D. Harvey - Modern Analytical Chemistry (794078), страница 62
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Repeat forthe analysis of Fe when Al is an interferent.SOLUTIONTo find a suitable masking agent, we look for a species that binds with theinterferent but does not bind with the analyte. Oxalate, for example, is aninappropriate choice because it binds with both Al and Fe. From Table 7.6 wefind that thioglycolic acid is a selective masking agent for Fe in the presence ofAl and that F– is a selective masking agent for Al in the presence of Fe.As shown in Example 7.13, a masking agent’s effectiveness can be judged byconsidering the equilibrium constants for the analytical and masking reactions.EXAMPLE 7.13Show that CN– is an appropriate masking agent for Ni2+ in a method in whichnickel’s complexation with EDTA is an interference.SOLUTIONThe relevant reactions and equilibrium constants from Appendix 3C areNi2+ + Y4– → NiY2–Kf = 4.2 × 1018Ni2+ + 4CN– → Ni(CN)42–β4 = 1.7 × 1030in which Y4– is an abbreviation for ethylenediaminetetraacetic acid (EDTA).Cyanide is an appropriate masking agent because the formation constant for1400-CH07 9/8/99 4:03 PM Page 209Chapter 7 Obtaining and Preparing Samples for Analysis209Ni(CN) 4 2– is greater than that for the Ni–EDTA complex.
In fact, theequilibrium constant for the reaction in which EDTA displaces the maskingagentNi(CN)42– + Y4–K =t NiY2– + 4CN–Kf4.2 × 1018= 2.5 × 10 −12=β41.7 × 1030is very small, indicating that Ni(CN)42– is relatively inert in the presence ofEDTA.7F.4 Separations Based on a Change of StateSince an analyte and interferent are usually in the same phase, a separation oftencan be effected by inducing a change in one of their physical or chemical states.Changes in physical state that have been exploited for the purpose of a separationinclude liquid-to-gas and solid-to-gas phase transitions.
Changes in chemical stateinvolve one or more chemical reactions.TemperatureChanges in Physical State When the analyte and interferent are miscible liquids, a separation based on distillation may be possible if theirboiling points are significantly different. The progress of a distillationis outlined in Figure 7.13, which shows a plot of temperature versusthe vapor-phase and liquid-phase composition of a mixture consistingof a low-boiling analyte and a high-boiling interferent. The initialmixture is indicated by the point labeled A. When this solution isbrought to its boiling point, a vapor phase with the composition indi0cated by the point labeled B is in equilibrium with the original liquid100phase.
This equilibrium is indicated by the horizontal tie-line betweenpoints A and B. When the vapor phase at point B condenses, a newliquid phase with the same composition as the vapor phase (point C)results. The liquid phase at point C boils at a lower temperature, with an equilibrium established with the vapor-phase composition indicated by point D. Thisprocess of repeated vaporization and condensation gradually separates the analyte and interferent.Two examples of the equipment used for distillations are shown in Figure 7.14.The simple distillation apparatus shown in Figure 7.14a does not produce a very efficient separation and is useful only for separating a volatile liquid from nonvolatileliquids or for separating liquids with boiling points that differ by more than 150 °C.A more efficient separation is achieved by a fractional distillation (Figure 7.14b).Packing the distillation column with a high-surface-area material, such as a steelsponge or glass beads, provides more opportunity for the repeated process of vaporization and condensation necessary to effect a complete separation.When the sample is a solid, a separation of the analyte and interferent by sublimation may be possible.
The sample is heated at a temperature and pressure belowits triple point where the solid vaporizes without passing through the liquid state.The vapor is then condensed to recover the purified solid. A good example of theuse of sublimation is in the isolation of amino acids from fossil mollusk shells anddeep-sea sediments.14VaporBADCLiquidMole % interferentMole % analyte1000Figure 7.13Boiling points versus composition diagramfor a near-ideal solution, showing theprogress of a distillation.1400-CH07 9/8/99 4:04 PM Page 210210Modern Analytical Chemistry(b)(a)ThermometerThermometerDistillationadaptorDistillationadaptorFigure 7.14Typical equipment for a(a) simple distillation; and a(b) fractional distillation.CondenserCondenserFractionatingcolumnDistillationflaskReceiving flaskReceiving flaskDistillationflaskAnother approach for purifying solids is recrystallization.
