D. Harvey - Modern Analytical Chemistry (794078), страница 102
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The end point for the titration reactionAg+(aq) + SCN–(aq)t AgSCN(s)is the formation of the reddish colored Fe(SCN)2+ complex.SCN–(aq) + Fe3+(aq)t Fe(SCN)2+(aq)The titration must be carried out in a strongly acidic solution to achieve the desiredend point.A third end point is evaluated with Fajans’ method, which uses an adsorptionindicator whose color when adsorbed to the precipitate is different from that whenit is in solution. For example, when titrating Cl– with Ag+ the anionic dye dichlorofluoroscein is used as the indicator. Before the end point, the precipitate of AgCl hasa negative surface charge due to the adsorption of excess Cl–. The anionic indicatoris repelled by the precipitate and remains in solution where it has a greenish yellowcolor.
After the end point, the precipitate has a positive surface charge due to theadsorption of excess Ag+. The anionic indicator now adsorbs to the precipitate’ssurface where its color is pink. This change in color signals the end point.Finding the End Point Potentiometrically Another method for locating the endpoint of a precipitation titration is to monitor the change in concentration for theanalyte or titrant using an ion-selective electrode. The end point can then be foundfrom a visual inspection of the titration curve. A further discussion of potentiometry is found in Chapter 11.9E.3 Quantitative ApplicationsPrecipitation titrimetry is rarely listed as a standard method of analysis, but maystill be useful as a secondary analytical method for verifying results obtained byother methods.
Most precipitation titrations involve Ag+ as either an analyte or1400-CH09 9/9/99 2:14 PM Page 355Chapter 9 Titrimetric Methods of AnalysisTable 9.22Representative Examplesof Precipitation TitrationsAnalyteTitrantaEnd PointbAsO43Br –AgNO3, KSCNAgNO3AgNO3, KSCNAgNO3AgNO3, KSCNAgNO3, KSCNAgNO3, KSCNAgNO3, KSCNAgNO3AgNO3, KSCNAgNO3, KSCNAgNO3, KSCNAgNO3, KSCNVolhardMohr or FajansVolhardMohr or FajansVolhard*Volhard*Volhard*Volhard*FajansVolhardVolhard*Volhard*VolhardCl–CO32C2O42CrO42I–PO43S2SCN–355aWhentwo reagents are listed, the analysis is by a back titration.
The firstreagent is added in excess, and the second reagent is used to back titratethe excess.bFor Volhard methods identified by an asterisk (*) the precipitated silversalt must be removed before carrying out the back titration.titrant. Those titrations in which Ag+ is the titrant are called argentometric titrations. Table 9.22 provides a list of several typical precipitation titrations.Quantitative Calculations The stoichiometry of a precipitation reaction is given bythe conservation of charge between the titrant and analyte (see Section 2C); thusMoles of chargemoles of charge× moles titrant =× moles analytemole titrantmole analyteExample 9.17 shows how this equation is applied to an analysis based on a directtitration.EXAMPLE 9.17A mixture containing only KCl and NaBr is analyzed by the Mohr method. A0.3172-g sample is dissolved in 50 mL of water and titrated to the Ag2CrO4end point, requiring 36.85 mL of 0.1120 M AgNO3. A blank titration requires0.71 mL of titrant to reach the same end point.
Report the %w/w KCl andNaBr in the sample.SOLUTIONThe volume of titrant reacting with the analytes isVAg = 36.85 mL – 0.71 mL = 36.14 mLConservation of charge for the titration requires thatMoles Ag+ = moles KCl + moles NaBrargentometric titrationA precipitation titration in which Ag+ isthe titrant.1400-CH09 9/9/99 2:14 PM Page 356356Modern Analytical ChemistryMaking appropriate substitutions for the moles of Ag+, KCl, and NaBr gives thefollowing equation.M AgVAg =g KClg NaBr+FW KCl FW NaBrSince the sample contains just KCl and NaBr, we know thatg NaBr = 0.3172 g – g KClandM AgVAg =g KCl0.3172 g – g KCl+FW KClFW NaBrSolving, we find(0.1120 M)(0.03614 L) =0.3172 g − g KClg KCl+74.551 g/mol102.89 g/mol4.048 × 10 −3 = 1.341 × 10 −2 (g KCl) + 3.083 × 10 −3 − 9.719 × 10 −3 (g KCl)3.691 × 10 −3 (g KCl) = 9.650 × 10 −4g KCl = 0.2614 gthat there is 0.2614 g of KCl andg NaBr = 0.3172 g – 0.2614 g = 0.0558 g0.0558 g of NaBr.
