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3. 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 solution
is cooled to below room temperature before adding NaOH; (2) the digestion
flask is quickly connected to the distillation apparatus after adding NaOH; and
(3) the condenser tip of the distillation apparatus is placed below the surface of
the HCl to ensure that the ammonia will react with the HCl before it can be lost
through volatilization.
2+
4. How does K 2 S remove Hg , and why is this important?
Adding sulfide precipitates the Hg 2+ as HgS. This is important because NH 3
2+
forms stable complexes with many metal ions, including Hg . Any NH 3 that is
complexed with Hg 2+ will not be collected by distillation, providing another
source of determinate error.

9B.5 Quantitative Applications
Although many quantitative applications of acid–base titrimetry have been replaced
by other analytical methods, there are several important applications that continue
to be listed as standard methods. In this section we review the general application of
acid–base titrimetry to the analysis of inorganic and organic compounds, with an
emphasis 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 not
readily available as primary standards and must be standardized before they can be
used in a quantitative analysis. Standardization is accomplished by titrating a
known amount of an appropriate acidic or basic primary standard.
The majority of titrations involving basic analytes, whether conducted in aque-
ous or nonaqueous solvents, use HCl, HClO 4 , or H 2 SO 4 as the titrant. Solutions of
these titrants are usually prepared by diluting a commercially available concentrated
stock solution and are stable for extended periods of time. Since the concentrations
of concentrated acids are known only approximately,* the titrant’s concentration is
determined by standardizing against one of the primary standard weak bases listed
in Table 9.7.
The most common strong base for titrating acidic analytes in aqueous solutions
is NaOH. Sodium hydroxide is available both as a solid and as an approximately
50% w/v solution. Solutions of NaOH may be standardized against any of the pri-
mary weak acid standards listed in Table 9.7. The standardization of NaOH, how-
ever, is complicated by potential contamination from the following reaction be-
–
tween CO 2 and OH .
–
2–
CO 2 (g) + 2OH (aq) ® CO 3 (aq)+H 2 O(l) 9.7
When CO 2 is present, the volume of NaOH used in the titration is greater than that
–
needed to neutralize the primary standard because some OH reacts with the CO 2 .

*The nominal concentrations are 12.1 M HCl, 11.7 M HClO 4, and 18.0 M H 2SO 4.