D. Harvey - Modern Analytical Chemistry (794078), страница 99
Текст из файла (страница 99)
In the Walden reductor the columnis filled with granular Ag metal. The solution containing the analyte is acidified withHCl and passed through the column where the oxidation of AgAg(s) + Cl–(aq)Walden reductorA reduction column using granular Ag asa reducing agent.t AgCl(s) + e–provides the necessary electrons for reducing the analyte. Examples of both reduction columns are shown in Table 9.19.Several reagents are commonly used as auxiliary oxidizing agents, includingammonium peroxydisulfate, (NH4)2S2O8, and hydrogen peroxide, H2O2.
Ammonium peroxydisulfate is a powerful oxidizing agentS2O82–(aq) + 2e–t 2SO42–(aq)capable of oxidizing Mn2+ to MnO4–, Cr3+ to Cr2O72–, and Ce3+ to Ce4+. Excess peroxydisulfate is easily destroyed by briefly boiling the solution. The reduction of hydrogen peroxide in acidic solutionH2O2(aq) + 2H3O+(aq) + 2e–t 4H2O(l)auxiliary oxidizing agentA reagent used to oxidize the analytebefore its analysis by a redox titration.1400-CH09 9/9/99 2:13 PM Page 342342Modern Analytical ChemistryTable 9.19Selected Reductions Using Metal Reductor ColumnsOxidizedSpeciesWalden ReductorJones ReductorCr3+Cu2+Fe3+TiO2+MoO22+VO2+––aCu2+ + e– → Cu+Fe3+ + e– → Fe2+—aMoO22+ + e– → MoO2+VO2+ + 2H3O+ + e– → VO2+ + 3H2OCr3+ + e– → Cr2+Cu2+ + 2e– → CuFe3+ + e– → Fe2+TiO2+ + 2H3O+ + e– → Ti3+ + 3H2OMoO22+ + 4H3O+ + 3e– → Mo3+ + 6H2OVO2+ + 4H3O+ + 3e– → V2+ + 6H2OaNoreaction.provides another method for oxidizing an analyte.
Excess H2O2 also can be destroyed by briefly boiling the solution.Selecting and Standardizing a Titrant In quantitative work the titrant’s concentration must remain stable during the analysis. Since titrants in a reduced state aresusceptible to air oxidation, most redox titrations are carried out using an oxidizingagent as the titrant. The choice of which of several common oxidizing titrants is bestfor a particular analysis depends on the ease with which the analyte can be oxidized.Analytes that are strong reducing agents can be successfully titrated with a relativelyweak oxidizing titrant, whereas a strong oxidizing titrant is required for the analysisof analytes that are weak reducing agents.The two strongest oxidizing titrants are MnO4– and Ce4+, for which the reduction half-reactions aret Mn2+(aq) + 12H2O(l)Ce4+(aq) + e– t Ce3+(aq)MnO4–(aq) + 8H3O+(aq) + 5e–Solutions of Ce4+ are prepared from the primary standard cerium ammonium nitrate, Ce(NO3)4 2NH4NO3, in 1 M H2SO4.
When prepared from reagent gradematerials, such as Ce(OH)4, the solution must be standardized against a primarystandard reducing agent such as Na2C2O4 or Fe2+ (prepared using Fe wire). Ferroinis a suitable indicator when standardizing against Fe2+ (Table 9.20). Despite itsavailability as a primary standard and its ease of preparation, Ce4+ is not as frequently used as MnO4– because of its greater expense.Solutions of MnO4– are prepared from KMnO4, which is not available as a primary standard.
Aqueous solutions of permanganate are thermodynamically unstable due to its ability to oxidize water.⋅4MnO4–(aq) + 2H2O(l)t 4MnO2(s) + 3O2(g) + 4OH–(aq)This reaction is catalyzed by the presence of MnO2, Mn2+, heat, light, and the presence of acids and bases. Moderately stable solutions of permanganate can be prepared by boiling for an hour and filtering through a sintered glass filter to removeany solid MnO2 that precipitates. Solutions prepared in this fashion are stable for1–2 weeks, although the standardization should be rechecked periodically. Standardization may be accomplished using the same primary standard reducing agentsthat are used with Ce4+, using the pink color of MnO4– to signal the end point(Table 9.20).1400-CH09 9/9/99 2:13 PM Page 343Chapter 9 Titrimetric Methods of AnalysisTable 9.20Standardization Reactions forSelected Redox TitrantsTitration ReactionCe4+ + Fe2+ → Ce3+ + Fe3+2Ce4+ + H2C2O4 + 2H2O → 2Ce3+ + 2CO2 + 2H3O+MnO4– + 5Fe2+ + 8H3O+ → Mn2+ + 5Fe3+ + 12H2O2MnO4– + 5H2C2O4 + 6H3O+ → 2Mn2+ + 10CO2 + 14H2OI3– + 2S2O32– → 3I– + S4O62–I2 + 2S2O32– → 2I– + S4O62–Potassium dichromate is a relatively strong oxidizing agent whose principal advantages are its availability as a primary standard and the long-term stability of itssolutions.