The solid is dissolvedin a minimum volume of solvent, for which the analyte’s solubility is significantwhen the solvent is hot, and minimal when the solvent is cold. The interferentsmust be less soluble in the hot solvent than the analyte, or present in much smalleramounts. A portion of the solvent is heated in an Erlenmeyer flask, and smallamounts of sample are added until undissolved sample is visible.
Additional heatedsolvent is added until the sample is again dissolved or until only insoluble impurities remain. The process of adding sample and solvent is repeated until the entiresample has been added to the Erlenmeyer flask. If necessary, insoluble impuritiesare removed by filtering the heated solution. The solution is allowed to cool slowly,promoting the growth of large, pure crystals, and then cooled in an ice bath to minimize solubility losses. The purified sample is isolated by filtration and rinsed to remove soluble impurities.
Finally, the sample is dried to remove any remainingtraces of the solvent. Further purification, if necessary, can be accomplished by additional recrystallizations.Changes in Chemical State Distillation, sublimation, and recrystallization use achange in physical state as a means of separation. Chemical reactivity also can beused in a separation by effecting a change in the chemical state of the analyte or interferent. For example, SiO 2 can be separated from asample by reacting with HF. The volatile SiF4 that formsTable 7.7 Selected Examples of theis easily removed by evaporation. In other cases distillaApplication of Distillation to thetion may be used to remove a nonvolatile inorganic ionSeparation of Inorganic Ionsafter chemically converting it to a more volatile form.
Forexample, NH4+ can be separated from a sample by makAnalyte or InterferentTreatmentaing the solution basic, resulting in the formation of NH3.CO32–CO32– + 2H3O+ → CO2 + 3H2OThe ammonia that is produced can then be removed byNH4+NH4+ + OH– → NH 3 + H2Odistillation. Other examples are listed in Table 7.7.SO32–SO32– + 2H3O+ → SO 2 + 3H2OOther types of reactions can be used to chemicallyS2–S2– + 2H3O+ → H 2 S + 2H2Oseparate an analyte and interferent, including precipitation, electrodeposition, and ion exchange. Two imporaUnderlined species is removed by distillation.tant examples of the application of precipitation are the1400-CH07 9/8/99 4:04 PM Page 211Chapter 7 Obtaining and Preparing Samples for Analysis211pH-dependent solubility of metal oxides and hydroxides, and the solubility ofmetal sulfides.Separations based on the pH-dependent solubility of oxides and hydroxides areusually accomplished using strong acids, strong bases, or NH3/NH4Cl buffers.
Mostmetal oxides and hydroxides are soluble in hot concentrated HNO3, although a fewoxides, such as WO3, SiO2, and SnO2 remain insoluble even under these harsh conditions. In determining the amount of Cu in brass, for example, an interferencefrom Sn is avoided by dissolving the sample with a strong acid. An insoluble residueof SnO2 remains that can then be removed by filtration.Most metals will precipitate as the hydroxide in the presence of concentratedNaOH. Metals forming amphoteric hydroxides, however, remain soluble in concentrated NaOH due to the formation of higher-order hydroxo-complexes.
For example, Zn2+ and Al3+ will not precipitate in concentrated NaOH due to the formationof Zn(OH)3– and Al(OH)4–. The solubility of Al 3+ in concentrated NaOH is used toisolate aluminum from impure bauxite, an ore of Al2O3. The ore is powdered andplaced in a solution of concentrated NaOH where the Al 2O3 dissolves to formAl(OH)4–. Other oxides that may be present in the ore, such as Fe2O3 and SiO2, remain insoluble. After filtering, the filtrate is acidified to recover the aluminum as aprecipitate of Al(OH)3.The pH of an NH3/NH4Cl buffer (pKa = 9.24) is sufficient to ensure the precipitation of most metals as the hydroxide. The alkaline earths and alkaline metals,however, will not precipitate at this pH.
In addition, metal ions that form solublecomplexes with NH3, such as Cu2+, Zn2+, Ni2+, and Co2+, also will not precipitateunder these conditions.Historically, the use of S2– as a precipitating reagent is one of the earliest examples of a separation technique. In Fresenius’s 1881 text, A System of Instruction inQuantitative Chemical Analysis,15 sulfide is frequently used as a means for separating metal ions from the remainder of the sample matrix.
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