D. Harvey - Modern Analytical Chemistry (794078), страница 98
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When added to a solution containing the analyte, the indicator imparts a color that depends on the solution’s electrochemical potential. Since theindicator changes color in response to the electrochemical potential, and not tothe presence or absence of a specific species, these compounds are called generalredox indicators.The relationship between a redox indicator’s change in color and the solution’selectrochemical potential is easily derived by considering the half-reaction for theindicatort Inredwhere Inox and Inred are, respectively, the indicator’s oxidized and reducedforms.
The Nernst equation for this reaction is°E = EIn−ox / In red0.05916[In ]log red[Inox ]nIf we assume that the indicator’s color in solution changes from that of Inox tothat of Inred when the ratio [Inred]/[Inox] changes from 0.1 to 10, then the endpoint occurs when the solution’s electrochemical potential is within the range°E = EIn±ox / In red0.05916n1.8001.6001.4001.2001.0000.8000.6000.4000.2000.000FerroinDiphenylaminesulfonic acid080204060Volume of titrant (mL)100Figure 9.37A partial list of general redox indicators is shown in Table 9.18.
Examples ofappropriate and inappropriate indicators for the titration of Fe2+ with Ce4+are shown in Figure 9.37.Finding the End Point Potentiometrically Another method for locating theend point of a redox titration is to use an appropriate electrode to monitor thechange in electrochemical potential as titrant is added to a solution of analyte.The end point can then be found from a visual inspection of the titrationcurve. The simplest experimental design (Figure 9.38) consists of a Pt indicator electrode whose potential is governed by the analyte’s or titrant’s redoxhalf-reaction, and a reference electrode that has a fixed potential. A furtherdiscussion of potentiometry is found in Chapter 11.Table 9.18redox indicatorA visual indicator used to signal the endpoint in a redox titration.Potential (V)Inox + ne–339Titration curve for 50.00 mL of 0.0500 MFe2+ with 0.0500 M Ce4+ showing the rangeof E and volume of titrant over which theindicators ferroin and diphenylamine sulfonicacid are expected to change color.BuretReferenceelectrodePotentiometerPt indicatorelectrodeSelected General Redox IndicatorsIndicatorOxidized ColorReduced ColorE° (V)indigo tetrasulfonatemethylene bluediphenylaminediphenylamine sulfonic acidtris(2,2’-bipyridine)ironferrointris(5-nitro-1,10-phenanthroline)ironbluebluevioletred-violetpale bluepale bluepale bluecolorlesscolorlesscolorlesscolorlessredredred-violet0.360.530.750.851.1201.1471.25Figure 9.38Experimental arrangement for recording apotentiometric redox titration curve.1400-CH09 9/9/99 2:13 PM Page 340340Modern Analytical Chemistry9D.3 Representative MethodThe photo in Colorplate 8c shows theindicator’s color change for this titration.Representative MethodsAlthough every redox titrimetric method has its own unique considerations, the following description of the determination of total residual chlorine in water providesan instructive example of a typical procedure.Method 9.3Determination of Total Chlorine Residual13Description of the Method.
The chlorination of public water supplies results inthe formation of several chlorine-containing species, the combined concentration ofwhich is called the total chlorine residual. Chlorine may be present in a variety ofstates including free residual chlorine, consisting of Cl2, HOCl, and OCl–, andcombined chlorine residual, consisting of NH2Cl, NHCl2, and NCl3. The total chlorineresidual is determined by using the oxidizing power of chlorine to convert I– to I3–.The amount of I3– formed is then determined by a redox titration using S2O32– as atitrant and starch as an indicator.
