D. Harvey - Modern Analytical Chemistry (794078), страница 88
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If acetic acid is placed in asolvent that is a stronger base than water, such as ammonia, then the reactionCH3COOH + NH3t NH4+ + CH3COO–proceeds to a greater extent. In fact, HCl and CH3COOH are both strong acids inammonia.All other things being equal, the strength of a weak acid increases if it is placedin a solvent that is more basic than water, whereas the strength of a weak base increases if it is placed in a solvent that is more acidic than water. In some cases, however, the opposite effect is observed.
For example, the pKb for ammonia is 4.76 inwater and 6.40 in the more acidic glacial acetic acid. In contradiction to our expectations, ammonia is a weaker base in the more acidic solvent. A full description ofthe solvent’s effect on a weak acid’s pKa or on the pKb of a weak base is beyond thescope of this text. You should be aware, however, that titrations that are not feasiblein water may be feasible in a different solvent.9B.4 Representative MethodAlthough each acid–base titrimetric method has its own unique considerations, thefollowing description of the determination of protein in bread provides an instructive example of a typical procedure.1400-CH09 9/9/99 2:12 PM Page 297Representative MethodsChapter 9 Titrimetric Methods of AnalysisMethod 9.1297Determination of Protein in Bread4Description of the Method. This quantitative method of analysis for proteinsis based on a determination of the %w/w N in the sample.
Since different cerealproteins have similar amounts of nitrogen, the experimentally determined%w/w N is multiplied by a factor of 5.7 to give the %w/w protein in the sample (onaverage there are 5.7 g of cereal protein for every gram of nitrogen). As describedhere, nitrogen is determined by the Kjeldahl method. The protein in a sample ofbread is oxidized in hot concentrated H2SO4, converting the nitrogen to NH4+. Aftermaking the solution alkaline, converting NH4+ to NH3, the ammonia is distilled intoa flask containing a known amount of standard strong acid. Finally, the excessstrong acid is determined by a back titration with a standard strong base titrant.Procedure.
Transfer a 2.0-g sample of bread, which has previously been airdried and ground into a powder, to a suitable digestion flask, along with 0.7 g ofHgO as a catalyst, 10 g of K2SO4, and 25 mL of concentrated H2SO4. Bring thesolution to a boil, and continue boiling until the solution turns clear, and for atleast an additional 30 min. After cooling to below room temperature, add 200 mLof H2O and 25 mL of 4% w/v K2S to remove the Hg2+ catalyst.
Add a few Zngranules to serve as boiling stones, and 25 g of NaOH. Quickly connect the flask toa distillation apparatus, and distill the NH3 into a collecting flask containing aknown amount of standardized HCl. The tip of the condenser should be placedbelow the surface of the strong acid.
After the distillation is complete, titrate theexcess strong acid with a standard solution of NaOH, using methyl red as a visualindicator.Questions1. Oxidizing the protein converts the nitrogen to NH4+. Why is the amount ofnitrogen not determined by titrating the NH4+ with a strong base?There are two reasons for not titrating the ammonium ion. First, NH4+ is a veryweak acid (Ka = 5.7 × 10–10) that yields a poorly defined end point when titratedwith a strong base.
Second, even if the end point can be determined withacceptable accuracy and precision, the procedure calls for adding a substantialamount of H2SO4. After the oxidation is complete, the amount of excess H2SO4will be much greater than the amount of NH4+ that is produced. The presenceof two acids that differ greatly in concentration makes for a difficult analysis.
Ifthe titrant’s concentration is similar to that of H2SO4, then the equivalencepoint volume for the titration of NH4+ may be too small to measure reliably. Onthe other hand, if the concentration of the titrant is similar to that of NH4+, thevolume needed to neutralize the H2SO4 will be unreasonably large.2. Ammonia is a volatile compound as evidenced by the strong smell of evendilute solutions. This volatility presents a possible source of determinate error.Will this determinate error be negative or positive?The conversion of N to NH3 follows the following pathwayN → NH4+NH4+ → NH3Any loss of NH3 is loss of analyte and a negative determinate error.—ContinuedThe photo in Colorplate 8a shows theindicator’s color change for this titration.1400-CH09 9/9/99 2:12 PM Page 298298Modern Analytical ChemistryContinued from page 2973.
Discuss the steps taken in this procedure to minimize this determinate error.Three specific steps are taken to minimize the loss of ammonia: (1) the solutionis cooled to below room temperature before adding NaOH; (2) the digestionflask is quickly connected to the distillation apparatus after adding NaOH; and(3) the condenser tip of the distillation apparatus is placed below the surface ofthe HCl to ensure that the ammonia will react with the HCl before it can be lostthrough volatilization.4. How does K2S remove Hg2+, and why is this important?Adding sulfide precipitates the Hg2+ as HgS. This is important because NH3forms stable complexes with many metal ions, including Hg2+. Any NH3 that iscomplexed with Hg2+ will not be collected by distillation, providing anothersource of determinate error.9B.5 Quantitative ApplicationsAlthough many quantitative applications of acid–base titrimetry have been replacedby other analytical methods, there are several important applications that continueto be listed as standard methods.
