Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods437strength to that of H 3 O + . In a different solvent HCl and HNO 3 may notbehave as strong acids.If we place acetic acid in water the dissociation reaction+ −CH COOH( aq) + HO() l HO ( aq) + CH COO ( aq )3 2 3 3does not proceed to a significant extent because CH 3 COO – is a strongerbase than H 2 O, and H 3 O + is a stronger acid than CH 3 COOH. If we placeacetic acid in a solvent, such as ammonia, that is a stronger base than water,then the reactionCH COOH + NH NH + CH COO+ −3 3 4 3proceeds to a greater extent. In fact, both HCl and CH 3 COOH are strongacids in ammonia.All other things being equal, the strength of a weak acid increases ifwe place it in a solvent that is more basic than water, and the strength of aweak base increases if we place it in a solvent that is more acidic than water.In some cases, however, the opposite effect is observed. For example, thepK b for NH 3 is 4.75 in water and it is 6.40 in the more acidic glacial aceticacid. In contradiction to our expectations, NH 3 is a weaker base in themore acidic solvent. A full description of the solvent’s effect on the pK a ofweak acid or the pK b of a weak base is beyond the scope of this text. Youshould be aware, however, that a titration that is not feasible in water maybe feasible in a different solvent.Representative Method 9.1Description o f t h e Me t h o dDetermination of Protein in BreadThis method is based on a determination of %w/w nitrogen using theKjeldahl method. The protein in a sample of bread is oxidized to NH 4+using hot concentrated H 2 SO 4 . After making the solution alkaline, whichconverts the NH 4 + to NH 3 , the ammonia is distilled into a flask containinga known amount of HCl. The amount of unreacted HCl is determinedby a back titration with standard strong base titrant. Becausedifferent cereal proteins contain similar amounts of nitrogen, multiplyingthe experimentally determined %w/w N by a factor of 5.7 gives the %w/wprotein in the sample (on average there are 5.7 g protein for every gramof nitrogen).The best way to appreciate the theoreticaland practical details discussed in this sectionis to carefully examine a typical acid–base titrimetric method. Although eachmethod is unique, the following descriptionof the determination of protein inbread provides an instructive example ofa typical procedure. The description hereis based on Method 13.86 as published inOfficial Methods of Analysis, 8th Ed., Associationof Official Agricultural Chemists:Washington, D. C., 1955.Pr o c e d u r eTransfer a 2.0-g sample of bread, which has previously been air-dried andground into a powder, to a suitable digestion flask, along with 0.7 g of aHgO catalyst, 10 g of K 2 SO 4 , and 25 mL of concentrated H 2 SO 4 . Bringthe solution to a boil. Continue boiling until the solution turns clear andthen boil for at least an additional 30 minutes. After cooling the solution