Regardless of its form, the total chlorine residual iscalculated as if all the chlorine were available as Cl2, and is reported as parts permillion of Cl.Procedure. Select a volume of sample requiring less than 20 mL of S2O32– to reachthe end point. Using glacial acetic acid, acidify the sample to a pH in the range of 3to 4, and add about 1 g of KI. Titrate with Na2S2O3 until the yellow color due to I3–begins to disappear. Add 1 mL of a starch indicator solution, and continue titratinguntil the blue color of the starch–I3– complex disappears. The volume of titrantneeded to reach the end point should be corrected for reagent impurities byconducting a blank titration.Questions1. Is this an example of a direct or an indirect analysis?This is an indirect method of analysis because the chlorine-containing speciesdo not react with the titrant. Instead the total chlorine residual oxidizesI– to I3–, and the amount of I3– is determined by the redox titration withNa2S2O3.2.
Why is the procedure not carried out directly using KI as a titrant?The redox half-reaction when I– is used as a titrant is3I–tI–3+ 2e–Because the product of the reaction, I3–, is itself colored, the color of thesolution containing the analyte changes with each addition of titrant. For thisreason it is difficult to find a suitable visual indicator for the titration’s endpoint.3. Both oxidizing and reducing agents can interfere with this analysis. Explainwhat effect each of these interferents will have on the result of an analysis.An interferent that is an oxidizing agent will convert additional I– to I3–. Thisextra I3– requires an additional volume of S2O32– to reach the end point,overestimating the total chlorine residual. If the interferent is a reducing agent,it reduces some of the I3–, produced by the reaction between the total chlorineresidual and iodide, back to I–.
The result is an underestimation of the totalchlorine residual.1400-CH09 9/9/99 2:13 PM Page 341Chapter 9 Titrimetric Methods of Analysis3419D.4 Quantitative ApplicationsAs with acid–base and complexation titrations, redox titrations are not frequentlyused in modern analytical laboratories.
Nevertheless, several important applicationscontinue to find favor in environmental, pharmaceutical, and industrial laboratories. In this section we review the general application of redox titrimetry. We begin,however, with a brief discussion of selecting and characterizing redox titrants, andmethods for controlling the analyte’s oxidation state.Adjusting the Analyte’s Oxidation State If a redox titration is to be used in aquantitative analysis, the analyte must initially be present in a single oxidation state.For example, the iron content of a sample can be determined by a redox titration inwhich Ce4+ oxidizes Fe2+ to Fe3+.
The process of preparing the sample for analysismust ensure that all iron is present as Fe2+. Depending on the sample and themethod of sample preparation, however, the iron may initially be present in boththe +2 and +3 oxidation states. Before titrating, any Fe3+ that is present must be reduced to Fe2+. This type of pretreatment can be accomplished with an auxiliary reducing or oxidizing agent.Metals that are easily oxidized, such as Zn, Al, and Ag, can serve as auxiliary reducing agents.
The metal, as a coiled wire or powder, is placed directly in the solution where it reduces the analyte. Of course any unreacted auxiliary reducing agentwill interfere with the analysis by reacting with the titrant. The residual auxiliary reducing agent, therefore, must be removed once the analyte is completely reduced.This can be accomplished by simply removing the coiled wire or by filtering.An alternative approach to using an auxiliary reducing agent is to immobilize itin a column. To prepare a reduction column, an aqueous slurry of the finely dividedmetal is packed in a glass tube equipped with a porous plug at the bottom (Figure9.39). The sample is placed at the top of the column and moves through the columnunder the influence of gravity or vacuum suction. The length of the reduction column and the flow rate are selected to ensure the analyte’s complete reduction.Two common reduction columns are used.
In the Jones reductor the columnis filled with amalgamated Zn prepared by briefly placing Zn granules in a solutionof HgCl2 to form Zn(Hg). Oxidation of the amalgamated ZnZn(Hg)(s)AqueoussampleSolidreductantPorousplugFigure 9.39Schematic diagram of a reductor column.auxiliary reducing agentA reagent used to reduce the analytebefore its analysis by a redox titration.Jones reductorA reduction column using a Zn amalgamas a reducing agent.t Zn2+(aq) + Hg(l) + 2e–provides the electrons for reducing the analyte.
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