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Chapter 9 Titrimetric Methods of Analysis 327

2. Why is a small amount of Mg –EDTA complex added to the buffer?
2+
The titration’s end point is signaled by the indicator calmagite, which gives a
good end point with magnesium, but a poor end point with other cations such
as calcium. If the sample does not contain any Mg 2+ as a source of hardness,
then the titration will have a poorly defined end point and inaccurate results
2+
will be obtained. By adding a small amount of Mg –EDTA to the buffer, a
source of Mg 2+ is ensured. When the buffer is added to the sample, the Mg 2+ is
2+
displaced by Ca , because Ca 2+ forms a stronger complex with EDTA. Since the
displacement is stoichiometric, the total concentration of hardness cations
remains unchanged, and there is no change in the amount of EDTA needed to
reach the equivalence point.
3. Why does the procedure specify that the titration take no longer than 5 min?
The presence of a time limitation suggests that there must be a kinetically
controlled interference, possibly arising from a competing chemical reaction. In
this case the interference is the possible precipitation of CaCO 3 .

9C.5 Quantitative Applications
With a few exceptions, most quantitative applications of complexation titrimetry
have been replaced by other analytical methods. In this section we review the gen-
eral application of complexation titrimetry with an emphasis on selected applica-
tions from the analysis of water and wastewater. We begin, however, with a discus-
sion of the selection and standardization of complexation titrants.

Selection and Standardization of Titrants EDTA is a versatile titrant that can be
used for the analysis of virtually all metal ions. Although EDTA is the most com-
monly employed titrant for complexation titrations involving metal ions, it cannot
be used for the direct analysis of anions or neutral ligands. In the latter case, stan-
2+
+
dard solutions of Ag or Hg are used as the titrant.
Solutions of EDTA are prepared from the soluble disodium salt,
Na 2 H 2 Y•2H 2 O. Concentrations can be determined directly from the known mass
of EDTA; however, for more accurate work, standardization is accomplished by
titrating against a solution made from the primary standard CaCO 3 . Solutions of
+
Ag and Hg 2+ are prepared from AgNO 3 and Hg(NO 3 ) 2 , both of which are sec-
ondary standards. Standardization is accomplished by titrating against a solution
prepared from primary standard grade NaCl.

Inorganic Analysis Complexation titrimetry continues to be listed as a standard
2+
–
–
method for the determination of hardness, Ca , CN , and Cl in water and waste-
water analysis. The evaluation of hardness was described earlier in Method 9.2. The
2+
determination of Ca 2+ is complicated by the presence of Mg , which also reacts
2+
with EDTA. To prevent an interference from Mg , the pH is adjusted to 12–13,
precipitating any Mg 2+ as Mg(OH) 2 . Titrating with EDTA using murexide or Eri-
2+
ochrome Blue Black R as a visual indicator gives the concentration of Ca .
Cyanide is determined at concentrations greater than 1 ppm by making the
sample alkaline with NaOH and titrating with a standard solution of AgNO 3 ,
–
forming the soluble Ag(CN) 2 complex. The end point is determined using
p-dimethylaminobenzalrhodamine as a visual indicator, with the solution turn-
+
ing from yellow to a salmon color in the presence of excess Ag .