Page 292 - Modern Analytical Chemistry

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Chapter 9 Titrimetric Methods of Analysis 275

–
This reaction occurs quickly and is of known stoichiometry. A titrant of SCN is
easily prepared using KSCN. To indicate the titration’s end point we add a small
amount of Fe 3+ to the solution containing the analyte. The formation of the red-
colored Fe(SCN) 2+ complex signals the end point. This is an example of a direct
titration since the titrant reacts with the analyte.
If the titration reaction is too slow, a suitable indicator is not available, or there
is no useful direct titration reaction, then an indirect analysis may be possible. Sup-
pose you wish to determine the concentration of formaldehyde, H 2 CO, in an aque-
–
ous solution. The oxidation of H 2 CO by I 3
–
–
–
–
H 2 CO(aq) + 3OH (aq)+I 3 (aq) t HCO 2 (aq)+3I (aq)+2H 2 O(l)
is a useful reaction, except that it is too slow for a direct titration. If we add a known
–
amount of I 3 , such that it is in excess, we can allow the reaction to go to comple-
2–
–
tion. The I 3 remaining can then be titrated with thiosulfate, S 2 O 3 .
–
2–
–
2–
I 3 (aq)+2S 2 O 3 (aq) t S 4 O 6 (aq)+3I (aq)
This type of titration is called a back titration. back titration
Calcium ion plays an important role in many aqueous environmental systems. A titration in which a reagent is added to
a solution containing the analyte, and
A useful direct analysis takes advantage of its reaction with the ligand ethylenedi- the excess reagent remaining after its
4–
aminetetraacetic acid (EDTA), which we will represent as Y . reaction with the analyte is determined
4–
2–
2+
Ca (aq)+Y (aq) t CaY (aq) by a titration.
Unfortunately, it often happens that there is no suitable indicator for this direct
2+
2+
titration. Reacting Ca with an excess of the Mg –EDTA complex
2–
2–
2+
2+
Ca (aq) + MgY (aq) t CaY (aq)+Mg (aq)
2+
2+
releases an equivalent amount of Mg . Titrating the released Mg with EDTA
2–
4–
2+
Mg (aq)+Y (aq) t MgY (aq)
gives a suitable end point. The amount of Mg 2+ titrated provides an indirect mea-
sure of the amount of Ca 2+ in the original sample. Since the analyte displaces a
species that is then titrated, we call this a displacement titration. displacement titration
When a suitable reaction involving the analyte does not exist it may be possible A titration in which the analyte displaces
to generate a species that is easily titrated. For example, the sulfur content of coal can a species, usually from a complex, and
the amount of the displaced species is
be determined by using a combustion reaction to convert sulfur to sulfur dioxide.
determined by a titration.
S(s)+O 2 (g) ® SO 2 (g)
Passing the SO 2 through an aqueous solution of hydrogen peroxide, H 2 O 2 ,
SO 2 (g)+H 2 O 2 (aq) ® H 2 SO 4 (aq)
produces sulfuric acid, which we can titrate with NaOH,
2–
–
H 2 SO 4 (aq) + 2OH (aq) t SO 4 (aq)+2H 2 O(l)
providing an indirect determination of sulfur.
9 3 Titration Curves
A.
To find the end point we monitor some property of the titration reaction that has a
well-defined value at the equivalence point. For example, the equivalence point for
a titration of HCl with NaOH occurs at a pH of 7.0. We can find the end point,