the original sample as %w/w Na3PO3.SOLUTIONThis is an example of an indirect analysis since the isolated precipitate, Hg2Cl2,does not contain the analyte, Na3PO3.
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