Monograph - Metrohm

30 Practical thermometric titrimetry
Thermometric titrimetry offers an opportunity to observe the influence of micelles on
the conduct of the determination. To avoid the influence of micelle-related phenomena
on the outcome of the titration, it is sometimes necessary to add a suitable solvent such
as acetonitrile.
7.4.6. Titration of non-ionic surfactants
When an excess of Ba 2+ is added to a non-ionic surfactant of the alkyl propylene oxide
derivative type, a pseudo-cationic complex is formed. This may be titrated with standard
sodium tetraphenylborate (see Figure 22). Two moles tetraphenylborate react with one
mole of the Ba 2+ / non-ionic surfactant complex.
Fig. 22: Thermometric titration of a
non-ionic surfactant in a formulation
containing an anionic surfactant.
7.5. Miscellaneous aqueous titrations
7.5.1. Titration of fluoride with boric acid
Acidic solutions of fluoride (including hydrofluoric acid) can be determined by a simple
thermometric titration with boric acid.
B(OH) 3 + 3 F – + 3 H +
BF 3 + 3 H 2 O
The titration plot illustrated in Figure 23 shows that the endpoint is quite rounded, suggesting
that the reaction might not proceed to stoichiometric equilibrium. However,
since the regions of the temperature curve immediately before and after the endpoint
are quite linear, the second derivative of this curve (representing the intersection of tangents)
will accurately locate the endpoint. Indeed, excellent precision can be obtained
with this titration, with a CV of less than 0.1.