It is not, however, as strong an oxidizing agent as MnO4– or Ce4+, whichprevents its application to the analysis of analytes that are weak reducing agents. Itsreduction half-reaction isCr2O72–(aq) + 14H3O+(aq) + 6e–t 2Cr3+(aq) + 21H2O(l)Although solutions of Cr2O72– are orange and those of Cr3+ are green, neither coloris intense enough to serve as a useful indicator.
Diphenylamine sulfonic acid, whoseoxidized form is purple and reduced form is colorless, gives a very distinct endpoint signal with Cr2O72–.Iodine is another commonly encountered oxidizing titrant. In comparison withMnO4–, Ce4+, and Cr2O72–, it is a weak oxidizing agent and is useful only for theanalysis of analytes that are strong reducing agents. This apparent limitation, however, makes I2 a more selective titrant for the analysis of a strong reducing agent inthe presence of weaker reducing agents.
The reduction half-reaction for I2 isI2(aq) + 2e–t 2I–(aq)Because of iodine’s poor solubility, solutions are prepared by adding an excessof I–. The complexation reactionI2(aq) + I–(aq)t I3–(aq)increases the solubility of I2 by forming the more soluble triiodide ion, I3–. Eventhough iodine is present as I3– instead of I2, the number of electrons in the reduction half-reaction is unaffected.I3–(aq) + 2e–t 3I–(aq)Solutions of I3– are normally standardized against Na2S2O3 (see Table 9.20) usingstarch as a specific indicator for I3–.Oxidizing titrants such as MnO4–, Ce4+, Cr2O72– and I3–, are used to titrate analytes that are in a reduced state.
When the analyte is in an oxidized state, it can bereduced with an auxiliary reducing agent and titrated with an oxidizing titrant. Alternatively, the analyte can be titrated with a suitable reducing titrant. Iodide is arelatively strong reducing agent that potentially could be used for the analysis of analytes in higher oxidation states. Unfortunately, solutions of I– cannot be used as adirect titrant because they are subject to the air oxidation of I– to I3–.3I–(aq)t I3–(aq) + 2e–3431400-CH09 9/9/99 2:13 PM Page 344344Modern Analytical ChemistryInstead, an excess of KI is added, reducing the analyte and liberating a stoichiometric amount of I3–. The amount of I3– produced is then determined by a back titration using Na2S2O3 as a reducing titrant.2S2O32–(aq)t S4O62–(aq) + 2e–Solutions of Na2S2O3 are prepared from the pentahydrate and must be standardized before use.
Standardization is accomplished by dissolving a carefullyweighed portion of the primary standard KIO3 in an acidic solution containing anexcess of KI. When acidified, the reaction between IO3– and I–IO3–(aq) + 8I–(aq) + 6H3O+(aq)t 3I3–(aq) + 9H2O(l)liberates a stoichiometric amount of I3–. Titrating I3– using starch as a visual indicator allows the determination of the titrant’s concentration.Although thiosulfate is one of the few reducing titrants not readily oxidizedby contact with air, it is subject to a slow decomposition to bisulfite and elemental sulfur. When used over a period of several weeks, a solution of thiosulfate should be restandardized periodically.
Several forms of bacteria are able tometabolize thiosulfate, which also can lead to a change in its concentration.This problem can be minimized by adding a preservative such as HgI 2 to thesolution.Another reducing titrant is ferrous ammonium sulfate, Fe(NH4)2(SO4)2 6H2O,in which iron is present in the +2 oxidation state. Solutions of Fe2+ are normallyvery susceptible to air oxidation, but when prepared in 0.5 M H2SO4 the solutionmay remain stable for as long as a month. Periodic restandardization with K2Cr2O7is advisable. The titrant can be used in either a direct titration in which the Fe2+ isoxidized to Fe3+, or an excess of the solution can be added and the quantity of Fe3+produced determined by a back titration using a standard solution of Ce 4+ orCr2O72–.⋅Inorganic Analysis Redox titrimetry has been used for the analysis of a wide rangeof inorganic analytes.
Although many of these methods have been replaced bynewer methods, a few continue to be listed as standard methods of analysis. In thissection we consider the application of redox titrimetry to several important environmental, public health, and industrial analyses. Other examples can be found inthe suggested readings listed at the end of this chapter.One of the most important applications of redox titrimetry is in evaluating thechlorination of public water supplies.
In Method 9.3 an approach for determiningthe total chlorine residual was described in which the oxidizing power of chlorine isused to oxidize I– to I3–. The amount of I3– formed is determined by a back titrationwith S2O32–.The efficiency of chlorination depends on the form of the chlorinatingspecies. For this reason it is important to distinguish between the free chlorineresidual, due to Cl2, HOCl, and OCl–, and the combined chlorine residual. Thelatter form of chlorine results from the reaction of ammonia with the free chlorine residual, forming NH2Cl, NHCl2, and NCl3. When a sample of iodide-freechlorinated water is mixed with an excess of the indicator N,N-diethyl-pphenylenediamine (DPD), the free chlorine oxidizes a stoichiometric portion ofDPD to its red-colored form.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
