Chemistry Reference
In-Depth Information
Ce(SO
4
)2
2H
2
O). Salts of this type are complex by name as
well as by formula, but in reality behave as
2(NH
4
)
2
SO
4
Ce
4
e
1
Ce
3
in solution. In the case of cerium, only one electron is transferred, and
calculation of the equivalent relationship is very straightforward.
A good example of a back titration involving iodine and thiosulfate is
the assay of resorcinol in Resorcinol Solution BP. Resorcinol is an antiseptic
that was widely used in the past, although less so now. The assay of resor-
cinol involves a quantitative electrophilic aromatic substitution reaction
using bromine as the reagent, as shown in Figure 6.4.
OH
OH
Br
Br
+ 3 Br
2
+ 3HBr
OH
OH
Br
Figure 6.4
The reaction of resorcinol with bromine.
Bromine is a volatile liquid at room temperature and pressure and so
cannot be measured accurately by pipette. It is also an extremely corrosive
compound, irritant to eyes, lungs and mucous membranes. To overcome
these difficulties, the bromine required for reaction with the resorcinol is
generated
in situ
by reaction of potassium bromate and potassium bromide
in the presence of strong mineral acid.
KBrO
3
5KBr
6HCl
3Br
2
3H
2
O
6KCl
1
To ensure that the bromination reaction proceeds quantitatively to the
right-hand side, an excess of bromine is generated and the volume of
bromine that does not react with resorcinol is determined by back titration.
Bromine cannot be titrated easily, so the excess bromine is determined by
addition of an excess of potassium iodide and titration of the liberated
iodine with sodium thiosulfate, to give sodium iodide and sodium
tetrathionate.
Br
2
2KI
I
2
2KBr
1
I
2
Na
2
S
4
O
6
This assay is great fun to do because the whole titration is carried out
using a special type of conical flask called an
iodine flask
. This type of flask
2Na
2
S
2
O
3
1
2NaI