The weight percents for the two analytes, therefore, are0.2614 g KCl× 100 = 82.41% w/w KCl0.3172 g0.0558 g NaBr× 100 = 17.59% w/w NaBr0.3172 gThe analysis for I– using the Volhard method requires a back titration. A typicalcalculation is shown in the following example.EXAMPLE 9.18The %w/w I– in a 0.6712-g sample was determined by a Volhard titration. Afteradding 50.00 mL of 0.05619 M AgNO3 and allowing the precipitate to form, theremaining silver was back titrated with 0.05322 M KSCN, requiring 35.14 mLto reach the end point. Report the %w/w I– in the sample.SOLUTIONConservation of charge for this back titration requires thatMoles Ag+ = moles I– + moles SCN–Making appropriate substitutions for moles of Ag+, I–, and SCN– leaves us withM AgVAg =g I–+ M SCNVSCNAW I –1400-CH09 9/9/99 2:14 PM Page 357Chapter 9 Titrimetric Methods of Analysis357Solving for the grams of I–, we findg I– = (AW I–)(MAgVAg – MSCNVSCN)= (126.9 g/mol)[(0.05619 M)(0.05000 L) – (0.05322 M)(0.03514 L)]= 0.1192 gthat there is 0.1192 g of iodide.
The weight percent iodide, therefore, ispAg0.1192 g× 100 = 17.76% w/w I −0.6712 g16.0014.0012.0010.008.006.004.002.000.0009E.4 Evaluation of Precipitation TitrimetryThe scale of operations, accuracy, precision, sensitivity, time, and cost of methodsinvolving precipitation titrations are similar to those described earlier in the chapterfor other titrimetric methods. Precipitation titrations also can be extended to theanalysis of mixtures, provided that there is a significant difference in the solubilitiesof the precipitates.
Figure 9.43 shows an example of the titration curve for a mixtureof I– and Cl– using Ag+ as a titrant.20406080Volume AgNO3 (mL)100Figure 9.43Titration curve for a mixture of I– and Cl–using AgNO3 as a titrant.9F KEY TERMSacid–base titration (p. 278)acidity (p.
301)alkalinity (p. 300)argentometric titration (p. 355)auxiliary complexing agent (p. 316)auxiliary oxidizing agent (p. 341)auxiliary reducing agent (p. 341)back titration (p. 275)buret (p. 277)complexation titration (p. 314)conditional formation constant (p. 316)displacement titration (p. 275)end point (p. 274)equivalence point (p. 274)formal potential (p. 332)Gran plot (p.
293)indicator (p. 274)Jones reductor (p. 341)Kjeldahl analysis (p. 302)leveling (p. 296)metallochromic indicator (p. 323)precipitation titration (p. 350)redox indicator (p. 339)redox titration (p. 331)titrant (p. 274)titration curve (p. 276)titration error (p. 274)titrimetry (p. 274)Walden reductor (p. 341)9G SUMMARYIn a titrimetric method of analysis the volume of titrant reactingstoichiometrically with the analyte provides quantitative information about the amount of analyte in a sample. The volume oftitrant required to achieve this stoichiometric reaction is calledthe equivalence point.
Experimentally we determine the titration’s end point using a visual indicator that changes color nearthe equivalence point. Alternatively, we can locate the end pointby recording a titration curve showing the titration reaction’sprogress as a function of the titrant’s volume. In either case,the end point must closely match the equivalence point if atitration is to be accurate. Knowing the shape of a titrationcurve is critical to evaluating the feasibility of a proposed titrimetric method.Many titrations are direct, in which the titrant reacts with theanalyte. Other titration strategies may be used when a direct reaction between the analyte and titrant is not feasible.
In a back titration a reagent is added in excess to a solution containing the analyte. When the reaction between the reagent and the analyte iscomplete, the amount of excess reagent is determined titrimetrically. In a displacement titration the analyte displaces a reagent,usually from a complex, and the amount of the displaced reagentis determined by an appropriate titration.1400-CH09 9/9/99 2:14 PM Page 358358Modern Analytical ChemistryTitrimetric methods have been developed using acid–base,complexation, redox, and precipitation reactions. Acid–base titrations use a strong acid or strong base as a titrant.
The most common titrant for a complexation titration is EDTA. Because of theirstability against air oxidation, most redox titrations use an oxidizing agent as a titrant. Titrations with reducing agents also are possible. Precipitation titrations usually involve Ag+ as either the analyte or titrant.Experiments9H Suggested EXPERIMENTSThe following experiments may be used to illustrate the application of titrimetry to quantitative, qualitative, orcharacterization problems. Experiments are grouped into four categories based on the type of reaction(acid–base, complexation, redox, and precipitation). A brief description is included with each experimentproviding details such as the type of sample analyzed, the method for locating end points, or the analysis of data.Additional experiments emphasizing potentiometric electrodes are found in Chapter 11.The first block of experiments cover acid–base titrimetry.Castillo, C. A.; Jaramillo, A.
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