Page 314 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:12 PM Page 297

Chapter 9 Titrimetric Methods of Analysis 297

Representative Methods Description of the Method. This quantitative method of analysis for proteins
Method 9.1
4
Determination of Protein in Bread
is based on a determination of the %w/w N in the sample. Since different cereal
proteins 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 (on
average there are 5.7 g of cereal protein for every gram of nitrogen). As described
here, nitrogen is determined by the Kjeldahl method. The protein in a sample of
bread is oxidized in hot concentrated H 2 SO 4 , converting the nitrogen to NH 4 . After
+
+
making the solution alkaline, converting NH 4 to NH 3 , the ammonia is distilled into
a flask containing a known amount of standard strong acid. Finally, the excess
strong 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 air
dried and ground into a powder, to a suitable digestion flask, along with 0.7 g of
HgO as a catalyst, 10 g of K 2 SO 4 , and 25 mL of concentrated H 2 SO 4 . Bring the
solution to a boil, and continue boiling until the solution turns clear, and for at The photo in Colorplate 8a shows the
least an additional 30 min. After cooling to below room temperature, add 200 mL indicator’s color change for this titration.
of H 2 O and 25 mL of 4% w/v K 2 S to remove the Hg 2+ catalyst. Add a few Zn
granules to serve as boiling stones, and 25 g of NaOH. Quickly connect the flask to
a distillation apparatus, and distill the NH 3 into a collecting flask containing a
known amount of standardized HCl. The tip of the condenser should be placed
below the surface of the strong acid. After the distillation is complete, titrate the
excess strong acid with a standard solution of NaOH, using methyl red as a visual
indicator.

Questions
+
1. Oxidizing the protein converts the nitrogen to NH 4 . Why is the amount of
+
nitrogen not determined by titrating the NH 4 with a strong base?
+
There are two reasons for not titrating the ammonium ion. First, NH 4 is a very
weak acid (K a = 5.7 ´10 –10 ) that yields a poorly defined end point when titrated
with a strong base. Second, even if the end point can be determined with
acceptable accuracy and precision, the procedure calls for adding a substantial
amount of H 2 SO 4 . After the oxidation is complete, the amount of excess H 2 SO 4
will be much greater than the amount of NH 4 that is produced. The presence
+
of two acids that differ greatly in concentration makes for a difficult analysis. If
the titrant’s concentration is similar to that of H 2 SO 4 , then the equivalence
+
point volume for the titration of NH 4 may be too small to measure reliably. On
+
the other hand, if the concentration of the titrant is similar to that of NH 4 , the
volume needed to neutralize the H 2 SO 4 will be unreasonably large.
2. Ammonia is a volatile compound as evidenced by the strong smell of even
dilute solutions. This volatility presents a possible source of determinate error.
Will this determinate error be negative or positive?

The conversion of N to NH 3 follows the following pathway
+
N ® NH 4
+
NH 4 ® NH 3
Any loss of NH 3 is loss of analyte and a negative determinate error.

—Continued