Analytical Chem istry - DePauw University

438 Analytical Chemistry 2.0below room temperature, remove the Hg 2+ catalyst by adding 200 mL ofH 2 O and 25 mL of 4% w/v K 2 S. Add a few Zn granules to serve as boilingstones and 25 g of NaOH. Quickly connect the flask to a distillationapparatus and distill the NH 3 into a collecting flask containing a knownamount of standardized HCl. The tip of the condenser must be placedbelow the surface of the strong acid. After the distillation is complete,titrate the excess strong acid with a standard solution of NaOH usingmethyl red as an indicator (Figure 9.18).Qu e s t i o n s1. Oxidizing the protein converts all of its nitrogen to NH 4 + . Why isthe amount of nitrogen not determined by titrating the NH 4 + witha strong base?There are two reasons for not directly titrating the ammonium ion.First, because NH 4 + is a very weak acid (its K a is 5.6 10 –10 ), itstitration with NaOH yields a poorly defined end point. Second, evenif the end point can be determined with acceptable accuracy and precision,the solution also contains a substantial larger concentrationof unreacted H 2 SO 4 . The presence of two acids that differ greatly inconcentration makes for a difficult analysis. If the titrant’s concentrationis similar to that of H 2 SO 4 , then the equivalence point volumefor the titration of NH 4 + is too small to measure reliably. On theother hand, if the titrant’s concentration is similar to that of NH 4 + ,the volume needed to neutralize the H 2 SO 4 is unreasonably large.2. Ammonia is a volatile compound as evidenced by the strong smell ofeven dilute solutions. This volatility is a potential source of determinateerror. Is this determinate error negative or positive?Any loss of NH 3 is loss of nitrogen and, therefore, a loss of protein.The result is a negative determinate error.3. Discuss the steps in this procedure that minimize this determinateerror.Three specific steps minimize the loss of ammonia: (1) the solution iscooled below room temperature before adding NaOH; (2) after add-Figure 9.18 Methyl red’s endpoint for the titration of a strong acidwith a strong base; the indicator is: (a) red prior to the end point;(b) orange at the end point; and (c) yellow after the end point.