In this section we review the general application ofacid–base titrimetry to the analysis of inorganic and organic compounds, with anemphasis on selected applications in environmental and clinical analysis. First,however, we discuss the selection and standardization of acidic and basic titrants.Selecting and Standardizing a Titrant Most common acid–base titrants are notreadily available as primary standards and must be standardized before they can beused in a quantitative analysis. Standardization is accomplished by titrating aknown amount of an appropriate acidic or basic primary standard.The majority of titrations involving basic analytes, whether conducted in aqueous or nonaqueous solvents, use HCl, HClO4, or H2SO4 as the titrant.
Solutions ofthese titrants are usually prepared by diluting a commercially available concentratedstock solution and are stable for extended periods of time. Since the concentrationsof concentrated acids are known only approximately,* the titrant’s concentration isdetermined by standardizing against one of the primary standard weak bases listedin Table 9.7.The most common strong base for titrating acidic analytes in aqueous solutionsis NaOH.
Sodium hydroxide is available both as a solid and as an approximately50% w/v solution. Solutions of NaOH may be standardized against any of the primary weak acid standards listed in Table 9.7. The standardization of NaOH, however, is complicated by potential contamination from the following reaction between CO2 and OH–.CO2(g) + 2OH–(aq) → CO32–(aq) + H2O(l)9.7When CO2 is present, the volume of NaOH used in the titration is greater than thatneeded to neutralize the primary standard because some OH– reacts with the CO2.*The nominal concentrations are 12.1 M HCl, 11.7 M HClO4, and 18.0 M H2SO4.1400-CH09 9/9/99 2:12 PM Page 299Chapter 9 Titrimetric Methods of AnalysisTable 9.7Selected Primary Standards for the Standardizationof Strong Acid and Strong Base TitrantsStandardization of Acidic TitrantsPrimary StandardTitration ReactionNa2CO3 + 2H3O+ → H2CO3 + 2Na+ + 2H2O(HOCH2)3CNH2 + H3O+ →(HOCH2)3CNH3+ + H2ONa2B4O7 + 2H3O+ + 3H2O → 2Na+ + 4H3BO3Na2CO3TRISNa2B4O7CommentabStandardization of Basic TitrantsPrimary StandardKHC8H4O4C6H5COOHKH(IO3)2Titration ReactionKHC8H4O4 + OH– → K+ + C8H4O42– + H2OC6H5COOH + OH– → C6H5COO– + H2OKH(IO3)2 + OH– → K+ + 2IO3– + H2OCommentcdaTheend point for this titration is improved by titrating to the second equivalence point, boiling thesolution to expel CO2, and retitrating to the second equivalence point.
In this case the reaction isNa2CO3 + 2H3O+ → CO2 + 2Na+ + 3H2ObTRIS stands for tris-(hydroxymethyl)aminomethane.cKHC H O is also known as potassium hydrogen phthalate, or KHP.8 4 4dDue to its poor solubility in water, benzoic acid is dissolved in a small amount of ethanol before beingdiluted with water.The calculated concentration of OH–, therefore, is too small. This is not a problemwhen titrations involving NaOH are restricted to an end point pH less than 6.Below this pH any CO32– produced in reaction 9.7 reacts with H3O+ to form carbonic acid.CO32–(aq) + 2H3O+(aq) → H2CO3(aq) + 2H2O(l)9.8Combining reactions 9.7 and 9.8 gives an overall reaction ofCO2(g) + H2O(l) → H2CO3(aq)which does not include OH–.
Under these conditions the presence of CO2 does notaffect the quantity of OH– used in the titration and, therefore, is not a source of determinate error.For pHs between 6 and 10, however, the neutralization of CO32– requires onlyone protonCO32–(aq) + H3O+(aq) → HCO3–(aq) + H2O(l)and the net reaction between CO2 and OH– isCO2(g) + OH–(aq) → HCO3–(aq)Under these conditions some OH– is consumed in neutralizing CO2.
The result is adeterminate error in the titrant’s concentration. If the titrant is used to analyze ananalyte that has the same end point pH as the primary standard used during standardization, the determinate errors in the standardization and the analysis cancel,and accurate results may still be obtained.Solid NaOH is always contaminated with carbonate due to its contact with theatmosphere and cannot be used to prepare carbonate-free solutions of NaOH. Solutions of carbonate-free NaOH can be prepared from 50% w/v NaOH sinceNa2CO3 is very insoluble in concentrated NaOH. When CO2 is absorbed, Na2CO32991400-CH09 9/9/99 2:12 PM Page 300300Modern Analytical Chemistryprecipitates and settles to the bottom of the container, allowing access to thecarbonate-free NaOH. Dilution must be done with water that is free from dissolvedCO2.
Briefly boiling the water expels CO2 and, after cooling, it may be used to prepare carbonate-free solutions of NaOH. Provided that contact with the atmosphereis minimized, solutions of carbonate-free NaOH are relatively stable when storedin polyethylene bottles. Standard solutions of sodium hydroxide should not bestored in glass bottles because NaOH reacts with glass to form silicate.Inorganic Analysis Acid–base titrimetry is a standard method for the quantitativeanalysis of many inorganic acids and bases. Standard solutions of NaOH can beused in the analysis of inorganic acids such as H3PO4 or H3AsO4, whereas standardsolutions of HCl can be used for the analysis of inorganic bases such as Na2CO3.Inorganic acids and bases too weak to be analyzed by an aqueous acid–basetitration can be analyzed by adjusting the solvent or by an indirect analysis.
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